Earth and Space Science #5

Oceanic ridge system

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Asthenosphere

- (asthenos = weak, sphere = a ball) The lithosphere overlies a weak region in the mantle known as the asthenosphere. The temperature and pressure in the upper asthenosphere (100 - 200 kilometers in depth) are such that the rocks there are very near their melting temperatures and, hence, respond to stress by flowing. As a result, Earth's rigid outer shell is effectively detached from the layers below, which permit it to move independently.

Convergent boundaries

- (con = together, verge = to move) New lithosphere is constantly being produced at the oceanic ridges; however, our planet is not growing larger - its total surface area remains constant. A balance is maintained because older, denser portions of oceanic lithosphere descend into the mantle at a rate equal to seafloor production. This activity occurs along convergent boundaries, where two plates move toward each other and the leading edge of one is bent downward, as it slides beneath the other.

Transform fault

- (conservative margins) - where two plates grind past each other without the production or destruction of lithosphere. (20% of all boundaries)

Divergent

- (constructive margins) - were two plates move apart, resulting in upwelling of hot material from the mantle to create new seafloor.

Convergent

- (destructive margins) - where two plates move together, resulting in oceanic lithosphere descending beneath an overriding plate, eventually to be reabsorbed into the mantle, or possibly in the collision of two continental blocks to create a mountain system. (Divergent + convergent boundaries = 40% of all boundaries)

Divergent boundaries

- (di = apart, vergere = to move) Divergent boundaries are located along the crests of oceanic ridges and can be thought of as constructive plate margins since this is where new ocean floor is generated. Divergent boundaries are also called spreading centers, because seafloor spreading occurs at these boundaries. Here, two adjacent plates are moving away from each other, producing long, narrow fractures in the ocean crust. As a result, hot rock from the mantle below migrates upward to fill the voids left as the crust is being ripped apart. This molten material gradually cools to produce new slivers of seafloor.

Volcanic island arc

- (island arc) an arc-shaped chain of volcanic islands created by oceanic - oceanic convergence.

Pangaea

- (meaning "all lands") the supercontinent that Wegener believed existed before the separation into the continents of today.

Fossil magnetism

- (or paleomagnetism) Rocks that formed thousands of millions of years ago and contain a "record" of the direction of the magnetic poles at the time of their formation are said to possess fossil magnetism.

Paleoclimate data (as evidence)

- (pale = ancient, climatic = climate) Wegener suspected that paleoclimatic data might also support the idea of mobile continents. His assertion was bolstered when he learned that evidence for a glaciation that dated to the late Paleozic had been discovered in southern Africa, South America, Australia. and India. This meant that about 300 million years ago, vast ice sheets covered extensive portions of the Southern Hemisphere as well as India. Much of the land area that contains evidence of the period of Paleozoic glaciation presently lies within 30 degrees of the equator in subtropical or tropical climates. Wegener suggested that a plausible explanation for the late Paleozoic glaciation was provided by the supercontinent Pangaea.

Transform fault boundaries

- (trans = across, forma = form) Along transform fault boundaries, plates slide horizontally past one another without the production of destruction of lithosphere (conservative plate margins). The nature of transform faults was discovered in 1965 by Canadian geologist J. Tuzon Wilson, who proposed that these large faults connected two spreading centers (divergent boundaries), or less commonly two trenches (convergent boundaries). Most transform faults are found on the ocean floor. Here they offset segments of the oceanic ridge system, producing a step-like plate margin.

Magnetic reversal

- Additional evidence surfaced when geophysicists discovered that over periods of hundreds of thousands of years, Earth's magnetic field periodically reverses polarity. During a magnetic reversal the north magnetic pole becomes the south magnetic pole, and vice versa. Lava solidifying during a period of reverse polarity will be magnetized with the polarity opposite that of volcanic rocks being formed today.

Rift valley

- Along the axis of some ridge segments is a deep down-faulted structure called a rift valley. This structure is evidence that tensional forces are actively pulling the ocean crust apart at the ridge crest.

Changes in plates and plate boundaries

- Although the total surface area of Earth does not change, the size and shape of individual plates are constantly changing. For example, the African and Antarctic plates are mainly bounded by divergent boundaries - sites of seafloor production. Thus, the African and Antarctic plates are continually growing in size as new lithosphere is added to their margins. By contrast, the Pacific plate is being consumed into the mantle along its northern and western flanks much faster than it is growing along the East Pacific Rise and this is diminishing in size.

Oceanic - oceanic convergence

- An oceanic - oceanic convergent boundary has many features in common with oceanic - continental plate margins. Where two oceanic slabs converge, one descends beneath the other, initiating volcanic activity by the same mechanism that operates at all subductions zones. Water squeezes from the subducting slab of oceanic lithosphere triggers melting in the hot wedge of mantle rock above. In this setting, volcanoes grow up from the ocean floor, rather than upon a continental platform. When subduction is sustained, it will eventually build a chain of volcanic structures large enough to emerge as islands.

Curie point

- Basaltic lavas erupt at the surface at temperatures greater than 100 degrees Celsius, exceeding a threshold temperature for magnetism know as the Curie point (about 585 C).

Water Derives Plate Motion

- Convection flow, warm, buoyant rock rises and cooler, denser material sinks.

Subduction zones

- Convergent boundaries are also referred to as subduction zones, because they are sites where lithosphere is descending (being subducted) into the mantle. Subduction occurs because the density of the descending tectonic plate is greater than the density of the underlying asthenosphere, whereas continental lithosphere is less dense and resists subduction. As a consequence, only oceanic lithosphere will subduct to greater depths.

Continental rifting

- Divergent boundaries can also develop within a continent, in which case the landmass may split into two or more smaller segments separated by an ocean basin. Continental rifting occurs where opposing tectonic forces act to pull the lithosphere apart. The initial stage of rifting tends to include mantle upwelling that is associated with broad upwarping of the overlying lithosphere. As a result, the lithosphere is stretched, causing the brittle crustal rocks to break into large slabs. As the tectonic forces continue to pull the crust apart, these crustal fragments sink, generating an elongated depression called a continental rift.

Plate tectonics

- Earthquake studies conducted in the western Pacific demonstrated that tectonic activity was occurring at great depths beneath deep-ocean trenches. Of equal importance was the fact that dredging of the seafloor did not bring up any oceanic crust that was older than 180 million years. Further, sediment accumulations in the deep-ocean basins were found to be thin, not the thousands of meters that were predicted. By 1968, these developments, among others, led to the unfolding of a far more encompassing theory that continental drift, known as plate tectonics.

Rock Types and Geologic Features

- Evidence

The Continental Jigsaw Puzzle

- Evidence of the continental drift, Wegener suspected that the continents might once have been joined when he noticed the remarkable similarity between the coastlines on opposite sides of the Atlantic Ocean.

Geophysicists/Geochemists

- Following WWII, modern instruments replaced rock hammers as the tools of choice for many researchers. Armed with these more advanced tools, geologists and a new breed of researchers including geophysicists and geochemists made several surprising discoveries that began to rekindle interest in the drift hypothesis. By 1968 these developments led to the unfolding of a far more encompassing explanation known as the theory of plate tectonics.

Mantle plume

- Most researchers are in agreement that cylindrically shaped upwelling of hot rock, called mantle plume, is located beneath the island of Hawaii. As the hot, rocky plume ascends through the mantle the confining pressure drops, which triggers partial melting, (this process is called decompression melting).

Continental volcanic arcs

- Mountain systems such as the Andes, which are produced in part by volcanic activity associated with the subduction of oceanic lithosphere, are called continental volcanic arcs.

Magnetometers

- Oceanographers had begun to do magnetic surveys of the ocean floor in conjunction with their efforts to construct detailed maps of seafloor topography. These magnetic surveys were accomplished by towing very sensitive instruments, called magnetometers, behind research vessels. The goal of these geophysical surveys was to map variations in the strength of Earth's magnetic field that arise from differences in the magnetic properties of the underlying crustal rocks.

Magnetic time scale

- Once the concept of magnetic reversals was confirmed, researchers set out to establish a time scale for the occurrences of magnetic reverse. The task was to measure the magnetic polarity of hundreds of lava flows and use rediometric dating techniques to establish the age of each flow. The result was the magnetic time scale.

Plate Boundaries

- One of the main tenets of the plate tectonic theory is that plates move as semicoherent units relative to all other plates. As plates move, the distance between two location on different plates gradually changes whereas the distance between two sites on the same plate remains relatively constant. Because plates are in constant motion relative to each other, most major interactions among them (and, therefor, most deformation) occur along their boundaries. Plate boundaries were first established by plotting the locations of earthquakes and volcanoes. Plates are bounded by three distinct types of boundaries, which are different by the type of movement they exhibit.

The Scientific revolution of the continental drift

- Prior to the 1960s most geologist held the view that the ocean basins and continents had fixed geographic positions and were of great antiquity. Less than a decade later researchers came to realize the Earth's continents are not static, instead they gradually migrate across the globe. Because of these movements, blocks of continental material collide, deforming the intervening crust, thereby creating Earth's great mountain chains. Furthermore, landmasses occasionally split apart. As the continental blocks separate, a new ocean basin emerges between them. Meanwhile, other portions of the seafloor plunge into the mantle. In short, a dramatically different model of Earth's tectonic processes emerged. This profound reversal in scientific thought has been appropriately described as a scientific revolution, called the continental drift.

Reverse polarity

- Rocks exhibiting the opposite magnetism are said to have reverse polarity.

Partial melting

- Sediment and oceanic crust contain a large amount of water that is carried to great depths by a subducting plate. As the plate plunges downward, heat and pressure drive water from the voids in the rock. At a depth of roughly 100 kilometers, the wedge of mantle rock is sufficiently hot that the introduction of water from the slab below leads to some melting. This process is called partial melting, and is thought to generate about 10% molten material, which is intermixed with unmelted mantle rock. Being less dense than the surrounding mantle, this hot mobile material gradually rises toward the surface. Depending on the environment, these mantle-derived masses of molten rock may ascend through the crust and give rise to a volcanic eruption. Much of this material never reaches the surface; rather, it solidifies at depth - a process that thickens the crust.

Evidence from ocean drilling

- Some of the most convincing evidence for seafloor spreading came from the Deep Sea Drilling Project, which operated from 1968 until 1983. One of the early goals was to gather samples of the ocean floor in order to establish its age. To accomplish this, the Glomar Challenger, a drilling ship capable of working in water thousands of meters deep, was built. Hundreds of holes were drilled through the layers of sediments that blanket the ocean crust, as well as into the basaltic rocks bellow. Rather than radiometrically dating the crustal rocks, researchers used the fossil remains of microorganisms found in the sediments resting directly on the crust to date the seafloor at each site. When the oldest sediment from each drill site was plotted against its distance from the ridge crest, the plot showed that the sediment increased in age with increasing distance from the ridge. This finding supported the seafloor-spreading hypothesis, which predicted that the youngest oceanic crust would be found at the ridge rest, the site of seafloor production, and the oldest oceanic crust would be located adjacent to the continents.

Lithosphereic/tectonic plates

- The lithosphere is composed of about 20 segments having irregular sizes and shapes called lithospheric or tectonic plates that are in constant motion with respect to one another. Seven major lithospheric plates are recognized. These plates, which account for 94% of Earth's surface area, include the North American, South American, Pacific, African, Eurasian, Australian-Indian, and Antarctic plates. The largest Pacific plate, which encompasses a significant portion of the Pacific Ocean basin. Other plates are the Caribbean, Nazca, Philippine, Arabian, Cocos, Scotia, and Juan de Fuca plates. These plates are composed of mostly oceanic lithosphere.

Seafloor spreading

- The mechanism that operates along the oceanic ridge system to create new seafloor is appropriately called seafloor spreader. Rates of spreading average around 5 centimeters (2 inches) per year.

Hot spot

- The surface manifestation of decompression melting, mantle plume, is a hot spot, an area of volcanism, high heat flow, and crustal uplifting that is a few hundred kilometers across. As the Pacific plate moved over the hot spot, a chain of volcanic structures known as a hot-spot track was built (the age of each volcano indicates how much time has elapsed since it was situated over the mantle plume).

Continental - continental convergence

- The third type of convergence boundary results when one landmass moves toward the margin of another because of subduction of the intervening seafloor. Whereas oceanic lithosphere tends to be dense and sink into the mantle, the buoyancy of continental material inhibits it form being subducted. Consequently, a collision between two converging continental fragments ensues . This event folds and deforms the accumulation of sediments and sedimentary rocks along the continental margins as if they had been placed in a gigantic vise. The result is the formation of a new mountain range composed of deformation sedimentary and metamorphic rocks that often contains slivers of oceanic crust.

Lithosphere

- The uppermost mantle, along with the overlying crust, behave as a strong, rigid layer, known as the lithosphere (lithos = stone, sphere = a ball) which is broken into segments commonly referred to as plates. The lithosphere is thinnest in the oceans where it varies from as little as a few kilometers along the axis of the oceanic ridge system to about 100 kilometers (60 miles) in the deep-ocean basin. In contrast, continental lithosphere is generally thicker than 100 kilometers and may extend to depths of 200 to 300 kilometers in some regions.

Mesosaurus (as evidence)

- To add credibility to his argument, Wegener documented cases of several fossil organisms that were found on different landmasses despite the unlikely possibility that their living form could have crossed the vast ocean presently separating them. A classic example is Mesosaurus, an aquatic fish-catching reptile whose fossil remains are limited to black shales of the Permain period in eastern South America and southwestern Africa. If Mesosaurus had been able to make the long journey across the South Atlantic, its remains would likely be more widely distributed. As this is not case, Wegener asserted that South America and Africa must have been joined during that period of Earth history.

Fractured zones

- Typically, transform faults are part of prominent linear breaks in the seafloor known as fractured zones, which include both the active transform faults as well as their inactive extensions into the plate interior. Active transform faults lie ONLY BETWEEN the two offset ridge segments and are generally defined by weak, shallow earthquakes. Here seafloor produced at one ridge axis moves in the opposite direction as seafloor produced at an opposing ridge segment. Between the ridge segments these adjacent slabs of oceanic crust are grinding past each other along a transform fault. Beyond the ridge crests are inactive zones, where the fractures are preserved as linear topographic depressions. The trend of these fracture zones roughly parallels the direction of plate motion at the time of their formation. Thus, these structures are useful in mapping the direction of plate motion in the geologic past.

Glossopteris (as evidence)

- Wegener also cited that distribution of the fossil "seed fern" Glossopteris as evidence for the existence of Pangaea. This plant, identified by its tongue-shaped leaves and seeds that ere too large to be carried by the wind, was known to be widely dispersed among Africa, Australia, India, and South America. Later, fossil remains of Glossopteris were also discovered in Antarctica. Wegener also learned that these seed ferns and associated flora grew only in subpolar climate. Therefor, he concluded that when these landmasses were joined, there were located much closer to the South Pole.

Supercontinent

- Wegener suggested that a single supercontinent consisting of Earth's landmasses once existed. He named this giant landmass Pangaea.

Fossils match across the sea

- Wegener's focus on the continental drift intensified when he discovered the similarities between fossils from both South America and Africa. He found that most paleontologists where in agreement that some type of land connection was needed to explain the existence of similar Mesozoic age life forms on widely separated landmasses.

Normal polarity

- When rocks exhibit the same magnetism as the present magnetic field, they are said to possess normal polarity.

Oceanic - continental convergence

- Whenever the leading edge of a plate capped with continental crust converges with a slab of oceanic lithosphere, the buoyant continental block remains "floating," and the denser oceanic slab sinks into the mantle. When a descending oceanic slab reaches a depth of about 100 kilometers (60 miles), melting is triggered within the wedge of hot asthenosphere that lies above it.

The Origin of Continents and Oceans

- Written by German meteorologist, Alfred Wegener, The Origin of Continents and Oceans set forth the basic outline of Wegener's hypothesis called continental drift...

Continental drift

- a hypothesis that challenged the long-held assumption that the continents and ocean basins had fixed and geographic positions.

Deep-ocean trenches

- are the surface manifestations produced as oceanic lithosphere descends into the mantle. These linear depressions are remarkably long and deep. Slabs of oceanic lithosphere descend into the mantle at angles that vary from a few degrees to nearly vertical (90 degrees). The angle at which oceanic lithosphere descends depends largely on its desity. As oceanic lithospher ages (gets farther from the spreading center), it gradually cools, which causes it to thicken and increase in density.

Ocean ridges

- elevated areas of the seafloor that are characterized by high heat flow and volcanism.

Microplates

- smaller tectonic plates.

Rafting, Island stepping stones, isthmian links

- were all ways those opposing the continental drift hypothesis explained the similar fossils on opposite coastlines, across the sea.