The Earth’s crust is broken up into a small number of relatively rigid plates that move slowly across the surface in response to forces generated beneath the crust, in themantle. Strictly speak- ing, the terms crust and mantle refer to chemical differences between the layers. It is more accurate to refer to the plates as comprising the Earth’slithosphere(rock-sphere), a rigid outer shell that rides on a hotter and plastic (not molten) layer called the asthenosphere(weak-sphere). The crust–mantle boundary
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Figure 9.6Magnetic record of seafloor spreading: (a) Magnetic anomaly pattern on the axis of the Juan de Fuca ridge, near Vancouver island.
Black indicates current magnetic field direction, white the reversed field. From Morgan (1968). (b) Interpretation of the cause of magnetic reversals, shown as a cross-section through the top of the Earth’s oceanic crust. Arrows indicate the sense of spreading. On the upper chart, the axis of the mid-ocean ridge is marked, and the relative strength of the magnetic field as a function of distance from the axis is given. Positive (gray) is the present direction of the field. From van Andel (1992).
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Figure 9.7Earthquake location and depth near the Tonga subduction region in the Pacific. A profile of the topography (exaggerated 13 times) is shown in the upper small panel, positioned so that the zero point is aligned with that on the big figure. Note that the position of the quakes with depth seems to outline quite nicely a subducting slab of crust moving diagonally under Earth’s surface. The symbols distinguish between earthquakes that occurred north of the station at Niumate, Tonga, (circles) and those occurring south of Niumate (triangles). The inset shows the deepest quake locations in more detail. From Isackset al. (1968).
is not the same as the lithosphere–asthenosphere boundary, but there is significant overlap between the mantle and the astheno- sphere. As discussed in Chapter 11, knowledge of the presence of a plastic layer beneath a rigid outer shell comes from observ- ing the slowing of earthquake waves, which must pass through this layer; the chemical differences are inferred from material erupted to the surface in certain volcanoes.
At the mid-ocean ridges, hot buoyant magma rises to the sur- face, cools, and freezes, forming new seafloor. As this seafloor moves laterally away from the ridges, it cools and eventually becomes dense enough to sink, forming a subduction zone. The sinking slab differs from the mid-ocean ridge material not only because of its temperature and hence density; it contains rock that has reacted chemically with seawater, as well as sediments delivered by rivers from the continents as well as the remains of countless shell-forming organisms. Some of the sediments are scraped off in the shallow part of the subduction zone, but
some survive to deeper levels. The sinking slab is subjected to increasingly higher temperatures and pressures as it descends, to the point where at least some of it is assimilated into the sur- rounding mantle material. How much of the slab is assimilated remains an active topic of debate: some computer models of subduction indicate that the majority of slab material stays cool enough to sink to the base of the mantle, forming a “graveyard"
of sunken slabs. Such debris might eventually absorb enough heat from the decay of its own radioactive elements, as well as from the underlying core, to rise again as the “mantle plumes”
that are responsible for the Hawaiian Islands and other island chains within plates. Other models, with different assumptions about the details of the chemistry and heat transfer, produce nearly complete assimilation of the slabs within the mantle.
The release of water from the heated slab triggers melting of overlying mantile material, leading to volcanism as described in Chapter 16.
oceanic plate
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shoreline
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oceanic plate spreading center volcano
spreading center
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and partial melting magma
chamber igneous
intrusions zone of mantle melting mantle
sea level
Figure 9.8Oblique view of Earth’s lithosphere illustrating the motion at mid-ocean ridges (spreading centers) and subduction zones. The spreading center is shown split by a transform fault.
Although subduction can occur at the boundary between two ocean plates, most subduction zones are on the margins of con- tinents, because continental crust is buoyant compared to ocean crust. The density difference is chemical in nature, ocean crust being basalt, which is a relatively dense rock compared to con- tinental granites. The origin of this difference, one of the key enablers of plate tectonics and a distinguishing feature of Earth compared with the other planets, is considered in Chapter 16.
Thus, ocean crust rides somewhat like rafts on a “sea” of oceanic crust, overriding the oceanic crust at subduction zones. Volcan- ism in the interior of a continent may then occur as heated slab material releases water to the upper mantle, melting it (Chapter 16). Other types of interactions, such as continent- continent collisions, or “obduction” in which a small piece of continental crust breaks off and is shoved into the subduction zone, with ocean crust riding over it and welding onto the edge of the continental boundary.
Figure 9.8 illustrates the nature of the basic plate bound- aries. Ridges and trenches where crust is brought up and slides back down, respectively, are not sufficient to accommodate plate motion on a spherical Earth with irregular continents. Instead, the ridges are sliced through by transformfaults, where lat- eral motion occurs. The San Andreas fault is part of a trans- form system, which connects the eastward-moving subduction of the Pacific plate on the west coast of South America with the northerly and northwesterly subduction occurring along western Canada across to Asia. A simplified map of ridges, trenches, and transform faults around the world is given in Figure 9.9.
The speeds at which plates move range from 1 to 10 centime- ters per year, corresponding to a thousand kilometers in 10 mil- lion to 100 million years. Aside from the geologic evidence that tells us, among other things, how long ago certain well-separated
rock formations were together, and magnetic reversal stripes on the seafloor that give us the rate from the calibrated history of reversals, the space age provided direct measurement capabil- ity. Astronauts placed reflectors on the surface of the Moon, which geologically appears to lack plates and to be a rigid sur- face. Bouncing laser beams off the Moon from various conti- nents on Earth allows the relative movements of the stations to be determined directly. More recently, Earth-orbiting satellites that calibrate their position by making accurate observations of deep-sky objects have been able to make similar direct determi- nations of relative movements of ground stations. Finally, large radio telescopes on Earth that also determine their position by staring at the sky, and are linked together by computers, can determine continental drift as well. The measured speeds are similar to inferred speeds at which the very plastic rock in the asthenosphere of Earth slowly turns over, removing heat to the surface.
Current plate motions and associated geology are complex (Figure 9.9). The concentration of earthquake and volcanic activ- ity along the Pacific Rim is the result of the northwestward move- ment of the Pacific plate, where it is taken up by subduction along the Aleutian islands and the east coast of Asia. Southward from Indonesia, the Pacific plate adjoins the Australian plate, which generally is moving northward. Although the plates are approx- imately rigid bodies, collision between continents on separate plates leads to compression and uplift of mountains or plateaus.
The collision of India on the Australian plate with Asia is the most dramatic example of this, pushing the Himalayan plateau up to the greatest altitudes above sea level anywhere on the planet.
On the east side of the Pacific, the plate’s northward motion is expressed by transverse sliding of the plate along the North American plate. The San Andreas fault system is where this
Arabian plate
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Antarctic plate South
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Fiji plate Philippine
sea plate
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convergent boundary;
teeth on overriding plate direction of fault motion divergent boundary,
offset by short transform fault Scotia plate
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Figure 9.9Schematic map of Earth’s system of ridges, trenches, and transform faults. Plates and the general directions of their motions are labeled. Adapted from Cloud (1988) by permission of W. W. Norton Company.
motion is accommodated on the continent, leading to the prodi- gious earthquake activity in California. Farther north, the Pacific plate undergoes subduction, the expression of which includes volcanoes such as Mount St. Helens. The material brought upward from the heating of subducting plates includes basalts, rhyolites (a volcanic form of granitic rocks), and a kind of hybrid between basaltic and granitic compositions, called andesite.
These volcanic products reflect a complex sequence of mixing of oceanic crust with scraped-off detritus of the underlying por- tions of the adjacent continent, followed by melting and eventual ascent to the surface.
Farther south, subduction of the Cocos and Nazca plates under South America is expressed in extensive mountain building and volcanic activity along the Andes. Spreading between the Cocos and Nazca plates, and between the Pacific plate and both of these smaller plates, creates a complex triple junction, the motion of which is accommodated by the subduction and transverse motions along the western end of North America.
The situation in the Atlantic is simpler and quieter. Spreading of that ocean along the mid-Atlantic ridge, dividing the African and Eurasian plates to the east and the South American and North American plates to the west, continues. The African plate is also rotating in such a way that the African continent is mov- ing northward, pushing the floor of the Mediterranean Sea into southern Europe and building the Alps. The Arabian landmass is moving into Asia, and a zone of spreading between Africa and
Asia includes the Arabian Sea and very substantial rift valleys on the east coast of Africa itself.