Chapter 15

Recall from Chapter 6 that about

250 million years ago all of Earth's continents were joined into the supercontinent called Pangea. Shortly thereafter, Pangea began to rift apart into the continents as we know them today. The Atlantic Ocean opened as the east coast of North America separated from Europe and Africa. As Pangea broke up, the continental crust fractured and thinned near the fractures (Figure 15.33A). Basaltic magma rose at the new spreading center, forming oceanic crust between North America and Africa (Figure 15.33B). All tectonic activity then focused on the continually spreading Mid-Atlantic Ridge, and no further tectonic activity occurred at the continental margins; hence the term "passive continental margin" (Figure 15.33C). (A) Continental crust fractured as Pangea began to rift. (B) Faulting and erosion thinned the crust as it separated. Rising basaltic magma formed new oceanic crust in the rift zone. (C) Sediment eroded from the continents formed broad continental shelves on the passive margins of North America and Africa.

atoll

A circular coral reef that forms a ring of islands around a central lagoon and that is bounded on the outside by the deep water of the open sea. Usually forms on top of a subsided seamount.

active continental margin

A continental margin that occurs at a convergent or transform plate boundary.

submarine canyons

A deep, V-shaped, steep-walled trough eroded into a continental slope and, in some cases, outer shelf. Funnels sediment from the continental shelf across the slope to the continental rise.

seismic reflection profiler

A device that emits a high-energy timed seismic signal that penetrates into and reflects from layers of sediment and rock beneath the seafloor; the data are used to construct an image of rock and sediment layers along with other geologic structures below the seafloor.

guyot

A flat-topped seamount, formed when the top of a sinking island, usually of volcanic origin, is eroded by wave energy.

turbidity currents

A highly turbulent mixture of sediment and water that flows rapidly downslope in a subaqueous setting. Capable of causing substantial subaqueous erosion.

bolide

A large piece of space debris, such as an asteroid, that crashes into a planet.

submarine fan

A large, fan-shaped accumulation of sediment deposited on the deep seafloor, usually within and beyond the mouth of submarine canyons.

oceanic trench

A long, narrow, steep-sided depression of the seafloor formed where a subducting oceanic plate sinks into the mantle, causing the seafloor to bend downward like a flexed diving board.

accreted terranes

A mappable, fault-bounded landmass that originates as an island arc or a microcontinent that is later added onto a continent.

passive continental margin

A margin that occurs where continental and oceanic crust are firmly joined together and where little tectonic activity occurs. Not a plate boundary.

chemosynthesis

A process in which bacteria produce energy from hydrogen sulfide or other inorganic compounds and thus are not dependent on photosynthesis.

seafloor drilling

A process in which drill rigs mounted on offshore platforms or on research vessels cut cylindrical cores from both the sediment and rock of the seafloor. Following their extraction, the cores are brought to the surface for study.

microwave radar

A satellite-based instrument that measures the travel time of microwave pulses that reflect off of the sea surface. The processed data allow for the detection of subtle swells and depressions on the sea surface, which are controlled by seafloor topography and can be used to map the seafloor.

continental shelf

A shallow, very gently sloping portion of the seafloor that extends from the shoreline to ~200 meters water depth at the top of the continental slope.

transform faults

A strike-slip fault between two offset segments of a mid-ocean ridge or along a strike-slip plate boundary.

oceanic island

A submarine mountain (seamount) that rises above sea level.

seamount

A submarine mountain, usually of volcanic origin, that rises 1 kilometer or more above the surrounding seafloor.

continental rise

An apron of sediment at the foot of the continental slope that merges with the deep seafloor.

echo sounder

An instrument that emits timed sound waves that reflect off the seafloor, return, and are recorded; the data are used to measure water depth and define the topography of the seafloor.

magnetometer

An instrument that measures the strength and, in some cases, the direction of a magnetic field.

rock dredge

An open-mouthed steel net dragged along the seafloor behind a research ship for the purpose of sampling rocks and sediment from the ocean floor.

carbonate platforms

Extensive accumulations of limestone, such as the Florida Keys and the Bahamas, formed in warm regions on an isolated continental shelf where terrigenous clastic sediment does not muddy the water and reef-building organisms thrive.

pillow basalt

Molten basaltic lava that solidified under water, forming spheroidal lumps of basalt that resemble a stack of pillows.

Pelagic sediment

Muddy ocean sediment made up of the skeletons of tiny marine organisms.

Terrigenous sediment

Sediment composed of sand, silt, and clay eroded from the continents.

abyssal plains

The flat, level, largely featureless parts of the ocean floor between the Mid-Oceanic Ridge and the continental rise.

continental slope

The relatively steep (averaging 3 degrees but varying between 1 degree and 10 degrees) submarine slope between the continental shelf and the continental rise.

sediment gravity flows

Underwater mixtures of sediment and water that flow downslope.

A seamount is

a submarine mountain that rises 1 kilometer or more above the surrounding seafloor. An oceanic island is a seamount that rises above sea level. Both are common in all ocean basins but are particularly abundant in the southwestern Pacific Ocean. Seamounts and oceanic islands sometimes occur as isolated peaks, but they are more commonly found in chains. Dredge samples show that seamounts, oceanic islands, and the ocean floor itself are all made of basalt. Many seamounts and oceanic islands are volcanoes that formed at a hot spot above a mantle plume, and most form within a tectonic plate rather than at a plate boundary. An isolated seamount or short chain of small seamounts probably formed over a plume that lasted for only a short time. In contrast, a long chain of large islands, such as the Hawaiian Island-Emperor Seamount chain, formed over a long-lasting plume. In this case, the lithospheric plate migrated over the plume as the magma continued to rise from a source beneath the lithosphere. Each volcano formed directly over the plume and then became extinct as the moving plate carried it away from the plume. As a result, the seamounts and oceanic islands become progressively younger toward the Island of Hawaii, located at the end of the chain and currently located over the active mantle plume (Figure 15.24). The Hawaiian Island-Emperor Seamount chain becomes older in a direction going away from the island of Hawaii. The numbers represent ages, in millions of years, of the oldest volcanic rocks of each island or seamount. After a volcanic island forms, it begins to sink, or subside. Three factors contribute to the sinking: If the mantle plume stops rising, it stops producing magma. Then the lithosphere beneath the island cools and becomes denser, and the island sinks. Alternatively, a moving plate may carry the island away from the hot spot. This also results in the cooling, contraction, and sinking of the island. The weight of the newly formed volcano causes isostatic sinking. Erosion lowers the top of the volcano. These three factors gradually transform a volcanic island to a seamount over geologic time (Figure 15.25). Calculations suggest that if the Pacific Ocean Plate continues to move at its present rate, the island of Hawaii may sink beneath the sea within 10 to 15 million years. In this case, sea waves likely will erode a horizontal upper surface on the sinking island, forming a flat-topped seamount called a guyot (pronounced "gee-o," after Swiss-born American geologist Arnold Henri Guyot), as illustrated in Figure 15.26. The Hawaiian Islands and Emperor Seamounts sink as they move away from the mantle plume currently located under the volcanically active Island of Hawaii. (A) A volcanic island rises above sea level. (B) Wave energy erodes a flat top on a sinking island to form a guyot. The South Pacific and portions of the Indian Ocean are dotted with numerous other islands called atolls. An atoll is a circular coral reef that forms a ring of islands around a central lagoon (Figure 15.27). Atolls vary from 1 to 130 kilometers in diameter and are surrounded by the deep water of the open sea. The Tetiaroa Atoll in French Polynesia formed by the process described in Figure 15.26. Over time, storm waves wash coral sand on top of the reef and vegetation grows on the sand, forming the individual islands in the atoll. If corals live only in shallow water, how did atolls form in the deep sea? Charles Darwin studied this question during his famous voyage on the Beagle from 1831 to 1836. He reasoned that a coral reef must have formed in shallow water on the flanks of a volcanic island. Eventually the island sank, but the reef continued to grow upward, so that the living portion always remained in shallow water (Figure 15.28). This proposal was not accepted at first because scientists could not explain how a volcanic island could sink. However, when scientists drilled into a Pacific atoll shortly after World War II and found volcanic rock hundreds of meters beneath the reef, Darwin's hypothesis was revived. It is considered accurate today, in light of our ability to explain why volcanic islands sink. (A) A fringing reef grows along the shore of a young volcanic island. (B) As the island sinks, the reef continues to grow upward to form a barrier reef that encircles the island. (C) Finally, the island sinks below sea level and the reef forms a circular atoll. This model of atoll formation was proposed by Charles Darwin in 1842, following his voyage on the Beagle.

Active continental margins form

along Andean-style subduction zones, where an oceanic plate converges with a continent, and along less common transform plate boundaries where oceanic crust is sliding past continental crust, as with the San Andreas Fault in California. In convergent margins, most of the sediment transported from a continent is swallowed up in the trench. As a result, an active margin commonly has a narrow continental shelf or none at all. The landward wall of the trench (the side toward the continent) is the continental slope of an active margin. It typically inclines at 4 degrees or 5 degrees in its upper part and steepens to 15 degrees or more near the bottom of the trench. The continental rise is absent or relatively small because sediment gravity flows generally transport sediment into the trench instead of across it to the ocean floor located over subducting plate (Figure 15.38). Along most active continental margins, an oceanic plate sinks beneath a continent, forming an oceanic trench. The continental shelf is narrow, the slope is steep, and the continental rise is small to nonexistent. Note that a trench can form wherever subduction occurs—where oceanic crust sinks beneath the edge of a continent, or where it sinks beneath another oceanic plate. However, the trench will be associated with a continental margin only when the overriding plate is made of continental lithosphere. At convergent boundaries, the oceanic plate sinks into the mantle at descent angles ranging from about 15 degrees to beyond vertical (Figure 15.39). Regardless of this remarkably wide variation in the descent angle of subducting slabs around Earth's ocean basins, an ocean trench is formed although in some regions the trench is filled with sediment and is not as obvious as elsewhere. For example, offshore of the Pacific Northwest, the Juan de Fuca plate is subducting below the North America plate, but the subduction is filled with sediment resulting from draining of the Cascades and northern Rockies. The Columbia River has been the largest deliverer of sediment to the trench, as huge volumes of water and sediment poured out of the Columbia River during and following each glaciation over the past 3 million years or so. In contrast, the Marianas trench off the eastern coast of the Philippines includes Earth's deepest ocean depth at 10,994 km—6.8 miles. A subducting slab will not usually descend into the mantle at the same angle as its dip angle. However, when it does occur (A above), as off the southern coast of Alaska, an subduction complex forms but the region behind the arc is neither compressed nor extended. (B) In situations where an old, cold ocean slab is sinking steeply into the mantle at an angle greater than its dip, the arc is subject to extensional stress, causing the overriding plate to break apart, as in the volcanic arc adjacent to the Marianas trench. When a young, hot and relatively buoyant oceanic plate subducts, it does so at an angle that is shallower than its dip, causing compression in the overriding plate and contractile deformation there as off the coast of Peru.

In many parts of the Pacific Ocean

and in some other ocean basins, two oceanic plates converge. One dives beneath the other, forming a subduction zone. The sinking plate pulls the seafloor downward, forming a long, narrow depression called an oceanic trench (Figure 15.20). The deepest place on Earth is in the Mariana Trench, north of New Guinea in the southwestern Pacific, where the ocean floor is nearly 11 kilometers below sea level. Depths of 8 to 10 kilometers are common in other trenches. An oceanic trench forms at a convergent boundary between two oceanic plates. One of the plates sinks and heats, generating magma that rises to form a chain of volcanic islands called an island arc. Huge amounts of magma are generated in the subduction zone. The magma rises and erupts at the seafloor to form submarine volcanoes next to the trench. The volcanoes eventually grow to become a chain of islands called an island arc (Figure 15.20), as we learned in Chapter 9. The western Aleutian Islands are an example of an island arc. Many others occur at the numerous convergent plate boundaries in the western Pacific (Figure 15.21). Photograph of an active stratovolcano on Pagan Island, part of the Commonwealth of the Northern Marianas in the western Pacific Ocean. The photo was taken on March 6, 2012, by an astronaut on the International Space Station, located about 480 kilometers southwest of the island at an altitude of about 400 kilometers. Pagan Island is part of the island arc formed by melting of old, cold subducting oceanic lithosphere of the Pacific Plate westward beneath oceanic crust of the Philippine Plate. The Marianas Trench is located immediately to the east of the arc and contains the deepest point of Earth's modern ocean at 10.9 kilometers. If subduction stops after an island arc forms, volcanic activity also ends. The island arc may then ride passively along with another tectonic plate until it arrives at another subduction zone. However, the density of island arc rocks is relatively low, making them too buoyant to sink into the mantle (Figure 15.22A and 15.22B). Instead, the island arc is mashed against the side of the overriding plate. (A) An island arc forms on a plate that is itself being subducted beneath a continent. (B) The island arc reaches the subduction zone but the arc's low density prevents it from subducting into the mantle. (C) The island arc is jammed onto the continental margin and becomes part of the continent. Both the subduction zone and trench jump to the seaward side of the island arc, thereby enlarging the continent. When an island arc and continent begin to slowly mash together, or "collide," the subducting plate commonly fractures on the seaward side of the island arc to form a new subduction zone. In this way, the island arc breaks away from the ocean plate and becomes part of the continent, enlarging it (Figure 15.22C). Much of the crust now underlying western California, Oregon, Washington, and western British Columbia was added to North America in this way between 180 million and about 50 million years ago, when several island arcs formerly in the eastern Pacific Ocean slowly collided with the western margin of the North American Plate. A simplified geologic map of these late additions to the North American continent, in general called accreted terranes, is shown in Figure 15.23. The accreted terranes of western North America are microcontinents and island arcs that were added to the continent from the Pacific Ocean.

Several devices

collect sediment and rock directly from the ocean floor. A rock dredge is an open-mouthed steel net dragged along the seafloor behind a research ship. The dredge breaks rocks from submarine outcrops and hauls them to the surface along with whatever other sediment and biota might be scooped up. Oceanographers sample seafloor mud by lowering a piston coring device to the bottom (Figure 15.4). This device consists of a steel core barrel with a sharp bit on the lower end, removable plastic tubing that fits inside the hollow core barrel, a large weight attached to the top of the barrel, and a trigger-and-piston mechanism. The trigger is released when the tip of the core barrel is only a meter or two above the seafloor, allowing the weighted core barrel to free-fall to the bottom, where it plunges bit-first into the mud. As the weight drives the core barrel downward, the piston inside the tubing is simultaneously drawn upward by a cable, in order to make way for and suck in the mud core. Once the barrel has stopped settling into the mud (usually less than a minute), the entire device is yanked out of the bottom sediments, winched to the surface, and brought on-board. There, scientists extracted, split, and describe the core. Illustration of a piston-coring device recovering a core of sediment from the seafloor. (A) The device is first lowered to the seafloor on a cable and consists of a steel piston core barrel with plastic core lining inside and a heavy lead weight attached to the top. The core barrel is connected through a trigger mechanism to a weighted cable that hangs down beyond the tip of the core barrel. (B) When the end of the weighted cable reaches the seafloor, the cable goes slack, allowing the spring-loaded trigger arm to rotate upward and release the core barrel. The core barrel and weight free fall a few meters to the seafloor. (C) The inertia of the falling, weighted core barrel drives it into the sediment of the seafloor. As the core barrel penetrates downward into the sediment, the piston within the core lining is pulled upward by a cable, making room for the sediment core and helping to suck it into the core barrel. (D) After a minute or so, friction stops the core barrel from penetrating any further into the sediment, and the entire apparatus is winched out of the seafloor and up to the surface. Once onboard the research vessel, the mud core is extracted from the core barrel, split into two halves, and described by scientists. Because piston coring involves driving the core barrel downward into the mud in one swift action, the length of the core itself is always limited by the strength of the barrel and its ability to penetrate downward into the sediment rather than fold over as the weight drives it downward. Most piston cores are not over 10 meters long. In contrast, seafloor drilling methods developed for oil exploration can recover continuous cores from seafloor sediment as well as lithified rock that are hundreds of meters long. Large drill rigs are mounted on offshore platforms and on research vessels. The drill cuts cylindrical cores from sediment and rock, which are then brought to the surface for study. Japanese scientists set world record for longest sediment-core recovery in response to 2011 Tōhoku earthquake/tsunami disaster Although the use of drilling platforms and research vessels to extract continuous cores of sediment from the deep ocean floor is expensive, the core sediments themselves represent the physical record of geologic history for that location and, as such, can be invaluable. Far from simply recovering a long tube of sediment or cylinder of rock for study, modern state-of-the-art deep-sea drilling involves simultaneously conducting numerous measurements of the physical conditions of the sediment and rock being cut by the drill bit. The simultaneous drilling and measuring or logging of these physical conditions, called logging while drilling (LWD) technology, provides real-time information about the sediment and rock being drilled, including its temperature and pressure, natural radioactivity, and ability to conduct electricity. LWD technology also provides real-time information about the drilling process, such as the drilling rate, orientation of the drill string, and circulation of fluids needed to drill the hole. An excellent example of the tremendous potential scientific and societal value of seafloor drilling, logging, and coring is the spring 2012 LWD campaign of the Japan Trench conducted by scientists aboard the Japanese research vessel Chikyu. The LWD work that took place in April 2012 was in direct response to the devastating Tōhoku earthquake and tsunami that struck only 13 months earlier on March 9, 2011. (See Chapter 7 for more on this earthquake and tsunami and the fault rupture that produced both.) The overarching research question behind the LWD program carried out aboard the Chikyu was this: "What mechanisms control the occurrence of destructive earthquakes, landslides, and tsunami?" More specifically, the research team sought to understand what controls the state of stress across faults that rupture to produce very large earthquakes and to assess whether all of this stress had been released during the 2011 Tōhoku Earthquake. In addition, the researchers sought to characterize the physical conditions associated with the main plate boundary fault that ruptured during the 2011 seismic event, using LWD technology in combination with recovery of continuous sediment/rock core and deployment of a permanent suite of temperature and pressure data sensors. After departing from the Japanese port of Shimizu on April 1, 2012, the Chikyu arrived at the drilling site (located about 250 kilometers east of the Sendai peninsula) two days later (Figure 15.5). While en route, the crew used the time to organize and prepare for deployment of the many sections of drill pipe that would be strung together to recover the core. After arriving on site and waiting out two days of bad weather, the crew deployed the ship's powerful stabilizers, which were needed to maintain the ship at a constant location and position throughout the drilling process. The stabilizers are controlled by computers that constantly monitor the ship's absolute position using a high-precision GPS system. On April 25, 2012, Japanese scientists aboard the Chikyu used a deep-sea drilling rig to recover a core 850 meters long from the outer portion of the subduction complex containing the fault that ruptured to produce the 2011 earthquake and tsunami. In doing so, the scientists set a world record by using a drill pipe with a total underwater length of 7.7 kilometers to recover the core in water 5.9 kilometers deep. Both the hole drilled and logged by the Japanese team and the core recovered from the hole provide direct access to the fault that ruptured to produce the devastating Tōhoku earthquake. An east-west-oriented seismic reflection profile shot before the drilling campaign clearly shows the main fault rupture surface within sediments of the subduction complex, in addition to imaging the basaltic top of the downgoing Pacific Plate and several normal faults that offset it (Figure 15.6). The normal faults are produced by extensional stress in the top of the downgoing slab as it is being bent downward into the subduction zone and are unrelated to the Tōhoku earthquake. Example of data collected from sediment and rock cut during the drilling process. This "logging while drilling" (LWD) technology provides onboard scientists with real-time information about the geology being reached by the tip of the drill string as drilling is taking place. Examples of the data collected during and after the drilling process are shown on the following page. Figure 15.6 is an example of the graphical data resulting from the logging process. Not only were various physical attributes of the sediment measured, such as its natural radioactivity, but an image of the sediment itself was captured by a camera system used in the logging process. The fault that ruptured on March 9, 2011, was reached by the Chikyu's drill bit at a total depth of ~820 meters below the seafloor. The fault puts a sequence of deformed, clayey to silty mudstones containing abundant terrigenous sediment and volcanic ash over a relatively undeformed sequence of pelagic mudstones inferred to have been deposited in deep water on the Pacific Plate (Figure 15.7). The fault itself is actually a 5-meter-thick fault zone consisting of highly sheared clay with numerous centimeter-scale curved fault surfaces that give the clay a scaly appearance. Seismic reflection profile showing the main geologic features associated with the Chikyu's drill site. The top of the basaltic oceanic crust of the downgoing Pacific Plate is clearly imaged on the profile, as is the ruptured fault surface that produced the Tōhoku earthquake. The fault was intersected by the Chikyu's drill bit approximately 820 meters below the seafloor. After extracting the sediment core from the drillhole, the scientists installed an array of extremely sensitive temperature and pressure sensors in order to measure the residual frictional heat left over from the fault rupture, to better quantify the amount of energy released by the earthquake, and to assess whether all energy was released by the fault rupture—which was estimated to involve 50 meters of offset at the drill site. An example of the temperature data is shown in Figure 15.8. The temperature peak coincides with the fault surface and represents heat still dissipating from the rupture. Temperature sensors installed in the borehole show a clear temperature spike across the ruptured fault zone. The higher temperatures result from heat still dissipating from frictional resistance during the 2011 fault rupture. These data help geoscientists understand how energy is released during fault ruptures that produce very large earthquakes. Though it will take scientists many years to slowly sift through the data generated by the Chikyu's LWD campaign and analyze the cores extracted from deep within the subduction complex, the information generated by this historic expedition is sure to reveal numerous additional insights into the geology behind the Tōhoku earthquake while furthering the scientific understanding of how extremely large earthquakes occur.

The primordial Earth

heated by the impacts of colliding planetesimals and the decay of radioactive isotopes, was molten, or near molten. The sky, without an atmosphere, was black. There were no oceans—no life. Today, we consider Earth in terms of four spheres: the geosphere, hydrosphere, atmosphere, and biosphere. Each sphere is as different from the others as a rock is different from a flowing stream, a breath of air, or a butterfly. For the moment, let's abandon our view of Earth's four spheres and think of only two kinds of Earth compounds: compounds that are volatile, or can evaporate rapidly and easily escape into the atmosphere, and compounds that are not. Water is a good example of a volatile compound; carbon dioxide is another. Most scientists agree that the surface of primordial Earth consisted mainly of rock and partially molten rock and contained few volatile compounds. How, then, did our thick atmosphere, vast oceans, and a global biosphere of living organisms form? For many years, geologists hypothesized that abundant volatiles, including water and carbon dioxide, were trapped within early Earth's interior. This reasoning was based on three observations and inferences. First, cosmogenic models showed that volatiles were evenly dispersed in the cloud of dust, gas, and planetesimals that coalesced to form the planets. It seemed likely that some of those volatiles would have become trapped within Earth as it formed. Second, scientists detected volatiles in modern comets, meteoroids, and asteroids. If volatiles were trapped within the small objects that passed through our neighborhood in space, it seemed logical to infer that they also accumulated in Earth's interior as the original cloud of dust and gas coalesced. Finally, modern volcanic eruptions eject gases and water vapor into the air. Geologists have inferred that these gases originate in the mantle and are remnants of the original volatiles trapped during Earth's formation. Geologists therefore concluded that some of these volatiles escaped during volcanic eruptions early in Earth's history and that they formed the atmosphere and the oceans, and ultimately gave rise to living organisms. Today, many scientists question this conclusion. To understand their questions, let's return to the cloud of dust and gas that coalesced to form the planets. Recall that our region of space heated up as dust, gas, and planetesimals collided to become planets. At the same time, hydrogen fusion began within the Sun and solar energy radiated outward to heat the inner Solar System. The newly born Sun also emitted a stream of ions and electrons, called the solar wind, that swept across the inner planets, blowing their volatile compounds into outer regions of the Solar System. As a result, most of Earth's volatile compounds essentially boiled off and were swept by the solar wind into the cold outer regions of the Solar System (Figure 15.1). As the Solar System formed volatiles boiled away from the inner planets. The solar wind then blew them into the cooler region beyond Mars. (B) The outer planets captured some of the volatiles, and some condensed to form comets in the frigid zone beyond Neptune. As a result, the inner four planets were left with little or no atmosphere or ocean and the outer planets grew to become gaseous giants. (C) Comets and meteorites crashed into Earth and other planets, returning some of the volatiles to their surfaces. Over time, the volatiles accumulated to form the atmosphere and oceans and therefore the foundation of life on Earth. (Sizes and distances in this drawing are not to scale.) According to a currently popular hypothesis, shortly after our planet formed and lost its volatiles, a Mars-sized object smashed into Earth. The cataclysmic impact blasted through the crust and deep into the mantle, ejecting huge quantities of pulverized rock into orbit. The fragments eventually coalesced to form the Moon. The impact also ejected most of Earth's remaining volatiles with enough velocity that they escaped Earth's gravity and disappeared into space. According to this hypothesis, Earth's surface then was left barren and rocky, with few volatiles either on the surface or in the deep mantle. Thus, Earth's early surface had neither water nor an atmosphere. The hot mantle churned and volcanic eruptions repaved the planet with lava, but these events brought relatively few volatiles to Earth's surface. According to one estimate, outgassing of the deep mantle accounted for no more than 10 percent of Earth's hydrosphere, atmosphere, and biosphere. If this scenario is correct, why do modern volcanoes release volatiles? According to one hypothesis, most of the gases given off by modern volcanoes are recycled from the surface. Water, carbon (in the form of carbonate rocks such as limestone), and other light compounds are carried into shallow parts of the mantle by subducting plates. There, the volatiles are incorporated into magma that forms within the volcanic arc associated with the subduction zone. The volatiles are returned to the surface when volcanoes forming the arc erupt. Thus, modern volcanic eruptions, like their primordial ancestors, do not deliver significant quantities of volatiles from the deep mantle to Earth's surface. Now let's return to the volatiles that streamed away from the hot, inner Solar System. As they flew away from the Sun, volatiles entered a cooler region beyond Mars. Most of the volatiles were captured by the outer planets—Jupiter, Saturn, Uranus, and Neptune—but some continued their journey toward the outer fringe of the Solar System (Figure 15.1B). There, beyond the orbits of the known planets, volatiles from the inner Solar System combined with residual dust and gas to form comets (Figure 15.2). A comet's nucleus has been compared to a dirty snowball because it is composed mainly of ice and rock. However, other compounds not common in snowballs exist in comets as well. These include frozen carbon dioxide, ammonia, and simple organic molecules. Volatiles are also abundant in certain types of meteoroids and asteroids in the region between Mars and Jupiter. Comet Hale-Bopp, shown here in 1997, was probably the most widely observed comet of the twentieth century. The early Solar System was crowded with comets, meteoroids, and asteroids; these bodies are composed of rock and condensed volatile materials such as ice and solid carbon dioxide. Astronomers calculate that the early Solar System was crowded with comets, meteoroids, and asteroids—space debris left over from planetary formation. Much of this material contained volatile compounds. When a large piece of space debris crashes into a planet, it is called a bolide. Early in the formation of the Solar System, a large number of bolides crashed into Earth, nearby planets, and moons. In this way, the bolides delivered and deposited a large mass of volatile compounds to the inner Solar System (Figure 15.1C). While falling space debris added less than 1 percent to Earth's total mass, it imported roughly 90 percent of its modern reservoir of volatiles. Upon entry and impact, the frozen volatiles in the bolides vaporized, releasing water vapor, carbon dioxide, ammonia, simple organic molecules, and other volatile compounds. As the planet cooled and atmospheric pressure increased, the water vapor condensed to liquid, forming the first oceans. The light molecules transported to Earth in bolides also provided gases that formed the atmosphere and the raw materials for life. (The formation and evolution of the atmosphere is discussed in Chapter 17.) Thus, at least some, and probably most, of the compounds necessary to produce the hydrosphere, the atmosphere, and the biosphere traveled to Earth from outer regions of the Solar System. The water that fills Earth's oceans came from interplanetary space. Later in Earth's history, impacts from outer space blasted rock and dust into the sky, causing mass extinctions and killing large portions of life on Earth. (See Chapter 4 for more on mass extinctions.) Ironically, extraterrestrial impacts may have provided the raw materials for the oceans, the atmosphere, and for life—but they later caused mass extinctions.

A thin layer of marine sedimentary rocks blankets

large areas of Earth's continents. These rocks tell us that those places must have been below sea level when the sediment accumulated. Tectonic activity can cause a continent to sink, allowing the sea to flood a large area. However, at particular times in the past (most notably during the Cambrian, Carboniferous, and Cretaceous Periods), marine sediments accumulated on low-lying portions of all continents simultaneously, indicating simultaneous global flooding of low-elevation regions on all continents. Although our plate tectonics model explains the sinking of individual continents, or parts of continents, it does not explain why all continents should sink at the same time. Therefore, we need to explain how sea level could rise globally by hundreds of meters to flood all continents simultaneously. Continental glaciers have advanced and melted numerous times in Earth's history. During the growth of continental glaciers, seawater evaporates and is frozen into the ice that rests on land. As a result, sea level drops. When glaciers melt, the water runs back into the oceans and sea level rises. The alternating growth and melting of glaciers during the Pleistocene Epoch caused sea level to fluctuate by as much as 150 meters. However, the ages of most marine sedimentary rocks on continents do not coincide with times of glacial melting. Therefore, we must look for a different cause to explain continental flooding. Recall that the new, hot lithosphere at a spreading center is buoyant, causing the Mid-Oceanic Ridge system to rise above the surrounding seafloor. This submarine mountain chain displaces a huge volume of seawater. If the Mid-Oceanic Ridge system were smaller, it would displace less seawater and sea level would fall. If it were larger, sea level would rise. The Mid-Oceanic Ridge rises highest at the spreading center, where new lithosphere rock is hottest and has the lowest density. The elevation of the ridge decreases on both sides of the spreading center because the lithosphere cools and shrinks as it moves outward. Now consider a spreading center where spreading is very slow (perhaps 1 to 2 centimeters per year). At such a slow rate, the newly formed lithosphere would cool before it migrated far from the spreading center. As a result, the ridge would be narrow and of low volume, as shown in Figure 15.19A. In contrast, rapid seafloor spreading of 10 to 20 centimeters per year would create a high-volume ridge because the newly formed, hot lithosphere would be carried a considerable distance away from the spreading center before it cooled and shrank (Figure 15.19B). This high-volume ridge would displace considerably more seawater than would a low-volume ridge, and the high-volume ridge would cause global sea level to rise. To summarize, rapid spreading produces a larger volume of hot rock and pushes aside more seawater, causing global sea level to go up. If spreading then slows down, a smaller volume of hot rock is produced, displacing less seawater and causing global sea level to fall. (A) Slow seafloor spreading creates a narrow, low-volume Mid-Oceanic Ridge that displaces less seawater and lowers sea level. (B) Rapid seafloor spreading creates a wide, high-volume ridge that displaces more seawater and raises sea level. Seafloor age data indicate that the rate of seafloor spreading has varied from about 2 to 16 centimeters per year over the past 200 million years ago, since the Jurassic Period. During Late Cretaceous times, between 110 and 85 million years ago, seafloor spreading was unusually rapid, and that rapid spreading caused the formation of an unusually high-volume Mid-Oceanic Ridge. As the total volume of the Mid-Oceanic Ridge grew with the increase in average global spreading rate, more and more seawater was displaced and low-lying portions of continents were inundated. Geologists have found marine sedimentary rocks of Late Cretaceous age on nearly all continents, indicating that Late Cretaceous time was, in fact, one of abnormally high global sea level. Thus, a process initiated by heat transfer deep within the geosphere profoundly affected the sea level of the hydrosphere and life throughout the biosphere. Unfortunately, because no oceanic crust is older than about 200 million years, the specific relationships between seafloor spreading rates and global sea level cannot be tested for earlier times when extensive marine sedimentary rocks accumulated on continents.

If you were to ask

most people to describe the difference between a continent and an ocean, they would almost certainly reply, "Why, obviously, a continent is land and an ocean is water!" This observation is true, of course, but to a geologist what is more important is that the rock beneath the ocean water is different from the rock beneath the land surface. The accumulation of seawater in the world's ocean basins is a result of that rock difference. Modern oceanic crust is dense basalt and varies from 4 to 7 kilometers thick. Continental crust is made of lower-density granite and averages 20 to 40 kilometers in thickness. In addition, the entire continental lithosphere is both thicker and less dense than oceanic lithosphere. As a result of these differences, the thick, lower-density continental lithosphere floats isostatically at high elevations, whereas oceanic lithosphere sinks to low elevations. Most of Earth's water flows downhill, to collect in the vast depressions formed by oceanic lithosphere. Even if no water existed on Earth's surface, oceanic crust would form deep basins and continental crust would form regions of higher elevation. Oceans cover about 71 percent of Earth's surface. The seafloor is about 5 kilometers deep in the central parts of the ocean basins, although it is only 2 to 3 kilometers deep above the Mid-Oceanic Ridge and plunges to 11 kilometers in the Mariana Trench (Figure 15.3). A schematic cross section of the continents and ocean basins. The vertical axis shows elevations relative to sea level. The horizontal axis shows the basic breakdown of Earth's main topographic surfaces. Thus, for example, roughly 30 percent, or about 150 million square kilometers, of Earth's surface lies above sea level. The ocean basins contain 1.4 billion cubic kilometers of water—18 times more than the volume of all land above sea level. So much water exists at Earth's surface that if Earth were a perfectly smooth sphere, it would be covered by a global ocean 2,000 meters deep. The size and shape of Earth's ocean basins have changed over geologic time. At present, the Atlantic Ocean is growing wider at a rate of a few centimeters each year as the seafloor spreads apart at the Mid-Atlantic Ridge and as the Americas move away from Europe and Africa. At the same time, the Pacific is shrinking at a similar rate, as oceanic crust sinks into subduction zones around its edges. In short, the Atlantic Ocean basin is now expanding at the expense of the Pacific. Global climate is profoundly influenced by the immense volume of Earth's oceans and the fact that the oceans consist of liquid water. On average, the ocean is about 3,800 meters deep, yet the thermal capacity of Earth's entire atmosphere is equivalent only to the uppermost 2.5 meters of ocean water (less than 0.05%). Ocean currents transport heat from the equator toward the poles, cooling equatorial climates and warming polar environments. The seas also absorb and store solar heat more efficiently than do rocks and soil. As a result, oceans are commonly warmer in winter and cooler in summer than adjacent land is. Most of the water that falls as rain or snow is water that evaporated from the seas. In these and other ways, the oceans have played a major role in controlling Earth's climate and the distribution of different climate zones through geologic time.

The discovery of the Mid-Oceanic Ridge system took

place over the course of several decades and was possible only by scientific analysis of large volumes of oceanographic data, much of which was collected for military purposes. Following World War I (1914-1918), oceanographers began using early versions of echo-sounding devices to measure ocean depths. Those surveys showed that the seafloor was much more rugged than previously thought, and they further identified the continuity and size of the Mid-Atlantic Ridge. During World War II, naval commanders needed topographic maps of the seafloor to support submarine warfare. Those detailed maps, made with early versions of the echo sounder, were kept secret by the military. When they became available to the public after peace was restored, scientists were surprised to learn that the ocean floor has at least as much topographic diversity and relief as the continents do. Broad plains, high peaks, and deep valleys form a varied and fascinating submarine landscape. In the 1950s, oceanographic surveys conducted by several nations led to the discovery that the Mid-Atlantic Ridge is just part of a great submarine mountain range, now called the Mid-Oceanic Ridge system. Recall from Chapter 6 that the Mid-Oceanic Ridge system is a continuous submarine mountain chain formed by the rifting apart of two oceanic plates. The Mid-Ocean Ridge system encircles the globe and has a total length exceeding 80,000 km (Figure 15.13). In some places it is more than 1,500 kilometers wide. The ridge system rises an average of 2 to 3 kilometers above the surrounding deep seafloor because heat from the rifting causes rocks in and around the plate boundary to be hot. The hot rocks of the ridge system take up more volume so float higher in the underlying asthenosphere. Although the Mid-Ocean Ridge lies almost exclusively beneath the sea surface, it is Earth's longest continuous mountain chain. This map shows that divergent plate boundaries, or spreading centers, coincide exactly with the Mid-Oceanic Ridge system in the world's oceans. The spreading centers are shown in double red lines; the single red lines are transform faults. A rift valley is an elongated depression that develops at a divergent plate boundary (Figure 15.14). In the Mid-Oceanic Ridge system, a rift valley 1 to 2 kilometers deep and several kilometers wide splits many segments of the ridge crest. Oceanographers in small research submarines have dived into the rift valley, where they have documented gaping vertical cracks up to 3 meters wide on the valley floor. In some cases, extremely hot hydrothermal water highly concentrated with dissolved metals and metal-bearing compounds streams out of the cracks. A cross-sectional view of the central rift valley in the Mid-Oceanic Ridge. As the plates separate, blocks of rock drop down along the fractures to form the rift valley, bounded by normal faults. Movements across these faults cause earthquakes. To understand the geologic significance of such cracks, recall that the Mid-Oceanic Ridge system is a spreading center, where two lithospheric plates are moving apart from each other. The cracks form as tensile stress at the ridge axis causes brittle oceanic crust to separate. Basaltic magma then rises through the resulting crack and flows onto the floor of the rift valley. This basalt becomes new oceanic crust as two lithospheric plates spread outward from the ridge axis. The new crust (and the underlying lithosphere) at the ridge axis is warmer, and therefore of relatively lower density, than older crust and lithosphere located farther from the ridge axis. Its buoyancy causes it to float high above the surrounding seafloor, elevating the Mid-Oceanic Ridge system 2 to 3 kilometers above the deep seafloor. The new lithosphere cools as it spreads away from the ridge. As a result of cooling, the lithosphere shrinks and cracks as it becomes denser and sinks to lower elevations, forming the deeper seafloor on both sides of the ridge (Figure 15.15). The seafloor sinks as it grows older. At the Mid-Oceanic Ridge, new lithosphere is buoyant because it is hot and of low density. As it moves away from the ridge, the lithosphere cools, thickens, and becomes denser, causing it to sink. On average, the seafloor lies at a depth of about 4 kilometers, relative to 2 to 3 kilometers at the Mid-Ocean Ridge. Normal faults and shallow earthquakes are common along the Mid-Oceanic Ridge system because oceanic crust fractures as the two plates separate. Blocks of crust drop downward along some of the seafloor cracks, forming faults that bound the rift valley. Thus, the cracks documented along the mid-ocean spreading ridge by oceanographers can be explained as the result of three primary causes: (1) separation of rock by tectonic tension associated with the spreading ridge; (2) decrease of rock volume by the cooling and contraction of newly formed basalt crust on the ocean floor; and (3) normal faulting in which rock is physically displaced across a crack. On the volcanically active Mid-Oceanic Ridge system, hot rocks heat seawater as it circulates through fractures in oceanic crust. The hot water dissolves metals and sulfur from the rocks. Eventually, the hot metal-and-sulfur-laden water rises back to the seafloor surface, spouting from fractures as a jet of cloudy, black water called a black smoker (described in Chapter 5). The black color is caused by precipitation of fine-grained metal sulfide minerals as the solutions cool on contact with seawater (Figure 15.16). A black smoker spouts from the East Pacific Rise. Seawater is heated as it circulates through the hot rocks of the rift zone, and it dissolves metals and sulfur from the rocks. The ions precipitate as "smoke," consisting of tiny mineral grains, when the hot suspension spews into cold ocean water. These scalding, sulfurous waters are as hot as 400*C , and seemingly would produce a sterile environment in which nothing could survive. Yet, remarkably, the deep seafloor around a black smoker teems with life. At the vents, bacteria produce energy from hydrogen sulfide in a process called chemosynthesis. Thus, the bacteria release energy from chemicals and are not dependent on photosynthesis. The chemosynthetic bacteria are the foundation of a deep-sea food chain: either larger vent organisms eat them or the larger organisms live symbiotically with the bacteria. For example, instead of a digestive tract, the red-tipped tube worm (Figure 15.17) has a special organ that hosts the chemosynthetic bacteria. In return for providing a home for the bacteria, the worm receives nutrition from the bacteria's wastes. Other vent organisms in this unique food chain include giant clams and mussels, eyeless shrimp, crabs, and fish. These red tubeworms are part of a thriving plant and animal community living near a submarine hydrothermal vent in the Guaymas Basin in the Gulf of California. In addition to cracks or faults located along the axis of the mid-oceanic ridges and rift valleys are hundreds of fractures, called transform faults, that cut across the rift valleys and ridges (Figure 15.18). These fractures extend through the entire thickness of the lithosphere and develop because the Mid-Oceanic Ridge system consists of many short segments. Each segment is slightly offset from adjacent segments by a transform fault. Transform faults are original features of the Mid-Oceanic Ridge; they form as an accommodation to Earth's spherical shape when lithospheric spreading begins. Transform faults offset segments of the Mid-Oceanic Ridge. Adjacent segments of the ridge may be separated by steep cliffs 3 kilometers high. Note the flat abyssal plain far from the ridge. Some transform faults displace the ridge by less than a kilometer, but others offset the ridge by hundreds of kilometers. In some cases, a transform fault can grow so large that it forms a transform plate boundary. The San Andreas Fault in California is a transform plate boundary that offsets both oceanic and continental crust.

A continental margin is a

place where continental lithosphere meets oceanic lithosphere. Two types of continental margins exist. A passive continental margin occurs where continental and oceanic lithosphere are firmly joined together. Because it is not a plate boundary, little tectonic activity occurs at a passive margin. Continental margins on both sides of the Atlantic Ocean are passive margins. In contrast, an active continental margin occurs at a convergent plate boundary, where oceanic lithosphere sinks beneath the continent in a subduction zone. The west coast of South America is an active continental margin, also called an Andean margin, as you learned in Chapter 9.

Remote sensing methods do not

require direct physical contact with the ocean floor, and for some studies this approach is both effective and economical. The echo sounder is an instrument commonly used to map seafloor topography. It emits a sound signal from a research ship and then records the signal after it bounces off the seafloor and travels back up to the ship. The water depth is calculated from the time required for the sound to make the round trip. A topographic map of the seafloor is constructed as the ship steers a carefully navigated course with the echo sounder operating continuously. Modern echo sounders, called sonar, transmit 1,000 signals at a time to create more-complete and accurate maps. A seismic reflection profiler works in the same way but uses a higher-energy signal that penetrates and reflects from layers of sediment and rock below the seafloor (Figure 15.9). This geophysical technique provides an image of the layering and structure of seafloor sediments, the rock layers and crust below, and major structures such as folds and faults that might be present. Seismic reflection profiling involves emitting source energy in the form of sound waves and recording that portion of the sound energy that reaches an array of detectors. The length of time required for the sound energy to travel downward to a particular layer of rock below the seafloor, bounce off of it, and travel farther to the detector depends on the depth of the rock layer and the velocity with which the sound waves travel to and from the layer. By using highly engineered source emitters and detectors, and carefully processing the returned sound recording through a computer, a seismic reflection profile is produced that is a visual representation of the geology below the seafloor as shown by example in Figures 15.7 and 15.10. Figure 15.10 is an example of a modern seismic reflection profile that was shot by the USGS in shallow water off the coast of central California. The red and blue layers in the image represent layers of sediment and sedimentary rock below the seafloor. Notice the presence of two major strike-slip fault zones, associated with the transform plate boundary between the North American and Pacific Plates. An example seismic reflection profile; note the horizontal and vertical scales. This profile was shot by the USGS offshore central California on the continental shelf. The red and blue lines in the profile reflect subsurface layers of sediment or rock from which significant returning sound energy was detected. Notice that the profile crosses two fault zones, both of which are associated with the transform boundary between the North American and Pacific Plates. A different type of remote sensing that involves the reflection of compression waves off the seafloor is side scan sonar imaging. This technique involves towing a heavy, torpedo-shaped vessel called a "tow-fish" behind a powerful ship. Inside the tow-fish is an array of transducers that emit, then receive, an acoustic pulse of energy of a specific frequency. Figure 15.11A is a cartoon showing a tow-fish emitting and receiving sonar information as it is being towed behind the research ship. Notice that the zone through which the acoustic pulse is sent and received is very narrow in the towing direction but very wide in the direction perpendicular to the towing track. This configuration is designed to maximize the return of sonar energy along the seafloor. As the ship travels and the tow-fish sends and receives sonar pulses with this configuration, an image called a sonogram is stitched together showing the topography along the seafloor (Figure 15.11B). Rougher areas return more sonar energy and so are bright on the sonogram, whereas smooth areas or shadows result in little sonar energy return and so are darker on the sonogram. Side scan sonar imaging. (A) A torpedoshaped "tow-fish" is towed behind a research vessel. As both the vessel and tow-ship move, a pulse of acoustic sonar energy (sometimes called a "chirp") is emitted, then received, by an array of transducers inside the tow-fish. Some of the sonar energy reflects off the seafloor and is returned to the tow-fish that measures the direction and magnitude of the returning pulse. The zone covered in a single "chirp-listen" cycle—highlighted in yellow in the figure to the left—is very short in the towing direction and very wide in the direction perpendicular to towing. (B) A sonogram is the image made by stitching together multiple individual cycles of emitting and receiving the sonar pulse. The sonogram shows the topography along the seafloor. Rougher surfaces reflect more sonar energy back to the tow-fish and are shown in bright shades. Smoother surfaces and shadows are shown in darker shades. This image is a composite of numerous sonograms shot by a remotely controlled submersible that used side scan sonar. Shown in the image is the bow section of the RMS Titanic, which sunk in the North Atlantic in the early hours of April 15, 1912, after hitting an ice berg. Also shown in the image is a portion of the debris field surrounding the wreck. Very similar to side scan, sonar imaging is multibeam imaging. Instead of involving a tow-fish, multibeam imaging systems are mounted directly next to the hull of the research vessel (Figure 15.12A). Multiple frequencies of acoustic sonar beams are emitted and received, and the resulting signal is processed on-board and shows the topographic information directly below the ship as it moves (Figure 15.12B). A major advantage of the multibeam system is that bubbles that form in the wake of the ship do not affect the sonar sending and receiving system because it is not being towed behind the ship. Multibeam sonar imaging. (A) Pulses of sonar energy of multiple different frequencies are emitted, then received, by an array of transducers located directly against the hull of the research vessel. Some of the sonar energy reflects off the seafloor and is returned to the ship and detected. As with side scan sonar, the zone covered in a single "chirp-listen" cycle—highlighted in yellow above-is very short in the towing direction and very wide in the direction perpendicular to towing. (B) By assembling the results from numerous multibeam surveys, scientists can produce a terrain model of the seafloor as shown in this example from the Gulf of Mexico. The tip of the Mississippi River Delta is depicted in the upper right, and the Mississippi Canyon and Mississippi Fan are shown on the right side of the image. The rest of the image shows the transition from the continental shelf shown in warm brown colors down the continental slope to the deep abyss on the floor of the Gulf of Mexico, shown in dark blue colors. The Sigsbee Escarpment marks the toe of the continental slope. The numerous pock-marks on the continental slope show mounds and circular basins caused by the mobilization of salt within the slope sediments. A magnetometer is an instrument that measures a magnetic field. Magnetometers towed behind research ships measure the magnetism of seafloor sediment and rock. Data collected by shipborne magnetometers resulted in the now-famous discovery of symmetric magnetic stripes on the seafloor. That discovery rapidly led to the development of the seafloor spreading hypothesis and of the theory of plate tectonics shortly thereafter, as described in Chapter 6. Satellite-based microwave radar instruments measure the echo of microwave pulses to detect subtle swells and depressions on the sea surface. These features reflect seafloor topography. For example, the mass of a seafloor mountain 4,000 meters high creates sufficient gravitational attraction to produce a gentle 6-meter-high swell on the sea surface directly above it. The microwave data are used to make seafloor maps.

Seventy-five years ago

scientists had better maps of the Moon than of the seafloor. The Moon is clearly visible in the night sky, and we can view its surface with a telescope. The seafloor, however, is deep, dark, and inhospitable to humans. Modern oceanographers use a variety of techniques to study the seafloor, including several types of sampling and remote sensing.

On all continents

streams and rivers deposit sediment in coastal deltas such as the Mississippi Delta. Much of this sediment is redistributed along the coastline by ocean currents and downslope as sediment-gravity flows—mixtures of sediment and water that flow downslope as a fluid, usually as a bottom-hugging current. Much of the sediment transported downslope from the shoreline and offshore forms a shallow, gently sloping underwater surface that marks the edge of the continent and is called a continental shelf (Figure 15.34). As sediment accumulates on a continental shelf, the edge of the continent sinks isostatically because of the added weight. This effect keeps the shelf generally below sea level. At the same time, the delivery and accumulation of more sediment from the weathering of the nearby continent causes the outer edge of the continental shelf to build outward, ultimately enlarging the size of the continent itself. In this way, mass is redistributed from the continental interior, where mountains are eroded, to the continental margins, which actively grow outward due to the accumulation of sediment there. A passive continental margin consists of a broad continental shelf, a slope, and a rise formed by accumulation of sediment eroded from the continent. Over millions of years, several kilometers of sediment accumulated on the passive east coast of North America, forming a broad continental shelf that projects outward along the entire coast. Beginning at the shoreline, the water depth of the North American Atlantic shelf increases gradually to about 200 meters at the outer shelf edge, which in most places is over 100 kilometers offshore. The average inclination of the continental shelf over this distance is about 0.1 degree. A continental shelf on a passive margin can be a very large feature. The shelf off the coast of southeastern Canada is about 500 kilometers wide (Figure 15.35), and parts of the shelves of Siberia and northwestern Europe are even wider. Color-coded map showing both onshore topography and seafloor bathymetry for part of New England, United States, as well as New Brunswick, Nova Scotia, and Newfoundland, Canada. See elevation/depth color key in the upper right. Notice that the St. Lawrence River flows into the Laurentian Channel which itself cuts across the continental shelf all the way to where it ends against the top of the continental slope. Sediment carried down the river and across the Laurentian Channel subsequently moves as turbidity currents and other forms of sediment gravity flows through one or more submarine canyons down across the slope to the continental rise. In some places, a supply of sediment may be lacking, either because no rivers bring sand, silt, or clay to the shelf or because ocean currents do not deliver sediment to the particular region. In warm regions where terrigenous sediment is either lacking or is delivered in very small quantities, carbonate reef-building organisms such as corals, mollusks, bryozoa, calcareous algae, and numerous others can thrive. When environmental conditions are optimal, the carbonate-generating ecosystem can produce very large volumes of carbonate sediment in very short periods of time. As a result, in tropical and subtropical latitudes where the rate of clastic sediment is low enough and water temperature and salinity is optimal, thick beds of limestone can accumulate. Limestone accumulations of this type may be hundreds of meters thick and hundreds of kilometers across and are called carbonate platforms. The Florida Keys and the Bahamas are modern-day examples of carbonate platforms on continental shelves (Figure 15.36). Clear skies and calm waters allowed the MODIS satellite to capture this stunning image of southern Florida, the Bahamas, and Cuba. In the center, Andros Island is surrounded by the bright blue halo of the Great Bahama Bank, a carbonate platform that was inundated by a rising sea level between 10,000 and 2,500 years ago as the last ice-age glaciers were melting. In most places, the water above the platform doesn't exceed 6 meters. Some of the world's richest petroleum reserves occur on the continental shelves of the North Sea between England and Scandinavia, in the Gulf of Mexico, and in the Beaufort Sea on the northern coast of Alaska and western Canada. In recent years, oil companies have explored and developed these offshore reserves. Deep drilling has revealed that granitic continental crust lies beneath the sedimentary rocks, confirming that the continental shelves are truly parts of the continents despite the fact that they are covered by seawater. The Continental Slope and Rise At the outer edge of a shelf, the seafloor gradually steepens to an average slope of about 3 degrees (though it can vary from 1 degree to 10 degrees) as it increases in depth from 200 meters to about 3 kilometers. This steep region of the seafloor averages about 50 kilometers wide and is called the continental slope. It is a surface formed by sediment accumulation, much like the shelf. Its steeper angle is due primarily to thinning of continental crust where it nears the junction with oceanic crust. Offshore seismic reflection profiling shows that the sedimentary layering is commonly disrupted where sediment has slumped and slid down the steep incline. A continental slope becomes less steep as it gradually merges with the deep ocean floor. This region, called the continental rise, consists of an apron of terrigenous sediment that was transported across the continental shelf and deposited on the deep ocean floor at the foot of the slope. The continental rise averages a few hundred kilometers wide. Typically, it joins the deep seafloor at a depth of about 5 kilometers. In essence, then, the shelf-slope-rise complex is a smoothly sloping, submarine surface on the edge of a continent formed by accumulation of sediment eroded from the continent. Submarine Canyons and Submarine Fans In many places, seafloor maps show deep valleys, called submarine canyons, eroded into the continental shelf and slope (Figures 15.34 and 15.35). They look like submarine stream valleys. A canyon typically starts on the outer edge of a continental shelf, usually beyond the outer reaches of a major river delta on the inner shelf, and continues across the slope to the rise. At its lower end, a submarine canyon commonly leads into a submarine fan (Figures 15.34 and 15.35), a large, fan-shaped accumulation of terrigenous sediment delivered down the submarine canyon to the continental rise by sediment gravity flows. Most submarine canyons occur downslope of a region where large rivers enter the sea. When they were first discovered, geologists thought the canyons had been eroded by rivers during the Pleistocene Epoch, when accumulation of glacial ice on land lowered sea level by as much as 150 meters. However, this explanation cannot account for the deeper portions of submarine canyons that cut erosionally into the lower continental slopes at depths of a kilometer or more. These deeper parts of the submarine canyons must have formed through erosion, and a submarine mechanism must be found to explain them. Geologists subsequently discovered that sediment gravity flows—underwater mixtures of sediment and water that flow downslope—can erode the continental shelf and slope to create or deepen the submarine canyons. Particularly efficient at eroding rock and sediment underwater are turbidity currents, a form of sediment gravity flow in which a turbulent mixture of sediment and seawater that is more dense than the seawater alone and so flows downslope along the bottom (Figure 15.37). You can create and observe a turbidity current at home by slowly pouring a cup of muddy water into a sloping basin of clear water. A turbidity current flowing down a laboratory flume filled with water. The metal hardware and wiring measure the velocity of the turbidity current and the density of the sediment-water mixture at different depths above the bed. Turbidity currents such as this deliver a tremendous volume of sediment to the continental rise and are responsible for much erosion in submarine canyons that cut across the continental slope. Turbidity currents can be triggered by an earthquake, a large storm, or simply by the oversteepening of the slope as sediment accumulates. (Recall the angle of repose for loose sediment, described in Chapter 10.) When the sediment starts to move, it mixes with water and flows across the shelf and down the slope as a turbulent, chaotic fluid. A turbidity current can travel at speeds greater than 100 kilometers per hour and for distances in excess of 1,200 kilometers. Sediment-laden water traveling at such speed has tremendous erosive power. Once a turbidity current cuts a small channel into the shelf and slope, subsequent currents follow the same channel, just as a stream uses the same channel year after year. Over time, the currents erode a deep submarine canyon into the shelf and slope. Turbidity currents slow down when they reach the deep seafloor. The sediment accumulates there to form a submarine fan. Most submarine canyons and fans form near the mouths of large rivers because the rivers supply the great amount of sediment needed to create turbidity currents and other types of sediment gravity flows. Most of the largest submarine fans form on passive continental margins. Submarine fans that form on active margins typically are much smaller because the trench swallows much of the sediment. Furthermore, most of the world's largest rivers drain toward passive margins. The largest known fan is the Bengal Fan, which covers about 4 million square kilometers beyond the mouth of the Ganges River in the Indian Ocean east of India. More than half of the sediment eroded from the rapidly rising Himalayas ends up in this fan. Interestingly, the Bengal Fan has no associated submarine canyon, perhaps because the sediment supply is so great that the rapid accumulation of sediment prevents erosion of a canyon.

Early oceanographers had believed

that the oceans are 4 billion years old, so mud on the ancient seafloor should have been very thick, having had so much time to accumulate. In 1947, however, scientists on the U.S. research ship Atlantis discovered that the mud layer on the bottom of the Atlantic Ocean is much thinner than they expected. Why is there so little mud on the seafloor? The answer to this question would be a crucial piece of evidence in the development of the theory of plate tectonics. Earth is 4.6 billion years old, and rocks as old as 4.04 billion years have been found on the North American continent. Because of its buoyancy, most continental crust remains near Earth's surface. In contrast, no parts of the seafloor are older than about 200 million years, because oceanic crust forms continuously at the Mid-Oceanic Ridge and then recycles into the mantle at subduction zones. As the theory of plate tectonics was emerging in the early 1960s, it became apparent that the presence of only a thin layer of mud covering the oceanic crust could be easily explained if the oldest seafloor is less than 200 million years old, not 4 billion years old as oceanographers had once advocated. This evidence supported the fledgling plate tectonics theory in its early days. Seismic reflection profiling and seafloor drilling show that oceanic crust is made of three layers. The uppermost layer consists of sediment, and the lower two are basalt (Figure 15.29). The three layers of oceanic crust. The uppermost layer consists of sediment. The middle layer consists of pillow basalt. The deepest layer is made of vertical dikes of basalt that merge downward into gabbro. Below this lowermost layer of oceanic crust is the upper mantle.

The uppermost layer of oceanic crust consists of

two types of sediment. Terrigenous sediment is sand, silt, and clay eroded from the continents and delivered to deep water mainly through submarine canyons that extend outward from the continental shelf. Thus, most terrigenous sediment is found close to the continents. Pelagic sediment, however, collects even on the deep seafloor far from continents. It is a gray and red-brown mixture of silt- and clay-sized particles that were transported by wind as dust derived from continental weathering and that lived in the ocean surface waters as sand-sized and smaller phytoplankton that photosynthesized, died, and sank (Figure 15.30). This scanning electron microscope image shows foraminifera, tiny organisms that float near the ocean surface. When these organisms die, their remains sink to the seafloor, to become part of the pelagic mud layer. Each of the fossils is the size of a fine sand grain. Pelagic sediment accumulates at a rate of about 2 to 10 millimeters per 1,000 years. Near the Mid-Oceanic Ridge system there is very little to virtually no sediment because the seafloor is so young. The sediment thickness increases with distance from the ridge because the seafloor becomes older as it spreads away from the ridge and, consequently, the sediment has had more time to accumulate. Close to shore, pelagic sediment becomes diluted by more and more terrigenous sediment, forming accumulations that can exceed 10 kilometers in thickness, though more typically are around four kilometers in thickness. The observation that the thickness of seafloor mud increased with distance from the ridge also supported the plate tectonics theory in its early days. Parts of the ocean floor beyond the Mid-Oceanic Ridge system are flat, level, essentially featureless surfaces called the abyssal plains. They are the flattest surfaces on Earth. Seismic profiling shows that the basaltic crust is rough and jagged throughout the ocean. On the abyssal plains, however, pelagic sediment buries this rugged surface, forming smooth surfaces. If you were to remove all of the sediment, you would see rugged topography similar to that of the Mid-Oceanic Ridge. Below the superficial layer of sediment on the ocean floor is a layer of basalt about 1 to 2 kilometers thick. It consists mostly of pillow basalt, which forms as hot magma oozes onto the seafloor. Contact with cold seawater causes newly erupting molten lava to rapidly form a solidified outer rind. Still-molten and pressurized magma immediately below the rind causes it to deform plastically into bulbous, pillow-shaped spheroids, called pillows (Figure 15.31). Underwater photo of pillow basalt off the island of Hawaii. Beneath the pillow basalt is the deepest and thickest layer of oceanic crust, consisting of between 3 and 5 kilometers of basalt that did not erupt onto the seafloor. This basalt directly overlies the mantle. The upper portion consists of vertical basalt dikes that formed as basaltic magma oozing toward the surface froze in the cracks of the rift valley. The lower portion consists of gabbro, the coarse-grained equivalent of basalt. The gabbro forms as pools of magma cool slowly, insulated by the basalt dikes above them. The pillow basalt, vertical basalt dikes, and gabbro all form at the Mid-Oceanic Ridge. These rocks make up the foundation of all oceanic crust because all oceanic crust forms at a ridge axis and then spreads outward. In some places, chemical reactions with seawater have altered the basalt to a soft, green rock called serpentinite that contains up to 13 percent water. Serpentinite is a fairly common rock type in parts of California and the Pacific Northwest due to the presence of abundant basalt and altered basalt that represents oceanic crust associated with the accreted terranes found there. Superb outcrops of serpentinite are found on U.S. Highway 1 south of Big Sur, California (Figure 15.32). Close-up of strongly sheared serpentinite exposed along the shoreline south of Big Sur, California. This serpentinite formed through the alteration of basalt that once was part of oceanic crust. The basalt was deformed as it was slowly mashed into the western part of the North American Plate, and that deformation continued after the basalt was altered to serpentinite. The car keys are for scale.