plate tectonics | Definition, Theory, Facts, & Evidence | misjon.info
Plate tectonics, theory dealing with the dynamics of Earth's outer . There are two types of crust, continental and oceanic, which differ in their .. Furthermore, the relationship between hotspots and plumes is hotly debated. The theory of plate tectonics accounts nicely for the slow and steady volcanic activity that occurs at .. Peridotite, a rock type commonly found deep in hot spot Note the relationship between temperature and mineral composition and stability. This was a major puzzle in relation to plate tectonics. A Canadian Adding to Wilson's theory, Jason Morgan furthered the idea of the hot spot. Morgan As the magma reaches the surface, it cools quickly and forms pillow lava. This pillow .
What causes hot spot volcanism? It is imagined that these plumes rise as a plastically deforming mass that has a bulbous plume head fed by a long, narrow plume tail. As the head impinges on the base of the lithosphere, it spreads outward into a mushroom shape. This causes decompressional melting of the hot mantle material, i. It is thought that the massive flood basalt provinces on earth are produced when large mantle plumes reach the lithosphere.
Note the bulbous plume heads, the narrow plume tails, and the flattened plume heads as they impinge on the outer sphere representing the base of the lithosphere. Illustration how the progressively older islands formed above the stationary mantle plume Courtesy of the USGS.
Mantle plumes appear to be largely unaffected by plate motions. While a plume that feeds hot spot volcanoes remains stationary relative to the mantle, the plate above it usually moves.
The result is that a chain of progressively older volcanoes are created on the overlying plate. During the late 20th and early 21st centuries, scientific understanding of the deep mantle was greatly enhanced by high-resolution seismological studies combined with numerical modeling and laboratory experiments that mimicked conditions near the core-mantle boundary.
At a depth of about 5, km 3, milesthe outer core transitions to the inner core. The polarity of the iron crystals of the OIC is oriented in a north-south direction, whereas that of the IIC is oriented east-west.
What is a Hot Spot? | Volcano World | Oregon State University
Earth's coreThe internal layers of Earth's core, including its two inner cores. Plate boundaries Lithospheric plates are much thicker than oceanic or continental crust. Their boundaries do not usually coincide with those between oceans and continentsand their behaviour is only partly influenced by whether they carry oceans, continents, or both. The Pacific Plate, for example, is entirely oceanic, whereas the North American Plate is capped by continental crust in the west the North American continent and by oceanic crust in the east and extends under the Atlantic Ocean as far as the Mid-Atlantic Ridge.
A general discussion of plate tectonics. In a simplified example of plate motion shown in the figure, movement of plate A to the left relative to plates B and C results in several types of simultaneous interactions along the plate boundaries.
At the rear, plates A and B move apart, or diverge, resulting in extension and the formation of a divergent margin. At the front, plates A and B overlap, or converge, resulting in compression and the formation of a convergent margin.
Along the sides, the plates slide past one another, a process called shear. As these zones of shear link other plate boundaries to one another, they are called transform faults. Theoretical diagram showing the effects of an advancing tectonic plate on other adjacent, but stationary, tectonic plates.
At the advancing edge of plate A, the overlap with plate B creates a convergent boundary. In contrast, the gap left behind the trailing edge of plate A forms a divergent boundary with plate B.
As plate A slides past portions of both plate B and plate C, transform boundaries develop. Divergent margins As plates move apart at a divergent plate boundarythe release of pressure produces partial melting of the underlying mantle. This molten material, known as magmais basaltic in composition and is buoyant. As a result, it wells up from below and cools close to the surface to generate new crust.
Because new crust is formed, divergent margins are also called constructive margins. Continental rifting Upwelling of magma causes the overlying lithosphere to uplift and stretch. Whether magmatism [the formation of igneous rock from magma] initiates the rifting or whether rifting decompresses the mantle and initiates magmatism is a matter of significant debate. If the diverging plates are capped by continental crust, fractures develop that are invaded by the ascending magma, prying the continents farther apart.
Settling of the continental blocks creates a rift valleysuch as the present-day East African Rift Valley. As the rift continues to widen, the continental crust becomes progressively thinner until separation of the plates is achieved and a new ocean is created. The ascending partial melt cools and crystallizes to form new crust. Because the partial melt is basaltic in composition, the new crust is oceanic, and an ocean ridge develops along the site of the former continental rift. Consequently, diverging plate boundaries, even if they originate within continents, eventually come to lie in ocean basins of their own making.
The Thingvellir fracture lies in the Mid-Atlantic Ridge, which extends through the centre of Iceland. Samples collected from the ocean floor show that the age of oceanic crust increases with distance from the spreading centre —important evidence in favour of this process.
These age data also allow the rate of seafloor spreading to be determined, and they show that rates vary from about 0. Seafloor-spreading rates are much more rapid in the Pacific Ocean than in the Atlantic and Indian oceans.
At spreading rates of about 15 cm 6 inches per year, the entire crust beneath the Pacific Ocean about 15, km [9, miles] wide could be produced in million years. Divergence and creation of oceanic crust are accompanied by much volcanic activity and by many shallow earthquakes as the crust repeatedly rifts, heals, and rifts again. Brittle earthquake -prone rocks occur only in the shallow crust.
Deep earthquakes, in contrast, occur less frequently, due to the high heat flow in the mantle rock. These regions of oceanic crust are swollen with heat and so are elevated by 2 to 3 km 1. The elevated topography results in a feedback scenario in which the resulting gravitational force pushes the crust apart, allowing new magma to well up from below, which in turn sustains the elevated topography.
Its summits are typically 1 to 5 km 0. This is accomplished at convergent plate boundaries, also known as destructive plate boundaries, where one plate descends at an angle—that is, is subducted—beneath the other. Because oceanic crust cools as it ages, it eventually becomes denser than the underlying asthenosphere, and so it has a tendency to subduct, or dive under, adjacent continental plates or younger sections of oceanic crust.The Earth's crust: tectonic plate movement, volcanoes, tsunami, earthquakes
The life span of the oceanic crust is prolonged by its rigidity, but eventually this resistance is overcome. Experiments show that the subducted oceanic lithosphere is denser than the surrounding mantle to a depth of at least km about miles.
The mechanisms responsible for initiating subduction zones are controversial. During the late 20th and early 21st centuries, evidence emerged supporting the notion that subduction zones preferentially initiate along preexisting fractures such as transform faults in the oceanic crust.
Irrespective of the exact mechanism, the geologic record indicates that the resistance to subduction is overcome eventually. Where two oceanic plates meet, the older, denser plate is preferentially subducted beneath the younger, warmer one. Where one of the plate margins is oceanic and the other is continental, the greater buoyancy of continental crust prevents it from sinking, and the oceanic plate is preferentially subducted. Continents are preferentially preserved in this manner relative to oceanic crust, which is continuously recycled into the mantle.
This explains why ocean floor rocks are generally less than million years old whereas the oldest continental rocks are more than 4 billion years old.
Before the middle of the 20th century, most geoscientists maintained that continental crust was too buoyant to be subducted. However, it later became clear that slivers of continental crust adjacent to the deep-sea trenchas well as sediments deposited in the trench, may be dragged down the subduction zone. The recycling of this material is detected in the chemistry of volcanoes that erupt above the subduction zone. Two plates carrying continental crust collide when the oceanic lithosphere between them has been eliminated.
Eventually, subduction ceases and towering mountain ranges, such as the Himalayasare created. See below Mountains by continental collision. Because the plates form an integrated system, it is not necessary that new crust formed at any given divergent boundary be completely compensated at the nearest subduction zone, as long as the total amount of crust generated equals that destroyed.
Subduction zones The subduction process involves the descent into the mantle of a slab of cold hydrated oceanic lithosphere about km 60 miles thick that carries a relatively thin cap of oceanic sediments.
The factors that govern the dip of the subduction zone are not fully understood, but they probably include the age and thickness of the subducting oceanic lithosphere and the rate of plate convergence. Most, but not all, earthquakes in this planar dipping zone result from compressionand the seismic activity extends to km to miles below the surface, implying that the subducted crust retains some rigidity to this depth.
At greater depths the subducted plate is partially recycled into the mantle. The site of subduction is marked by a deep trench, between 5 and 11 km 3 and 7 miles deep, that is produced by frictional drag between the plates as the descending plate bends before it subducts. The overriding plate scrapes sediments and elevated portions of ocean floor off the upper crust of the lower plate, creating a zone of highly deformed rocks within the trench that becomes attached, or accreted, to the overriding plate.
This chaotic mixture is known as an accretionary wedge. The rocks in the subduction zone experience high pressures but relatively low temperatures, an effect of the descent of the cold oceanic slab.
Under these conditions the rocks recrystallize, or metamorphose, to form a suite of rocks known as blueschists, named for the diagnostic blue mineral called glaucophanewhich is stable only at the high pressures and low temperatures found in subduction zones. See also metamorphic rock. At deeper levels in the subduction zone that is, greater than 30—35 km [about 19—22 miles]eclogiteswhich consist of high-pressure minerals such as red garnet pyrope and omphacite pyroxeneform.
The formation of eclogite from blueschist is accompanied by a significant increase in density and has been recognized as an important additional factor that facilitates the subduction process. Island arcs When the downward-moving slab reaches a depth of about km 60 milesit gets sufficiently warm to drive off its most volatile components, thereby stimulating partial melting of mantle in the plate above the subduction zone known as the mantle wedge.
Melting in the mantle wedge produces magmawhich is predominantly basaltic in composition. This magma rises to the surface and gives birth to a line of volcanoes in the overriding plate, known as a volcanic arctypically a few hundred kilometres behind the oceanic trench.
The distance between the trench and the arc, known as the arc-trench gap, depends on the angle of subduction. Steeper subduction zones have relatively narrow arc-trench gaps. A basin may form within this region, known as a fore-arc basin, and may be filled with sediments derived from the volcanic arc or with remains of oceanic crust.
If both plates are oceanic, as in the western Pacific Ocean, the volcanoes form a curved line of islandsknown as an island arcthat is parallel to the trench, as in the case of the Mariana Islands and the adjacent Mariana Trench. If one plate is continental, the volcanoes form inland, as they do in the Andes of western South America. Though the process of magma generation is similar, the ascending magma may change its composition as it rises through the thick lid of continental crust, or it may provide sufficient heat to melt the crust.
In either case, the composition of the volcanic mountains formed tends to be more silicon -rich and iron - and magnesium -poor relative to the volcanic rocks produced by ocean-ocean convergence. Back-arc basins Where both converging plates are oceanic, the margin of the older oceanic crust will be subducted because older oceanic crust is colder and therefore more dense.
This results in a process known as back-arc spreading, in which a basin opens up behind the island arc. The crust behind the arc becomes progressively thinner, and the decompression of the underlying mantle causes the crust to melt, initiating seafloor-spreading processessuch as melting and the production of basalt; these processes are similar to those that occur at ocean ridges. The geochemistry of the basalts produced at back-arc basins superficially resembles that of basalts produced at ocean ridgesbut subtle trace element analyses can detect the influence of a nearby subducted slab.
This style of subduction predominates in the western Pacific Oceanin which a number of back-arc basins separate several island arcs from Asia. However, if the rate of convergence increases or if anomalously thick oceanic crust possibly caused by rising mantle plume activity is conveyed into the subduction zone, the slab may flatten.
Such flattening causes the back-arc basin to close, resulting in deformationmetamorphismand even melting of the strata deposited in the basin. Mountain building If the rate of subduction in an ocean basin exceeds the rate at which the crust is formed at oceanic ridges, a convergent margin forms as the ocean initially contracts.
This process can lead to collision between the approaching continentswhich eventually terminates subduction.
Mountain building can occur in a number of ways at a convergent margin: Many mountain belts were developed by a combination of these processes. For example, the Cordilleran mountain belt of North America —which includes the Rocky Mountains as well as the Cascadesthe Sierra Nevadaand other mountain ranges near the Pacific coast—developed by a combination of subduction and terrane accretion.
As continental collisions are usually preceded by a long history of subduction and terrane accretion, many mountain belts record all three processes. Over the past 70 million years the subduction of the Neo-Tethys Seaa wedge-shaped body of water that was located between Gondwana and Laurasialed to the accretion of terranes along the margins of Laurasia, followed by continental collisions beginning about 30 million years ago between Africa and Europe and between India and Asia.
These collisions culminated in the formation of the Alps and the Himalayas. Jurassic paleogeographyDistribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the late Jurassic Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones. Subduction results in voluminous magmatism in the mantle and crust overlying the subduction zoneand, therefore, the rocks in this region are warm and weak. Although subduction is a long-term process, the uplift that results in mountains tends to occur in discrete episodes and may reflect intervals of stronger plate convergence that squeezes the thermally weakened crust upward.
For example, rapid uplift of the Andes approximately 25 million years ago is evidenced by a reversal in the flow of the Amazon River from its ancestral path toward the Pacific Ocean to its modern path, which empties into the Atlantic Ocean.
In addition, models have indicated that the episodic opening and closing of back-arc basins have been the major factors in mountain-building processes, which have influenced the plate-tectonic evolution of the western Pacific for at least the past million years.
Mountains by terrane accretion As the ocean contracts by subduction, elevated regions within the ocean basin—terranes—are transported toward the subduction zone, where they are scraped off the descending plate and added—accreted—to the continental margin.
Since the late Devonian and early Carboniferous periods, some million years ago, subduction beneath the western margin of North America has resulted in several collisions with terranes. The piecemeal addition of these accreted terranes has added an average of km miles in width along the western margin of the North American continentand the collisions have resulted in important pulses of mountain building.
The more gradual transition to the abyssal plain is a sediment-filled region called the continental rise. The continental shelf, slope, and rise are collectively called the continental margin. During these accretionary events, small sections of the oceanic crust may break away from the subducting slab as it descends. Instead of being subducted, these slices are thrust over the overriding plate and are said to be obducted.
Where this occurs, rare slices of ocean crust, known as ophiolitesare preserved on land.
They provide a valuable natural laboratory for studying the composition and character of the oceanic crust and the mechanisms of their emplacement and preservation on land.
A classic example is the Coast Range ophiolite of Californiawhich is one of the most extensive ophiolite terranes in North America.
- Intraplate (hot-spot) volcanism
- Plate Tectonics and the Hawaiian Hot Spot
- Hawaiian Islands and Hot Spots
These ophiolite deposits run from the Klamath Mountains in northern California southward to the Diablo Range in central California.