10.4 Plate, key Motions, and also Plate border Processes

Continental drift and also sea-floor spreading became widely accepted about 1965 as an ext and more geologists began thinking in these terms. Through the finish of 1967 the Earth’s surface had been mapped into a collection of plates (Figure 10.4.1). The significant plates room Eurasia, Pacific, India, Australia, phibìc America, south America, Africa, and also Antarctic. There are also numerous tiny plates (e.g., Juan de Fuca, Cocos, Nazca, Scotia, Philippine, Caribbean), and many very little plates or sub-plates. For example the Juan de Fuca bowl is in reality three separate plates (Gorda, Juan de Fuca, and Explorer) that all move in the same general direction but at slightly different rates.

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Figure 10.4.1 A map showing 15 the the earth tectonic plates and also the approximate rates and directions of plate motions.

Rates of movements of the major plates variety from less than 1 cm/y to end 10 cm/y. The Pacific plate is the fastest, adhered to by the Australian and also Nazca Plates. The phibìc American Plate is among the slowest, averaging roughly 1 cm/y in the southern up to practically 4 cm/y in the north.

Plates relocate as strictly bodies, so it may seem surprising the the north American Plate deserve to be moving at different rates in different places. The explanation is the plates move in a rotational manner. The phibìc American Plate, because that example, rotates counter-clockwise; the Eurasian plate rotates clockwise.

Boundaries between the plates room of 3 types: divergent (i.e., moving apart), convergent (i.e., relocating together), and transform (moving side by side). Before we talk around processes in ~ plate boundaries, it’s vital to point out that there are never gaps between plates. The key are consisted of of crust and the lithospheric component of the mantle (Figure 10.4.2), and also even despite they are moving all the time, and also in various directions, there is never a far-reaching amount of room between them. Plates are thought to move along the lithosphere-asthenosphere boundary, together the asthenosphere is the zone of partial melting. That is assumed that the relative lack of stamin of the partial melting zone facilitates the sliding of the lithospheric plates.

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Figure 10.4.2 The crust and also upper mantle. Tectonic bowl consist that lithosphere, which contains the crust and the lithospheric (rigid) component of the mantle.

At dispersing centres, the lithospheric mantle may be an extremely thin due to the fact that the upward convective motion of hot mantle product generates temperature that room too high for the presence of a significant thickness of rigid lithosphere (Figure 10.3.8). The fact that the plates encompass both crustal material and also lithospheric mantle material makes it possible for a single plate to be comprised of both oceanic and also continental crust. For example, the north American Plate has most of phibìc America, plus half of the northern Atlantic Ocean. Likewise the south American plate extends across the western part of the southern Atlantic Ocean, when the European and also African plates every include component of the east Atlantic Ocean. The Pacific plate is nearly entirely oceanic, yet it does encompass the part of California west the the san Andreas Fault.

Divergent Boundaries

Divergent borders are spreading boundaries, where brand-new oceanic crust is produced from magma acquired from partial melting of the mantle caused by decompression as warm mantle absent from depth is moved toward the surface ar (Figure 10.4.3). The triangle zone the partial melting close to the ridge comb is about 60 kilometres thick and also the ratio of magma is around 10% the the absent volume, thus developing crust the is around 6 km thick. Most divergent borders are located at the oceanic ridges (although some room on land), and the crustal material created at a dispersing boundary is constantly oceanic in character; in various other words, the is mafic igneous absent (e.g., basalt or gabbro, rich in ferromagnesian minerals). Spreading rates vary considerably, native 2 cm/y to 6 cm/y in the Atlantic, to between 12 cm/y and also 20 cm/y in the Pacific. (Note that spreading prices are typically twin the velocities of the 2 plates moving away from a ridge.)

Some of the processes occurring in this setting include:

Magma indigenous the mantle pushing approximately fill the voids left by aberration of the two platesPillow lavas creating where magma is moved out into seawater (Figure 10.4.4)Vertical sheeted dykes intruding into cracks resulting from the spreadingMagma cooling more slowly in the lower component of the new crust and also forming gabbro bodies
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Figure 10.4.3 The general processes the take location at a divergent boundary. The area in ~ the dashed white rectangle is shown in figure 10.4.4.
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Figure 10.4.4 explicate of the processes and materials created at a divergent boundary.

Spreading is hypothesized to start within a continental area through up-warping or doming pertained to an basic mantle plume or collection of mantle plumes. The buoyancy that the mantle plume material creates a dome in ~ the crust, causing it to fracture in a radial pattern, with three arms spaced at roughly 120° (Figure 10.4.5). When a collection of mantle feather exists in ~ a large continent, the result rifts might align and also lead to the formation of a rift sink (such as the present-day great Rift valley in eastern Africa). The is suggested that this form of valley eventually develops into a direct sea (such together the contemporary Red Sea), and also finally right into an ocean (such as the Atlantic). The is likely that as many as 20 mantle plumes, countless of which tho exist, were responsible because that the initiation that the rifting the Pangea follow me what is currently the mid-Atlantic ridge (see figure 10.3.10).

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Figure 10.4.5 relenten of the procedure of dome and three-part rift development (left) and also of continent rifting in between the African and South American parts of Pangea at approximately 200 Ma (right).Convergent Boundaries

Convergent boundaries, where 2 plates are relocating toward every other, space of 3 types, relying on whether oceanic or continent crust is existing on either side of the boundary. The types are ocean-ocean, ocean-continent, and continent-continent.

At one ocean-ocean convergent boundary, one of the key (oceanic crust and lithospheric mantle) is pushed, or subducted, under the other. Frequently it is the older and colder plate the is denser and also subducts beneath the younger and hotter plate. There is commonly an ocean trench along the boundary. The subducted lithosphere descends right into the hot mantle in ~ a fairly shallow angle close to the subduction zone, yet at steeper angles farther under (up to about 45°). As debated in the context of subduction-related volcanism in thing 4, the significant volume that water within the subducting material is released as the subducting late is heated. Most of this water is existing within the paper silicate mineral serpentine which is derived from change of pyroxene and olivine close to the spreading ridge soon after the rock’s formation. The is released as soon as the oceanic crust is heats and then rises and mixes v the overlying mantle. The enhancement of water to the warm mantle lowers the rocks’s melting allude and leader to the development of magma (flux melting) (Figure 10.4.6). The magma, i beg your pardon is lighter 보다 the neighboring mantle material, rises v the mantle and also the overlying oceanic crust to the ocean floor wherein it creates a chain of volcanic islands well-known as an island arc. A tires island arc develops into a chain the relatively huge islands (such as Japan or Indonesia) as more and an ext volcanic material is extruded and sedimentary rocks accumulate about the islands.

As described over in the context of Benioff area (Figure 10.3.6), earthquakes take ar close come the boundary in between the subducting crust and also the overriding crust. The largest earthquakes happen near the surface where the subducting plate is tho cold and strong.

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Figure 10.4.6 Configuration and also processes of one ocean-ocean convergent boundary.

Examples of ocean-ocean convergent zones space subduction that the Pacific Plate in ~ the phibìc America Plate south of Alaska (Aleutian Islands) and also beneath the Philippine bowl west that the Philippines, subduction the the India Plate beneath the Eurasian Plate south of Indonesia, and also subduction that the Atlantic Plate beneath the Caribbean key (see number 10.4.1).

At one ocean-continent convergent boundary, the oceanic bowl is moved under the continent plate in the same manner as at one ocean-ocean boundary. Sediment the has collected on the continental slope is thrust up into an accretionary wedge, and also compression leader to thrusting in ~ the continent plate (Figure 10.4.7). The mafic magma produced nearby to the subduction region rises come the basic of the continental crust and also leads to partial melting of the crustal rock. The result magma ascends with the crust, creating a hill chain with many volcanoes.

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Figure 10.4.7 Configuration and also processes of an ocean-continent convergent boundary.

Examples of ocean-continent convergent borders are subduction that the Nazca plate under south America (which has produced the Andes Range) and subduction the the Juan de Fuca plate under phibìc America (creating the mountains Garibaldi, Baker, St. Helens, Rainier, Hood, and Shasta, jointly known together the Cascade Range).

A continent-continent collision occurs as soon as a continent or large island that has actually been moved in addition to subducting oceanic tardy collides with one more continent (Figure 10.4.8). The colliding continental material will not be subducted because it is also light (i.e., because it is composed mainly of light continent rocks ), yet the source of the oceanic plate will at some point break off and sink into the mantle. Over there is incredible deformation that the pre-existing continental rocks, and also creation of hills from that rock, from any sediments that had accumulated along the shores (i.e., within geosynclines) the both continental masses, and also commonly also from some ocean crust and also upper mantle material.

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Figure 10.4.8 Configuration and processes that a continent-continent convergent boundary.

Examples that continent-continent convergent boundaries are the collision of the India Plate v the Eurasian Plate, developing the Himalaya Mountains, and the collision that the african Plate through the Eurasian Plate, producing the series of ranges extending from the Alps in Europe come the Zagros hills in Iran. The Rocky mountains in B.C. And Alberta are also a result of continent-continent collisions.

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Figure 10.4.9 The san Andreas error extends from the north end of the eastern Pacific rise in the Gulf the California to the southern finish of the Juan de Fuca Ridge. All of the red currently on this map space transform faults.

Transform limits exist where one plate slides past another without manufacturing or damage of crustal material. As defined above, many transform faults affix segments the mid-ocean ridges and are hence ocean-ocean plate limits (Figure 10.3.11). Some transform faults attach continental components of plates. An example is the mountain Andreas Fault, i m sorry extends native the southern finish of the Juan de Fuca Ridge to the northern end of the east Pacific increase (ridge) in the Gulf the California (Figures 10.28 and 10.29). The component of California west that the san Andreas error and every one of Baja California are on the Pacific Plate. Change faults do not just connect divergent boundaries. Because that example, the Queen Charlotte fault connects the north end of the Juan de Fuca Ridge, starting at the north end of Vancouver Island, to the Aleutian subduction zone.

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Figure 10.4.10 The san Andreas fault at Parkfield in main California. The human being with the orange shirt is standing on the Pacific Plate and the human at the much side that the bridge is top top the phibìc American Plate. The leg is designed come accommodate motion on the error by sliding on that foundation.