Chapter 10: Origin and Evolution of the Ocean Floor
Reef-building corals grow best in
75 F cannot live below 64 F or above 86 F require clear, sunlit water limited to 150 feet
deep sea fan
As fans from adjacent submarine canyons grow, they merge to produce a continuous wedge of sediment at the base of the continental slope, forming the continental rise.
It is important to note that the lithospheric mantle which makes up about 80 % of the descending oceanic slab, drives subduction.
Even when the oceanic crust is quite old, its density is still less than that of the underlying asthenosphere. Subduction, depends on lithospheric mantle that is colder and therefore denser than the underlying asthenosphere
The abyssal plain found off the coast of Argentina, has less than 10 feet of relief over a distance exceeding ___ miles. The monotonous topography of abyssal plains is interrupted by the protruding summit of a partially buried volcanic peak or ___.
800 miles seamount
Concept 10.4 Where do rift valleys form along the oceanic ridge system?
Along the axis of some segments of the oceanic ridge system are deep, down-faulted structures called rift valleys because of their striking similarity to the continental rift valleys found in East Africa.
By combining swaths of side-scan sonar data, oceanographers produced the first photograph-like images of the seafloor.
Although side-scan sonar provides valuable views of the seafloor, it does not provide bathymetric or water depth data
Particularly significant was the discovery of the global oceanic ridge system, a ____ ____ _____ that stands 2 to 3 kilometers higher than the adjacent deep-ocean basins and is the longest topographic feature on Earth.
broad elevated landform
The remaining melt tends to cool along the walls of magma chambers to form massive amounts of coarse grained ___. This portion accounts for up to 3 to 4 miles of ocean crust thickness.
gabbro
Where the lithosphere is thick, cool, and strong, rifts tend to be ____ --often less than a few hundred kilometers wide. examples of narrow continental rifts include: ____ ___ __, the __ ___ ___, the ___ __ and the -_ __.
narrow East African Rift the Rio Grande Rift the Baikal Rift Rhine Valley
Furthermore, the ridge system is broken into segments that range from a few to tens to hundreds of kilometers in ___. Each segment is offset from the adjacent segment by a ___ __.
length. transform fault
The most recent period of rifting began about 20 million years ago, as upwelling in the mantle intruded the base of the ___.
lithosphere
As plates diverge, fractures created in the oceanic crust fill with molten rock that gradually wells up from the hot ____ below.
mantle
These differences appear to be the result of differences in spreading rates --which largely determine the amount of melt generated at a rift zone. At fast spreading centers, more magma wells up from the ____ than at slow spreading centers. This difference in output causes differences in the structure and topography of various ridge segments.
mantle
Continental Slope
marking the seaward edge of the continental shelf is the continental slope, a relatively steep structure that marks the boundary between continental crust and oceanic crust. Although the inclination of the continental slope varies greatly from place to place, it averages about 5 degrees and in places exceeds 25 degrees.
Concept 10.3 4. What features on the ocean floor most resemble flood basalt provinces on the contients
massive oceanic plateaus
Concept 10.1 1. Define bathymetry.
measurement of ocean depths and charting of the shape or topography of the ocean floor
bathymetry
measurement of ocean depths and charting of the shape or topography of the ocean floor.
Most mountain ranges on land form when compressional forces associated with continental collisions fold and ____ thick sequences of sedimentary rocks, oceanic ridges form where ____ from the mantle generates new oceanic crust.
metamorphose upwelling
In 10.14, large sections of the oceanic ridge system have been named based on their locations within the various ocean basins. A ridge may run through the middle of an ocean basin, where it is called __ ___ __.
mid-ocean ridge examples: Mid-Atlantic Ridge Mid-Indian Ridge
The three remaining fragments of the once extensive Farallon plate include the ___, ___, and ___.
modern Juan de Fuca Cocos Nazca plates
active continental margins
most of the Pacific Ocean is bordered by active continental margins or subduction zones. Some of those active subduction zones lie far beyond the margins of the continents.
Oceanic ridges are as high as some ____ on the continents, but the similarities end there.
mountains
Despite their greater efficiency and enhanced detail, research vessels equipped with ____ sonar travel at a mere 6 to 12 miles per hour.
multibeam It would take at least 100 vessels outfitted with this equipment hundreds of years to map the entire seafloor.
In addition, because the depth of the ocean depends largely on the age of the seafloor, ridge segments that exhibit faster spreading rates tend to have more gradual profiles than ridges that have ____ spreading rates.
slower
Trenches are sites of plate convergence where slabs of oceanic lithosphere subduct and plunge back into the mantle. In addition to earthquakes being created as one plate scrapes against another, volcanic activity is also triggered by plate ___.
subduction
The westward migration of North America also caused a section of the East Pacific Rise to enter the ___ ___ that once lay off the coast of California. As this spreading center subducted, it was destroyed and replaced by a transform fault system that currently accommodates the differential motion between the North American and Pacific plates.
subduction zone
Most of the sediments are delivered to the seafloor by ___ ___.
turbidity currents, mixtures of sediment and water that periodically flow down submarine canyons
why one rift valley developed into a full-fledged active spreading center while others fail to develop is not fully ___.
understood.
Layer 1
upper layer is a sequence of deep-sea sediments or sedimentary rocks. Sediments are very thin near the axes of oceanic ridges but may be several kilometers thick next to continents.
Furthermore, ___ ___ ___, such as those making up portions of the Andes and Cascades are located parallel to subduction zones that lie adjacent to continental margins.
continental volcanic arcs
Continental shelves represent only ____% of the total ocean area, they have economic and political significance.
7.5 % oil natural gas fishing grounds
At the time of the breakup of __, the Farallon plate was situated on the eastern side of a spreading center as shown in figure 10.24a.
Pangaea
The opening of a new ocean basin begins with the formation of a ___ ___, an elongated depression along which the entire lithosphere is stretched and thinned.
continental rift
deep-ocean basins
found between the continental margin and the oceanic ridge lies the __ ___ __. the size of this region --almost 30% of Earth's surface is roughly comparable to the percentage of land above sea level.
Concept 10.3 Using Darwin's hypothesis, place these coral reefs in order from youngest to oldest:
fringing reef barrier reef atoll
The oceanic ridge system winds through all major oceans in a manner similar to the seam on a baseball, and it is the ____ topographic feature on Earth exceeding _ miles in length.
longest 43,000 miles
It is composed almost entirely of ___ rocks that are underlain by a layer of ultramafic rock peridotite, which forms the lithospheric mantle.
mafic
this activity continuously adds new basaltic rock to diverging plate margins, temporarily weldging them together; they are then broken as spreading continues. Along some ridges, outpourings of __ __ build submerged shield volcanoes or seamounts as well as elongated lava ridges.
pillow lava
In order to balance the amount of newly created lithosphere, there must be a process whereby oceanic ____ are destroyed.
plates.
Ocean is Earth's most ____ feature covering more than _______% of its surface.
prominent 70%
seismic reflection profilers
researches have determined that the relatively featureless topography of abyssal plains is due to thick accumulations of sediment that have buried an otherwise rugged ocean floor.
Along the axis of some segments of the oceanic ridge system are deep, down - faulted structures called ___ ____ because of their striking similarity to the continental rift valleys found in East Africa.
rift valleys
The vertical displacement of large slabs of oceanic crust along normal faults is responsible for the steep walls of the __ ___.
rift valleys
Because of these differences in topography, the gently sloping, less rugged portions of fast-spreading ridges are called ___
rises
Furthermore, volcanism produces numerous cones in the rift valley, which enhance the ___ ____ of the ridge crest.
rugged topography
Oceanic ridges that exhibit slow spreading rates from 1 to 5 centimeters per year have prominent rift valleys and ___ ___.
rugged topography ex: Mid-Atlantic Ridge ex: Mid-Indian Ridge
The term ridge is somewhat misleading because these features are not narrow and steep, as the term implies , but have widths from 600 to 2500 miles and the appearance of broad, elongated swells that exhibit varying degrees of ____.
ruggedness.
The greatest yield of magma , more than 60% of Earth's total yearly output is produced along the oceanic ridge system in association with ___ ___.
seafloor spreading
This molten material slowly cools and crystallizes, producing new slivers of ___. This process repeats in episodic bursts, generating new lithosphere that moves away from the ridge crest in a conveyor belt fashion.
seafloor.
Following World War II, the U.S. Navy developed ___ ___ to look for explosive devices that had been deployed in shipping lanes. Torpedo - shaped instruments are towed behind ships, where they send out a fan of sound extending on either side of the ship's path.
sidescan sonar
seamounts
submarine volcanoes may rise hundreds of meters above the surrounding topography. more than a million exist most common in the Pacific
Concept 10.2 4. Briefly explain how an accretionary wedge forms.
Along some subduction zones, sediments from the ocean floor and pieces of oceanic crust are scraped from the descending oceanic plate and plastered against the edge of the overriding plate. This chaotic accumulation of deformed sediment and scraps of oceanic crust is called an ___ __.
Concept 10.3 2 Why are abyssal plains more extensive on the floor of the Atlantic than on the floor of the Pacific?
Because it has few trenches to act as traps for sediment carried down the continental slopes.
Layer 2
Below the layer of sediments is a rock unit composed mainly of basaltic lavas that contain abundant pillowlike structures called pillow basalts.
Concept 10.3 3. How does a flat-topped seamount, called a guyot form?
During their existence, inactive volcanic islands are gradually but inevitably lowered to near sea level by the forces of weathering and erosion. As a moving plate slowly carries volcanic islands away from the elevated oceanic ridge or hot spot over which they formed, they gradually sink and disappear below the water surface. Submerged, flat-topped seamounts that form in this manner are called __.
Guyots
During their existence, inactive volcanic islands are gradually but inevitably lowered to near sea level by the forces of weathering and erosion. As a moving plate slowly carries volcanic islands away from the elevated oceanic ridge or hot spot over which they formed, they gradually sink and disappear below the water surface. Submerged, flat-topped seamounts that form in this manner are called __.
Atlantic Ocean
If spreading continues, the Red Sea will grow wider and develop an elevated oceanic ridge similar to the Mid-Atlantic Ridge. The Atlantic ocean shows what the Red Sea would eventually become over tens of millions of years. As the Atlantic formed, new oceanic crust was added to the diverging plates, and the rifted continental margins gradually receded from the region of upwelling. As a result, they cooled, contracted, and sank.
Partial melting of ultramafic mantle rock produces ____ magma that has a surprisingly consistent chemical composition.
basaltic
Oceanic ridges consist of layers and piles of newly formed ____ rocks that are buoyantly uplifted by the ___ mantle rocks from which they formed.
basaltic hot
The primary reason for the elevated position of the ridge system is that newly created oceanic lithosphere is hot and therefore less dense than cooler rocks of the deep -ocean ___.
basin
Studies conducted by submersibles along several segments of the oceanic ridge have photographed these metallic rich solution gushing from the seafloor to form particle-filled clouds called ___.
black smokers
Conversely, ___ ___ create slight depressions.
canyons and trenches
As rock rises, it experiences a decrease in confining pressure that may lead to ____ ____
decompressing melting.
When these muddy slurries emerge from the mouth of a canyon onto the relatively flat ocean floor, they deposit sediment that forms a _____ ____ ___.
deep sea fan.
This region contains __ __ __, which are extremely deep linear depressions in the ocean floor, remarkably flat areas known as ___ ___; tall volcanic peaks called ___ and ____; and large, elevated flood basalt provinces called oceanic plateaus.
deep-ocean trenches abyssal plains seamounts and guyots oceanic plateaus
Combined with traditional sonar depth measurements, the data are used to produce ___ ___ ___ ___.
detailed ocean floor maps
Because massive structures such as seamounts and ridges exert stronger than average ____ attraction, they produce elevated areas on the ocean surface.
gravitational
guyots
image
seismic reflection profiler
image
If a volcano grows large enough before it is carried from its magma source by plate motion, the structure may emerge as a ___ ____.
volcanic island examples: Easter Island, Tahiti, Bora Bora, the Galapagos, and the Canary Islands
By contrast, where the crust is thin, hot and weak, rifts can be more than ____ miles wide as exemplified by the ____ and ____ region in the Western United States.
600 miles Basin and Range
The breakup of Pangaea illustrates that ocean basins, such as the Atlantic, originate when continents break ___.
apart
Abyssal plains
are flat features of the deep-ocean floor they are likely the most level places on Earth.
Provinces of the ocean Floor
1. continental margins 2. deep ocean basins 3. oceanic or mid-ocean ridges Profiles of this type have their vertical dimension exaggerated many times or 40 times to make topographic features more conspicuous.
Concept 10.1 What are the three major topographic provinces of the ocean floor?
1. continental margins 2. deep-ocean basins 3. oceanic or mid-ocean ridges
The sediment consists of 3 materials :
1. fine sediments transported far out to sea by turbidity currents. 2. mineral matter that has precipitated out of seawater 3. shells and skeletons of microscopic marine organisms.
Challenger expendition
1. first comprehensive study of the global ocean 2. 79, 200 mile voyage 3. scientists traveled to every ocean except the Arctic 4. water depth by lowering long weighted lines overboard 5. measured the depth of the deepest spot on the seafloor in 1875, which is called Challenger Deep
The naturalist Charles Darwin was one of the first to formulate a hypothesis on the origin of ringed-shaped atolls. He did this during a 5 year period when he sailed aboard the British ship HMS Beagle on its famous global circumnavigation. Darwin noticed a progression in coral reef development from
1. fringing reef along the margins of a volcano to 2. a barrier reef with a volcano in the middle to 3. an atoll, consisting of a continuous or broken ring of coral reef surrounding a central lagoon.
Passive continental margins
1. most are wide and are sites where large quantities of sediments are deposited. 2. features comprising the passive continental margins include: continental shelf, continental slope, and the continental rise
Our knowledge of the oceanic ridge system comes from
1. soundings of the ocean floor 2. core samples from deep-sea drilling 3. firsthand inspection of slices of ocean floor that have been thrust onto dry land during continental collisions 4. visual inspections using submersibles like Alvin
What is found under the ocean if all the water could be drained from the ocean basins?
1. volcanic peaks, 2. deep trenches 3. extensive plains 4. linear ridges 5. large plateaus
Some rift valleys, including those along the rugged Mid-Atlantic Ridge are typically 20 to 30 miles wide and have walls that tower __ ___ above the valley floor. This makes them comparable to the deepest and widest part of Arizona's Grand Canyon
1600 to 8200 feet.
Prior to ___, information about the ocean floor was extremely limited.
1950's
Because of this change in plate geometry, the Pacific plate has captured a sliver of North America, the Baja Peninsula and a portion of southern California, and is carrying it northwestward toward Alaska at a rate of about ____ per year.
2.5 inches
In the late 1970's, geologists began using magnetic stripes and fracture zones on the ocean floor to reconstruct the past ____ million years of plate movement. This research showed that parts of , or even entire, ocean basins have been destroyed along subduction zones.
200
The ocean - floor topography did not unfold until the historic ____ year voyage of HMS Challenger .
3 and 1/2 year December 1872 - May 1876
The Challenger Deep, located in the Mariana trench, has been measured at ____ below sea level, making it the deepest known part of the world ocean.
36,069 feet
ocean crust consist of ____ layers
4 layers
Using the speed of sound waves in water, about ____ ____ _____, and measuring the time required for the energy pulse to reach the ocean floor and return, the ____ can be calculated.
4900 feet per second depth Depth = 1/2 (1500 meters/second x echo travel time)
This explains why only ___ percent of the seafloor has been mapped in detail.
5
Seismic soundings indicate that its thickness if only about __ miles.
5 miles
Concept 10.2 3. How are active continental margins related to plate tectonics?
Active continental margins are located along convergent plate boundaries, where oceanic lithosphere is being subducted beneath the leading edge of a continent. Deep-ocean trenches are a major topographic expression at convergent plate boundaries. These deep, narrow furrows surround most the Pacific Rim.
Mapping the Ocean Floor from Space
After compensating for waves, tides, currents, and atmospheric effects, it was discovered that the water's surface is not perfectly flat.
Some like the Hawaiian - Emperor Seamount chain stretches from the Hawaiian islands to the -_____ trench, form over volcanic hot spots. Others are born near oceanic ridges.
Aleutian
Concept 10.5 3. Why does the lithosphere thickens as it moves away from the ridge as a result of seafloor spreading?
As lithosphere is displaced away from the ridge crest, cooling also causes a gradual increase in lithospheric thickness. This happens because the boundary between the lithosphere and asthenosphere is a thermal temperature boundary. As material in the uppermost asthenosphere ages or cools, it becomes stiff and rigid. Thus, the upper portion of the asthenosphere is gradually converted to lithosphere simply by cooling.
The permeable and highly fractured lava of the upper oceanic crust allows seawater to penetrate to depths of 1 to 2 miles.
As seawater circulates through the hot crust, it is heated and chemically reacts with the basaltic rock in a process called hydrothermal metamorphism. This alteration causes the dark silicates olivine and pyroxene in basalt to form new metamorphic minerals such as chlorite and serpentine. Simultaneously, the hot seawater dissolves ions of silica, iron, copper, and silver and gold from the hot basalts
When an oceanic slab descends to about 250 miles, mineral phase changes , the transition from low density mineral to high density mineral, enhance subduction.
At this depth, the transition from olivine to its high density form that has the spinel-structure increases the density of the slab, which helps pull the plate into the subduction zone.
Abyssal plains are found in all oceans. __ __ has the most extensive abyssal plains because it has few trenches to act as traps for sediments carried down the continental slopes.
Atlantic
Continued seafloor spreading eventually results in the formation of a mature ocean basin bordered by rifted continental margins. example:___ ___
Atlantic ocean
Bathymetry determined from continuous monitoring of these echoes is plotted to obtain a profile of the ocean floor.
By laboriously combining numerous profiles, a detailed chart of the seafloor has been produced.
A similar event generating the Queen Charlotte transform fault, located off the west coast of __ and __ __.
Canada southeastern Alaska
Buoyant uplifting of the heating lithosphere led to doming and stretching of the crust.
Consequently, the upper crust was broken along high angle normal faults, producing down-faulted blocks, or grabbens, while the lower crust deformed by ductile stretching.
What causes atolls to form, and how do they attain such great thicknesses.
Corals are tiny animals that generally appear in large numbers and when linked form colonies. Most corals create a hard external skeleton made of calcium carbonate. Some build large calcium carbonate structures, called reefs, where new colonies grow atop the strong skeletons of previous colonies. Sponges and algae may attach to the reef, enlarging it further.
Concept 10.1 2. Average speed of sound waves in water is 4900 feet per second, determine the water depth if the signal sent out by an echo sounder requires 6 seconds to strike bottom and return to the recorder.
Depth = 4900 feet/second x 6 seconds = 29,400 feet
Volcanic Structures on the Ocean Floor
Dotting the sea floor are numerous volcanic structures of various sizes.
Because of this sea-level drop, rivers extended their courses, and land-dwelling plants and animals migrated to the newly exposed portions of the continents.
Dredging off the coast of N.America has retrieved the ancient remains of land dwellers such as mammoths, mastodons, and horses, providing further evidence that portions of the continental shelves were once above sea level.
For example, during the breakup of Pangaea on page 50, notice that the African plate moved northward, eventually colliding with Eurasia.
During this event, the floor of the intervening Tethys Ocean was almost entirely consumed into the mantle, leaving behind a few small remnants --The Eastern Mediterranean Sea and the Black Sea
By contrast, the __ ___ ___ is not a mid-ocean feature. Rather it is located in the eastern Pacific, far from the center of the ocean.
East Pacific Rise
echo sounders
Echo sounders work by transitting a sound wave called a ping into the water in order to produce an echo when it bounces off any object, such as a large marine organism or the ocean floor. A sensitive receiver intercepts the reflected echo, and a clock precisely measures the travel time to fractions of a second.
Beginning about 180 million years ago, the Americas were propelled westward, as Pagaea broke up and the Atlantic Ocean started to open. As a result, the ___ plate began subducting beneath the Americas faster than it was being generated, causing it to decrease in size.
Farallon
The spreading center, which generated both the ___ and ___ plates is the East Pacific Rise
Farallon and Pacific plates
Red Sea
Gradually, a rift valley will lengthen and deepen, eventually to the margin of the continent. At this point, the continental rift becomes a narrow linear sea with an outlet to the ocean, similar to the Red Sea The Red Sea formed when the Arabian Peninsula rifted from Africa beginning about 30 million years ago. Steep fault scarps that rise as much as 2 miles above sea level flank the margins of this water body. Thus, the escarpment surrounding the Red Sea are similar to the steep cliffs that border the East African Rift. Although the Red Sea reaches a oceanic depths up to 3 miles in only a few locations, symmetrical magnetic stripes indicate that typical seafloor spreading has been occurring for at least the past 5 million years.
Interactions Between Seawater
In addition to serving as a mechanism for the dissipation of Earth's internal heat, the interaction between seawater and the newly formed basaltic crust alters both the seawater and the crust.
The continental shelf tends to be relatively featureless, however, some areas are mantled by extensive glacial deposits and thus are quite rugged. In addition, some continental shelves are dissected by large valleys running from the coastline into deeper waters.
Many of these shelf valleys are the seaward extensions of river valleys on the adjacent landmass. They were eroded during the last Ice Age , Quaternary period, when enormous quantities of water were stored in vast ice sheets on the continents, causing sea level to drop at least 100 meters or 330 feet.
Failed Rifts
Not all continental rift valleys develop into full-fledged spreading centers. In the Central United States, a failed rift extends from Lake Superior into Kansas. This once-active rift valley is filled with clastic sedimentary and basaltic rocks that were extruded onto the crust more than a billion years ago.
Why Oceanic Lithosphere Subducts
Oceanic lithosphere will resist subduction unless its overall density is greater than that of the underlying asthenosphere. It takes at least 15 million years for a young slab of oceanic lithosphere to cool sufficiently to become denser than the supporting asthenosphere.
Apparently too buoyant to subduct, the ___ ___ ___ clogged the trench.
Ontong Java Plateau
Concept 10.2 Describe the difference between active and passive margins and give a geographic example of each.
Passive = nearly the entire Atlantic Ocean and a large portion of the Indian Ocean are surrounded by passive contiental margins. Active = most of the Pacific Ocean is bordered by active continental margins , subduction zones.
Concept 10.2 4. What is meant by subduction erosion?
Rather than sediment accumulating along the front of the overriding plate, sediment and rock are scraped off the bottom of the overriding plate and transported into the mantle by the subducting plate. Subduction erosion is effective where cold, dense oceanic lithosphere subducts at a steep angle as exemplified by the Mariana trench.
In settings where rifting continues, the rift system evolves into a young, narrow ocean basin, such as the present day __ ___.
Red Sea.
The volcanic activity associated with the trenches that surround the Pacific Ocean explains why this region is called the __ __ __.
Ring of Fire
Concept 10.1 3. Describe how satellites orbiting Earth can determine features on the seafloor without being able to directly observe them beneath several kilometers of seawater.
Satellites equipped with radar altimeters are able to measure subtle differences in sea level by bouncing microwaves off the sea surface. These devices can measure variations as small as a few centimeters.
The plate tectonics theory provide the most current scientific explanation of how volcanic islands become extinct and sink to great depths over long periods of time.
Some volcanic islands form over relatively stationary mantle plumes, causing the lithosphere to be warmed and buoyantly uplifted. Over spans of millions of years, these volcanic islands gradually sink as the moving plates carry them away from the region of hot-spot volcanism because the lithosphere cools, becomes denser, and sinks.
In areas where forced subduction occurs, a strong coupling develops between the overlying plate and the subducting plate that can result in particularly strong and frequent earthquakes.
Stated another way, plate motion generates horizontal compressional forces that cause the upper plate and underlying plate to grind against each other. the result can be folding and thickening of the upper plate and sometimes the formation of mountainous terrains like we see today in Andes. Shallow subduction and strong coupling have also been observed in the past decade along the Sunda subduction zone, off the coast of Sumatra, another region that experienced several major earthquakes.
Concept 10.7 Name a modern example of a continental rift.
The East African Rift The Rio Grande Rift The Baikal Rift The Rhine Valley
It has been determined that unusually thick units of oceanic crust, those that approach 20 miles in thickness, are likely to resist subduction.
The Ontong Java Plateau, for example, is a thick oceanic plateau, about the size of Alaska, loacted in the western Pacific. About 20 million years ago, this plateau reached the trench that forms the boundary between the subducting Pacitic plate and the overriding Australian - Indian plate.
sonar
The basic approach is to use sonar, sound navigation and ranging
Spontaneous Subduction Subduction zones can be subdivided into two basic types, based on the nature of the subducting plate.
The first type, referred to as a Marina type subduction zone is characterized by old, dense lithosphere sinking into the mantle by its own weight. The lithosphere entering the Mariana trench is about 185 million years old, some of the oldest and densest lithosphere in today's oceans. Along this trench, the subducting slab descends into the mantle at a steep angle that approaches 90 degrees. Steep subduction angles produce deep trenches, which account in part for the depth of the Challenger Deep, located at the southern limb of the Mariana trench. The Marina and most of the other subduction zone in the western Pacific involve cold, dense lithosphere and therefore exhibit spontaneous subduction.
Layer 4
The lowest unit is mainly gabbro, the coarse-grained equivalent of basalt, which crystallized deeper in the crust without erupting.
Layer 3
The middle, rocky layer is made up of numerous interconnected dikes that have a nearly vertical orientation, called the sheeted dike complex. These dikes are former pathways where magma rose to feed pillow basalts on the ocean floor.
How Does Oceanic Crust Form?
The molten rock that forms new oceanic crust originates from partial melting of the ultramafic mantle rock. This process generates basaltic melt that is less dense than the surrounding solid rock. The newly formed melt rises through the upper mantle along thousands of tiny conduits that feed into a few dozen larger, elongated channels.
accretionary wedge
This chaotic accumulation of deformed sediment and scraps of oceanic crust is called an ___ ___. Prolonged plate subduction can produce massive accumulations of sediment along active continental margins.
roughly 10 to 20 percent of the melt eventually erupts on the ocean floor. Because the surface of these submarine lava flows is chilled quickly by seawater, these flows generally travel no more than a few kilometers before completely solidifying. The forward motion occurs as lava accumulates behind the congealed margin and then breaks through. This process occurs repeatedly, as molten basalt is extruded -like toothpaste from a tightly squeezed tube.
The result is protuberances resembling large bed pillows stacked on atop the other, hence the name pillow basalts
Forced Subduction
The second type of subduction, called the Peru-Chili - type subduction zone, is characterized by younger, hotter, and less dense lithosphere that dips at shallower angles. Along Peru-Chile - type boundaries, the lithosphere is too buoyant to subduct spontaneosly; rather, it is forced beneath the overlying plate by compressional forces.
Concept Check 10.3 Explain how deep-ocean trenches are related to plate boundaries.
They are the site of plate convergence, where slabs of oceanic lithosphere subduct and plunge back into the mantle. In addition to earthquakes being created as one plate scrapes against another, volcanic activity is also triggered by plate subduction.
high-resolution multibeam instruments
This drawback was resolved with the development of __ ___ ___ ___. These systems use hull-mounted sound sources that send out a fan of sound and then record reflections from the seafloor through a set of narrowly focused receivers aimed at different angles.
New dikes intrude older dikes, which are still warm and weak, to form a sheeted dike complex.
This portion of the oceanic crust is 1 to 2 kilometers thick
ophilite complex
When fragments of oceanic crust and the underling mantle are discovered on land, they are called an ___. From studies of various ophilite complexes around the globe and related data, geologists have pieced together a scenario for the formation of the ocean floor.
At fast spreading centers greater than ____ centimeters per year, such as along much of the East Pacific Rise, rift valleys are generally ____.
absent
Passive Continental Margins
are geologically inactive regions located some distance from plate boundaries. They are not associated with strong earthquakes or volcanic activity. develop when continental blocks rift apart and are separated by continued. As a result, the continental blocks are firmly attached to the adjacent oceanic crust.
Active Continental margins
are located along convergent plate boundaries, where oceanic lithosphere is being subducted beneath the leading edge of a continent.
deep ocean trenches
are long, narrow creases in the seafloor that represent the deepest parts of the ocean floor. most are located in along the margins of the Pacific Ocean, where many exceed 6 miles in depth.
Coral atolls
are ring-shaped structures that extend from slightly above sea level to depths of several thousand meters.
deep - ocean trenches
are the major topographic expression at convergent plate boundaries. These deep, narrow furrows surround most of the Pacific Rim. One exception is the Puerto Rico trench, which forms the boundary between the Carribean Sea and the Atlantic Ocean.
continental margins
are the outer margins of the continents, where continental crust transitions to oceanic crust. passive continental margin active continental margin
Continental shelf
average inclination of the continental shelf is only about 1/10 of 1 degree, a slope so slight that it would appear to an observer to be a horizontal surface.
As the hot liquid 750F mixes with the cold, mineral-laden seawater, the dissolved minerals precipitate to form massive metallic sulfide deposits, some of which are economically important.. Occasionally these deposits grow upward to form underwater ____ ___ ____ equivalent in height to skyscrapers.
chimney-like structures
Continental Rise
continental slope merges into a more gradual incline known as the continental rise, which may extend seaward for hundreds of kilometers. consists of a thick accumulation of sediment that has moved down the continental slope and onto the deep-ocean floor
the lowest unit of the ocean crust develops from ____ within the central magma chamber itself. The first minerals to crystallize are olivine, pyroxene, and chromite , chromium oxide, which settle through the magma to form a layered zone near the floor of the reservoir.
crystalization
The newly formed melt seaparates from the mantle rock and rises toward the surface. Along some ridge segments, the melt collects in small, elongated reservoirs located just beneath the ridge crest. Eventually, about 10 to 20 percent of the melt migrates upward along fissures and erupts as lava flows on the ocean floor, while the remainder ____ at depth to form the lower crust.
crystallizes
passive continental margins
entire Atlantic Ocean and a large portion of the Indian Ocean are surrounded by passive continental margins
The first devices that used sound to measure water depth, called ___ ___, were developed early in the twentieth century.
echo sounders
Instead, the ridge axis is _____.
elevated. These elevated structures called swells, are built from lava flows up to 30 feet thick that have incrementally paved the ridge crest with volcanic rocks.
Concept 10.2 1. List the three major features of a passive continental margin. Which of these features is considered a flooded extension of the continent? continental shelf Who one has the steepest slope? continental slope
geologically inactive regions located some distance from plate boundaries. They are no associated with strong earthquake or volcanic activity. develop when continental blocks rift apart and are separated by continued seafloor spreading. wide and are sites where large quantities of sediments are deposited. The three features: a. continental shelf is an extension of the continent b. continental slope has the steepest slope c. continental rise
By contrast, along the Galapagos Ridge, and _____ spreading rate of 5 to 9 centimeters per year is the norm. As a result, the rift valleys that develop are relatively shallow---often less than 600 feet deep. In addition, their topography is more subdued compared to ridges that have slower spreading rates.
intermediate
East African Rift
is a continental rift that extends through eastern Africa for 2000 miles. It consists of several interconnected rift valleys that split into eastern and western sections around Lack Victoria Whether this rift will eventually develop into a spreading center, with the Somali subplate separating from the rest of the continent of Africa , is uncertain.
Continental shelf
is a gently sloping, submerged surface that extends from the shoreline toward the deep-ocean basin. consists mainly of continental crust capped with sedimentary rocks and sediments eroded from adjacent landmasses. varies greatly in width.
Reconstruction of the breakup of Pangaea also helped investigators understand the demise of the Farallon plate--a ____ ___ ___that once occupied much of the eastern Pacific basin.
large oceanic plate
Although new lithosphere is continually being produced at divergent plate boundaries, Earth's surface is not growing __.
larger.
The crest of the ridge typically stands above the adjacent deep-ocean basins and marks the plate boundary, where new_______ is created.
oceanic
Along well-developed divergent boundaries, the searfloor is elevated, forming a broad linear swell called the ____ ___ or ___ ___ __ or ___.
oceanic ridge mid-oceanic ridge or rise
Concept 10.4 Although the oceanic ridges can be as tall as some mountains, list some ways in which the features are different.
oceanic ridges form where upwelling from the mantle generates new oceanic crust. Oceanic ridges consist of layers and piles of newly formed basaltic rocks that are buoyantly uplifted by the hot mantle rocks from which they are formed.
Along some subduction zones, sediments from the ocean floor and pieces of oceanic crust are scraped from the descending oceanic plate and plastered against the edge of the _________________________plate.
overriding
Over time, continental margins subsided below sea level, and material that had eroded from the adjacent highlands blanketed this once-rugged topography. The result was a __ ___ ___ on the both sides of the Atlantic, consisting of rifted continental crust that has been covered by a thick wedge of relatively undisturbed sediment and sedimentary rock.
passive continental margins.
As the melt begins to accumulate in lens shaped reservoirs, it is blocked from continuing upward by the still overlying rocks. As the amount of melt entering the magma reservoirs increases, pressure rises.
periodically, the pressure exceeds the strength of the overlying rocks, which fracture and initiate a short episode of seafloor spreading.
Challenger Deep
picture of instrument used to measure the deepest spot on the seafloor in 1875
Although molten rock rises continuously from the mantle toward the surface, seafloor spreading occurs in ___ ___ bursts.
pulse-like
Satellites equipped with ___ ___ are able to measure subtle differences in sea level by bouncing microwaves off the sea surface. These devices can measure variations as small as a few centimeters. Such data have added to our knowledge of ocean-floor topography.
radar altimeters
Seafloor Spreading Harry Hess of Princeton University formulated the concept of seafloor spreading in the early 1960's. Later,geologists were able to verify Hess's view that seafloor spreading occurs along the crests of oceanic __.
ridges, where hot mantle rock rises to replace the material that has shifted horizontally.
Rather than obtain the depth of a single point every few seconds, this technique allows a survey ship to map a swath of ocean floor __ ____ ___ wide.
tens of kilometers wide. In addition, these systems collect bathymetric data of such high resolution that they can distinguish depths that differ by less than 3 feet. When multibeam sonar is used to map sections of seafloor, the ship travels in a regularly spaced back-and forth pattern known as mowing the lawn.
Only two trenches are located in the Atlantic Ocean:
the Puerto Rico trench adjacent to the Lesser Antilles arc and the South Sandwich trench
oceanic plateaus
the ocean floor also contains several massive oceanic plateaus, which resemble the flood basalt provinces found on the continents. 20 miles thick generated from vast outpourings of fluid basaltic lavas formed when the bulbous head of a rising mantle plume melts, producing a vast outpouring of basaltic lava
subduction erosion
the opposite process known as __, characterizes many other subduction zones. Rather than sediment accumulating along the front of the overriding plate, sediment and rock are scraped off the bottom of the overriding plate and transported into the mantle by the subducting plate. Subduction erosion is effective where cold, dense oceanic lithosphere subducts at a steep angle as exemplified by the Mariana trench.
When researchers studied various segments of the oceanic ridge system, it became clear that there were ____ differences.
topographic
At other locations, more voluminous lava flows pave the surface to create a relatively subdued ____.
topography
As more of the ridge subducted, the ___ ____ ___, which we now call the ___ ___ ___, increased in length. Today, the southern end of the __ ___ Fault connects to a young spreading center that is generating the Gulf of California.
transform fault system San Andreas Fault San Andreas Fault
The essence of Darwin's hypothesis is that in addition to being lowered by erosional forces, many volcanic islands gradually sink. In addition, Darwin hypothesized that corals responded to the gradual change in water depth caused by the subsiding volcano by building the reef complex __
upward. Eventually, the volcano would submerge beneath the sea, and its remnant would be covered by a type of reef we call an atoll.
In the early stages of rifting, magma generated by decompression melting of the rising mantle rocks intruded the crust. Occasionally, some of the magma migrated upward along fractures and erupted at the surface. This activity produced extensive basaltic flows within the rift as well as ___ ___
volcanic cones--some forming more than 60 miles from the rift axis. ex: Mount Kenya ex: Mount Kilimanjaro, the highest point in Africa, rising almost 20,000 feet above the Serengeti Plain
As a result, some trenches run parallel to an arc-shaped row of active volcanoes called a ___ ___ ___.
volcanic island arc
Today, sound energy is used to measure _____ _________. The basic approach employs sonar.
water depths
Concept 10.4 Briefly describe the oceanic ridge system
winds through all major oceans in a manner similar to the seam on a baseball, and it is the longest topographic feature on Earth exceeding 43, 000 miles in length.
