10.4 Plates, bowl Motions, and also Plate-Boundary Processes

Continental drift and sea-floor spreading became widely accepted roughly 1965 as much more and more geologists started thinking in these terms. Through the finish of 1967, Earth’s surface had been mapped right into a series of bowl (Figure 10.16). The significant plates are Eurasia, Pacific, India, Australia, north America, southern America, Africa, and also Antarctic. There are additionally numerous little plates (e.g., Juan de Fuca, Nazca, Scotia, Philippine, Caribbean), and many very little plates or sub-plates. For instance the Juan de Fuca plate is in reality three separate plates (Gorda, Juan de Fuca, and Explorer) the all relocate in the same general direction but at slightly various rates.

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Figure 10.16 A map mirroring 15 of the earth’s tectonic plates and also the approximate rates and also directions of key motions.

Rates of motions of the major plates variety from much less than 1 cm/y to over 10 cm/y. The Pacific key is the fastest at over 10 cm/y in some areas, followed by the Australian and Nazca Plates. The phibìc American Plate is one of the slowest, averaging around 1 cm/y in the southern up to nearly 4 cm/y in the north.

Plates relocate as strictly bodies, therefore it may seem surprising the the phibìc American Plate can be moving at different rates in different places. The explanation is the plates relocate in a rotational manner. The phibìc American Plate, for example, rotates counter-clockwise; the Eurasian plate rotates clockwise.

Boundaries between the plates are of 3 types: divergent (i.e., moving apart), convergent (i.e., relocating together), and also transform (moving next by side). Prior to we talk about processes at plate boundaries, it’s important to point out that there are never gaps in between plates. The plates are comprised of crust and the lithospheric part of the mantle (Figure 10.17), and also even despite they are relocating all the time, and in various directions, over there is never ever a far-reaching amount of room between them. Plates room thought to move along the lithosphere-asthenosphere boundary, together the asthenosphere is the zone the partial melting. The is assumed that the relative absence of toughness of the partial melting zone facilitates the slide of the lithospheric plates.

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Figure 10.17 The crust and also upper mantle. Tectonic plates consist of lithosphere, which has the crust and also the lithospheric (rigid) component of the mantle.

At dispersing centres, the lithospheric mantle might be very thin since the increase convective movement of warm mantle material generates temperature that room too high because that the existence of a far-reaching thickness of strictly lithosphere (Figure 10.12). The reality that the plates include both crustal material and lithospheric mantle product makes it possible for a solitary plate to be consisted of of both oceanic and also continental crust. For example, the phibìc American Plate includes most of north America, plus fifty percent of the north Atlantic Ocean. Likewise the south American plate extends throughout the western part of the southern Atlantic Ocean, if the European and African plates every include part of the eastern Atlantic Ocean. The Pacific bowl is nearly entirely oceanic, but it does include the component of California west of the san Andreas Fault.

Divergent Boundaries

Divergent boundaries are dispersing boundaries, where brand-new oceanic late is produced from magma obtained from partial melt of the mantle caused by decompression as warm mantle absent from depth is moved toward the surface ar (Figure 10.18). The triangular zone that partial melting near the ridge comb is roughly 60 km thick and also the proportion of magma is about 10% the the rock volume, thus developing crust the is around 6 km thick. Many divergent borders are located at the oceanic ridges (although some are on land), and the crustal material produced at a dispersing boundary is always oceanic in character; in other words, the is mafic igneous rock (e.g., basalt or gabbro, affluent in ferromagnesian minerals). Spreading prices vary considerably, from 1 cm/y to 3 cm/y in the Atlantic, to in between 6 cm/y and also 10 cm/y in the Pacific. Some of the processes occurring in this setup include:

Magma native the mantle pushing up to fill the voids left by divergence of the two platesPillow lavas developing where magma is moved out right into seawater (Figure 10.19)Vertical sheeted dykes intruding into cracks resulting from the spreadingMagma cooling much more slowly in the lower component of the brand-new crust and forming gabbro bodies
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Figure 10.18 The basic processes the take location at a divergent boundary. The area within the dashed white rectangle is shown in figure 10.19.

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Figure 10.19 relenten of the processes and materials formed at a divergent boundary

Spreading is hypothesized to start within a continental area through up-warping or doming related to an basic mantle plume or collection of mantle plumes. The buoyancy of the mantle plume product creates a dome in ~ the crust, bring about it to fracture in a radial pattern, with 3 arms spaced at about 120° (Figure 10.20). When a series of mantle feather exists in ~ a huge continent, the result rifts might align and also lead to the development of a rift valley (such as the present-day great Rift sink in eastern Africa). It is said that this form of sink eventually develops into a linear sea (such as the contemporary Red Sea), and also finally into an s (such together the Atlantic). The is likely that as countless as 20 mantle plumes, numerous of which quiet exist, were responsible for the initiation that the rifting of Pangea along what is now the mid-Atlantic ridge (see figure 10.14).

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Figure 10.20 relenten of the process of dome and also three-part rift formation (left) and of continental rifting in between the African and South American components of Pangea at around 200 Ma (right)

Convergent Boundaries

Convergent boundaries, where two plates are relocating toward each other, room of three types, depending on the type of crust present on either side of the border — oceanic or continental. The species are ocean-ocean, ocean-continent, and also continent-continent.

At one ocean-ocean convergent boundary, one of the bowl (oceanic crust and also lithospheric mantle) is pushed, or subducted, under the other. Frequently it is the older and colder plate that is denser and subducts beneath the younger and also hotter plate. There is generally an ocean trench follow me the boundary. The subducted lithosphere descends into the warm mantle in ~ a reasonably shallow edge close come the subduction zone, but at steeper angle farther down (up to about 45°). As questioned in the context of subduction-related volcanism in thing 4, the far-ranging volume the water in ~ the subducting product is released as the subducting tardy is heated. This water is mostly derived from alteration of pyroxene and also olivine come serpentine near the dispersing ridge shortly after the rock’s formation. It mixes v the overlying mantle, and also the addition of water to the hot mantle lowers the crust’s melting point and leads to the development of magma (flux melting). The magma, which is lighter than the bordering mantle material, rises v the mantle and the overlying oceanic crust to the s floor where it create a chain of volcanic islands known as an island arc. A maturation island arc creates into a chain of relatively huge islands (such as Japan or Indonesia) as much more and more volcanic material is extruded and also sedimentary rocks accumulate about the islands.

As described over in the context of Benioff zones (Figure 10.10), earthquakes take ar close come the boundary between the subducting crust and also the overriding crust. The biggest earthquakes take place near the surface ar where the subducting plate is still cold and also strong.

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Figure 10.21 Configuration and also processes of an ocean-ocean convergent boundary

Examples of ocean-ocean convergent zones space subduction that the Pacific Plate southern of Alaska (Aleutian Islands) and also west the the Philippines, subduction the the India Plate southern of Indonesia, and also subduction the the Atlantic Plate beneath the Caribbean plate (Figure 10.21).

At one ocean-continent convergent boundary, the oceanic bowl is driven under the continent plate in the same manner together at one ocean-ocean boundary. Sediment that has collected on the continental slope is thrust up into an accretionary wedge, and also compression leader to thrusting in ~ the continent plate (Figure 10.22). The mafic magma produced nearby to the subduction ar rises to the basic of the continental crust and leads to partial melt of the crustal rock. The resulting magma ascends v the crust, developing a mountain chain with countless volcanoes.

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Figure 10.22 Configuration and processes of an ocean-continent convergent border

Examples that ocean-continent convergent limits are subduction of the Nazca bowl under south America (which has created the Andes Range) and subduction the the Juan de Fuca key under north America (creating the hills Garibaldi, Baker, St. Helens, Rainier, Hood, and Shasta, collectively known together the Cascade Range).

A continent-continent collision occurs when a continent or big island that has been moved along with subducting oceanic tardy collides with one more continent (Figure 10.23). The colliding continental material will no be subducted since it is as well light (i.e., because it is composed greatly of light continental rocks ), yet the source of the oceanic bowl will at some point break off and also sink right into the mantle. There is incredible deformation that the pre-existing continental rocks, and also creation of hills from that rock, from any kind of sediments the had gathered along the shores (i.e., in ~ geosynclines) that both continental masses, and also commonly likewise from some ocean crust and upper mantle material.

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Figure 10.23 Configuration and processes of a continent-continent convergent border

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

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Transform borders exist where one plate slides past another without manufacturing or damage of crustal material. As explained above, many transform faults affix segments of mid-ocean ridges and are thus ocean-ocean plate boundaries (Figure 10.15). Some transform faults connect continental components of plates. An instance is the san Andreas Fault, which connects the southern end of the Juan de Fuca Ridge through the northern end of the eastern Pacific rise (ridge) in the Gulf that California (Figures 10.24 one 10.25). The component of California west the the mountain Andreas fault and all of Baja California are on the Pacific Plate. Transform faults do not just affix divergent boundaries. For example, the Queen Charlotte error connects the north finish of the Juan de Fuca Ridge, beginning at the north end of Vancouver Island, come the Aleutian subduction zone.

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Figure 10.24 The mountain Andreas fault extends from the north end of the east Pacific climb in the Gulf of California come the southern finish of the Juan de Fuca Ridge. All of the red currently on this map space transform faults.

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Figure 10.25 The san Andreas fault at Parkfield in central California. The person with the orange shirt is stand on the Pacific Plate and the human being at the far side that the bridge is top top the phibìc American Plate. The bridge is design to slide on its foundation.