What type of boundary causes the plates to move apart creating new ocean floor or rift valleys?
Chapter ten Plate Tectonics
10.4 Plates, Plate Motions, and Plate-Boundary Processes
Continental migrate and sea-floor spreading became widely accepted around 1965 as more and more geologists started thinking in these terms. By the end of 1967, Earth's surface had been mapped into a series of plates (Figure 10.xvi). The major plates are Eurasia, Pacific, India, Australia, Due north America, South America, Africa, and Antarctic. There are as well numerous modest plates (east.grand., Juan de Fuca, Nazca, Scotia, Philippine, Caribbean), and many very pocket-size plates or sub-plates. For example the Juan de Fuca Plate is actually three separate plates (Gorda, Juan de Fuca, and Explorer) that all movement in the same general management but at slightly different rates.
Rates of motions of the major plates range from less than 1 cm/y to over 10 cm/y. The Pacific Plate is the fastest at over 10 cm/y in some areas, followed by the Australian and Nazca Plates. The North American Plate is 1 of the slowest, averaging around 1 cm/y in the south up to almost 4 cm/y in the n.
Plates motion every bit rigid bodies, so it may seem surprising that the Northward American Plate can exist moving at dissimilar rates in dissimilar places. The explanation is that plates motion in a rotational manner. The North American Plate, for example, rotates counter-clockwise; the Eurasian Plate rotates clockwise.
Boundaries between the plates are of iii types: divergent (i.e., moving autonomously), convergent (i.e., moving together), and transform (moving adjacent). Before we talk most processes at plate boundaries, information technology's important to point out that there are never gaps between plates. The plates are made upwardly of chaff and the lithospheric part of the drape (Figure 10.17), and fifty-fifty though they are moving all the fourth dimension, and in unlike directions, there is never a significant amount of space between them. Plates are idea to move forth the lithosphere-asthenosphere boundary, equally the asthenosphere is the zone of fractional melting. It is assumed that the relative lack of strength of the partial melting zone facilitates the sliding of the lithospheric plates.
At spreading centres, the lithospheric mantle may be very thin because the upward convective motion of hot mantle material generates temperatures that are too high for the existence of a significant thickness of rigid lithosphere (Figure 10.12). The fact that the plates include both crustal fabric and lithospheric drape material makes information technology possible for a single plate to be made upwards of both oceanic and continental crust. For instance, the N American Plate includes most of North America, plus half of the northern Atlantic Ocean. Similarly the South American Plate extends across the western office of the southern Atlantic Sea, while the European and African plates each include office of the eastern Atlantic Body of water. The Pacific Plate is nigh entirely oceanic, but it does include the function of California west of the San Andreas Error.
Divergent Boundaries
Divergent boundaries are spreading boundaries, where new oceanic crust is created from magma derived from partial melting of the pall caused by decompression as hot drape rock from depth is moved toward the surface (Figure ten.18). The triangular zone of partial melting about the ridge crest is approximately 60 km thick and the proportion of magma is about 10% of the rock book, thus producing crust that is nigh 6 km thick. Nigh divergent boundaries are located at the oceanic ridges (although some are on country), and the crustal material created at a spreading boundary is always oceanic in grapheme; in other words, it is mafic igneous rock (e.m., basalt or gabbro, rich in ferromagnesian minerals). Spreading rates vary considerably, from 1 cm/y to 3 cm/y in the Atlantic, to betwixt vi cm/y and x cm/y in the Pacific. Some of the processes taking identify in this setting include:
- Magma from the drape pushing up to fill the voids left by divergence of the two plates
- Pillow lavas forming where magma is pushed out into seawater (Effigy ten.19)
- Vertical sheeted dykes intruding into cracks resulting from the spreading
- Magma cooling more slowly in the lower part of the new crust and forming gabbro bodies
Spreading is hypothesized to start within a continental area with up-warping or doming related to an underlying pall plume or series of curtain plumes. The buoyancy of the mantle feather material creates a dome within the crust, causing it to fracture in a radial design, with iii arms spaced at approximately 120° (Figure 10.20). When a serial of mantle plumes exists beneath a large continent, the resulting rifts may align and pb to the germination of a rift valley (such as the present-day Great Rift Valley in eastern Africa). It is suggested that this type of valley eventually develops into a linear body of water (such as the present-24-hour interval Ruddy Body of water), and finally into an ocean (such as the Atlantic). It is likely that as many as 20 mantle plumes, many of which however be, were responsible for the initiation of the rifting of Pangea along what is now the mid-Atlantic ridge (see Figure x.xiv).
Convergent Boundaries
Convergent boundaries, where two plates are moving toward each other, are of three types, depending on the type of crust present on either side of the purlieus — oceanic or continental. The types are ocean-bounding main, ocean-continent, and continent-continent.
At an ocean-ocean convergent boundary, one of the plates (oceanic crust and lithospheric pall) is pushed, or subducted, nether the other. Oftentimes it is the older and colder plate that is denser and subducts beneath the younger and hotter plate. In that location is commonly an ocean trench along the boundary. The subducted lithosphere descends into the hot pall at a relatively shallow angle close to the subduction zone, only at steeper angles farther downwardly (upwards to almost 45°). Every bit discussed in the context of subduction-related volcanism in Chapter 4, the meaning volume of water within the subducting material is released as the subducting crust is heated. This water is generally derived from alteration of pyroxene and olivine to serpentine nigh the spreading ridge shortly afterwards the rock's formation. Information technology mixes with the overlying mantle, and the add-on of water to the hot mantle lowers the crust's melting betoken and leads to the formation of magma (flux melting). The magma, which is lighter than the surrounding mantle material, rises through the mantle and the overlying oceanic crust to the ocean floor where it creates a chain of volcanic islands known as an island arc. A mature island arc develops into a chain of relatively large islands (such equally Nippon or Indonesia) as more and more than volcanic cloth is extruded and sedimentary rocks accumulate around the islands.
Equally described higher up in the context of Benioff zones (Figure x.10), earthquakes take place close to the boundary between the subducting chaff and the overriding crust. The largest earthquakes occur near the surface where the subducting plate is all the same cold and potent.
Examples of ocean-ocean convergent zones are subduction of the Pacific Plate due south of Alaska (Aleutian Islands) and west of the Philippines, subduction of the Republic of india Plate south of Indonesia, and subduction of the Atlantic Plate beneath the Caribbean Plate (Figure 10.21).
At an body of water-continent convergent boundary, the oceanic plate is pushed under the continental plate in the same manner as at an ocean-ocean boundary. Sediment that has accumulated on the continental slope is thrust up into an accretionary wedge, and pinch leads to thrusting within the continental plate (Figure 10.22). The mafic magma produced adjacent to the subduction zone rises to the base of operations of the continental crust and leads to fractional melting of the crustal rock. The resulting magma ascends through the crust, producing a mountain chain with many volcanoes.
Examples of ocean-continent convergent boundaries are subduction of the Nazca Plate under South America (which has created the Andes Range) and subduction of the Juan de Fuca Plate under Due north America (creating the mountains Garibaldi, Baker, St. Helens, Rainier, Hood, and Shasta, collectively known as the Pour Range).
A continent-continent collision occurs when a continent or large isle that has been moved forth with subducting oceanic crust collides with another continent (Figure 10.23). The colliding continental textile will non be subducted because information technology is also lite (i.e., because information technology is composed largely of light continental rocks [SIAL]), but the root of the oceanic plate volition eventually break off and sink into the mantle. At that place is tremendous deformation of the pre-existing continental rocks, and creation of mountains from that stone, from any sediments that had accumulated along the shores (i.e., inside geosynclines) of both continental masses, and usually also from some bounding main crust and upper mantle material.
Examples of continent-continent convergent boundaries are the collision of the India Plate with the Eurasian Plate, creating the Himalaya Mountains, and the collision of the African Plate with the Eurasian Plate, creating the series of ranges extending from the Alps in Europe to the Zagros Mountains in Iran. The Rocky Mountains in B.C. and Alberta are likewise a event of continent-continent collisions.
Transform boundaries exist where one plate slides past another without production or destruction of crustal textile. As explained above, most transform faults connect segments of mid-ocean ridges and are thus ocean-ocean plate boundaries (Effigy 10.fifteen). Some transform faults connect continental parts of plates. An example is the San Andreas Fault, which connects the southern end of the Juan de Fuca Ridge with the northern end of the E Pacific Rise (ridge) in the Gulf of California (Figures ten.24 an 10.25). The role of California due west of the San Andreas Fault and all of Baja California are on the Pacific Plate. Transform faults practise not just connect divergent boundaries. For case, the Queen Charlotte Error connects the n end of the Juan de Fuca Ridge, starting at the north end of Vancouver Island, to the Aleutian subduction zone.
Exercise ten.four A Different Type of Transform Fault
This map shows the Juan de Fuca (JDF) and Explorer Plates off the coast of Vancouver Island. We know that the JDF Plate is moving toward the North American Plate at around 4 cm/y to five cm/y. We call back that the Explorer Plate is also moving east, but we don't know the rate, and there is prove that it is slower than the JDF Plate.
The purlieus between the two plates is the Nootka Fault, which is the location of frequent small-to-medium earthquakes (up to magnitude ~5), equally depicted by the red stars. Explain why the Nootka Error is a transform fault, and show the relative sense of motion along the fault with two pocket-sized arrows.
As originally described past Wegener in 1915, the present continents were in one case all part of a supercontinent, which he termed Pangea (all land). More recent studies of continental matchups and the magnetic ages of ocean-floor rocks have enabled us to reconstruct the history of the break-upward of Pangea.
Pangea began to rift autonomously along a line betwixt Africa and Asia and between Northward America and South America at around 200 Ma. During the same period, the Atlantic Ocean began to open up between northern Africa and North America, and India bankrupt away from Antarctica. Between 200 and 150 Ma, rifting started betwixt South America and Africa and between North America and Europe, and Bharat moved north toward Asia. By eighty Ma, Africa had separated from South America, near of Europe had separated from North America, and India had separated from Antarctica. By fifty Ma, Australia had separated from Antarctic, and shortly after that, India collided with Asia. To see the timing of these processes for yourself get to: http://barabus.tru.ca/geol1031/plates.html.
Inside the past few 1000000 years, rifting has taken place in the Gulf of Aden and the Red Sea, and too within the Gulf of California. Incipient rifting has begun along the Bang-up Rift Valley of eastern Africa, extending from Federal democratic republic of ethiopia and Djibouti on the Gulf of Aden (Crimson Body of water) all the manner southward to Republic of malaŵi.
Over the adjacent 50 million years, information technology is likely that there will be full development of the east African rift and creation of new ocean flooring. Eventually Africa will divide apart. There volition as well be continued northerly motility of Australia and Indonesia. The western part of California (including Los Angeles and office of San Francisco) will split away from the rest of N America, and somewhen sail right by the west coast of Vancouver Island, en route to Alaska. Because the oceanic crust formed by spreading on the mid-Atlantic ridge is not currently being subducted (except in the Caribbean area), the Atlantic Ocean is slowly getting bigger, and the Pacific Ocean is getting smaller. If this continues without irresolute for another couple hundred million years, nosotros volition be back to where nosotros started, with ane supercontinent.
Pangea, which existed from near 350 to 200 Ma, was not the start supercontinent. It was preceded by Pannotia (600 to 540 Ma), by Rodinia (1,100 to 750 Ma), and by others earlier that.
In 1966, Tuzo Wilson proposed that in that location has been a continuous series of cycles of continental rifting and collision; that is, break-up of supercontinents, drifting, collision, and formation of other supercontinents. Now, North and South America, Europe, and Africa are moving with their respective portions of the Atlantic Sea. The eastern margins of North and S America and the western margins of Europe and Africa are called passive margins because there is no subduction taking place along them.
This state of affairs may not continue for also much longer, however. As the Atlantic Sea flooring gets weighed down effectually its margins past bully thickness of continental sediments (i.eastward., geosynclines), it will be pushed farther and farther into the mantle, and somewhen the oceanic lithosphere may interruption away from the continental lithosphere (Figure ten.26). A subduction zone will develop, and the oceanic plate will begin to descend nether the continent. One time this happens, the continents will no longer continue to movement autonomously because the spreading at the mid-Atlantic ridge will exist taken up by subduction. If spreading along the mid-Atlantic ridge continues to be slower than spreading within the Pacific Ocean, the Atlantic Ocean will start to shut upwardly, and eventually (in a 100 million years or more) Due north and South America volition collide with Europe and Africa.
There is strong testify around the margins of the Atlantic Ocean that this procedure has taken place before. The roots of aboriginal mountain belts, which are present along the eastern margin of N America, the western margin of Europe, and the northwestern margin of Africa, bear witness that these land masses once collided with each other to form a mountain concatenation, possibly as big equally the Himalayas. The apparent line of collision runs between Norway and Sweden, between Scotland and England, through Ireland, through Newfoundland, and the Maritimes, through the northeastern and eastern states, and across the northern finish of Florida. When rifting of Pangea started at approximately 200 Ma, the fissuring was forth a different line from the line of the earlier standoff. This is why some of the mountain bondage formed during the earlier collision can be traced from Europe to North America and from Europe to Africa.
That the Atlantic Body of water rift may have occurred in approximately the same place during two separate events several hundred million years autonomously is probably no coincidence. The series of hot spots that has been identified in the Atlantic Bounding main may also have existed for several hundred meg years, and thus may have contributed to rifting in roughly the same place on at least two dissever occasions (Figure 10.27).
Exercise 10.five Getting to K now the Plates and Their Boundaries
This map shows the boundaries between the major plates. Without referring to the plate map in Figure 10.16, or any other resources, write in the names of as many of the plates as you can. Start with the major plates, and so work on the smaller ones. Don't worry if you can't proper name them all.
Once you've named most of the plates, draw arrows to evidence the general plate motions. Finally, using a highlighter or coloured pencil, label as many of the boundaries as you tin as divergent, convergent, or transform. [map past SE]
Source: https://opentextbc.ca/geology/chapter/10-4-plates-plate-motions-and-plate-boundary-processes/
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