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The lithosphere

The Earth's outermost rigid, rocky layer is called the lithosphere. It is broken, like a slightly cracked eggshell, into about a dozen separate rigid blocks, or plates. There are two types of plates, oceanic and continental. An example of an oceanic plate is the Pacific Plate, which extends from the East Pacific Rise to the deep-ocean trenches bordering the western part of the Pacific basin. A continental plate is exemplified by the North American Plate, which includes North America as well as the oceanic crust between it and a portion of the Mid-Atlantic Ridge, an enormous submarine mountain chain that extends down the axis of the Atlantic basin, passing midway between Africa and North and South America.

The upper layer of the lithosphere is termed the crust. It is composed of low-density, easily melted rocks; the continental crust is predominantly granitic, while the oceanic crust is basaltic. Analyses of seismic waves, generated by earthquakes within the Earth's interior, show that crustal compositions extend beneath the continents to depths of about 50 kilometres, but only 5 or 10 kilometres beneath the ocean floors. The denser lithospheric plates (60 kilometres thick beneath the oceans and ranging from about 100–200 kilometres beneath the continents) ride on a weak, perhaps partially molten, layer of the upper mantle called the asthenosphere.

Slow convection currents deep within the mantle generated by radioactive heating of the interior are believed to drive the lateral movements of the plates (and the continents that rest on top of them) at a rate of several centimetres per year. The plates interact along their marginal zones, and these boundaries are classified into three general types on the basis of the relative motions of the adjacent plates: divergent, convergent, and transform (or strike slip).

In areas of divergence, two plates move in opposite directions. Buoyant upwelling motions force the plates apart at rift zones (such as along the middle of the Atlantic Ocean floor) where magmas from the underlying mantle rise to form new oceanic crustal rocks.

Lithospheric plates move toward each other along convergent plate boundaries. When a continental plate and an oceanic plate come together, the leading edge of the oceanic crust is forced beneath the continental plate (i.e., is subducted). Only the thinner, denser slabs of oceanic crust will subduct, however. When the thicker, more buoyant continents come together at convergent zones, they resist subduction and tend to buckle, producing great mountain ranges. The Himalayas, along with the adjacent Plateau of Tibet, were formed during such a continent-continent collision when India was carried into the Eurasian Plate by relative motion of the Indian-Australian Plate.

At the third type of plate boundary, the transform variety, two plates slide parallel to one another in opposite directions. These areas are often associated with high seismicity, as stresses that build up in the sliding crustal slabs are released to generate earthquakes. The San Andreas Fault in California is an example of this type of boundary, which is also known as a fault or fracture zone.

Most of the Earth's active tectonic processes, including nearly all earthquakes, occur near plate margins. Volcanoes form along zones of subduction, because the oceanic crust tends to be remelted as it moves into the hot mantle and then rises to the surface as molten lava. Chains of active, often explosive, volcanoes are thus formed in such places as the western Pacific and the west coasts of the Americas. Older mountain ranges, eroded by weathering and runoff, mark zones of earlier plate-margin activity. The oldest, most geologically stable parts of the Earth are the central cores of some continents (such as Australia, southern parts of Africa, and northern North America) where little mountain-building, faulting, or other tectonic processes have occurred for hundreds of millions to billions of years. Because of the stability, erosion has flattened the topography, and geologic evidence of crater scars from the rare, often ancient impacts of asteroids and comets is preferentially preserved. In contrast, much of the oceanic crust is substantially younger (tens of millions of years old), and none dates back more than 200 million years.

It is not known when the original continental cores formed or how long ago modern plate-tectonic processes began to operate. Certainly the processes of internal convection, thermal segregation of minerals by partial melting and fractional crystallization, and basaltic volcanism were operating even more extensively and thoroughly in early epochs. But the assembling of continental landmasses had to compete with giant impacts, which tended to disaggregate them until the impact rate decreased nearly four billion years ago. It is thought that a single supercontinent that had been created by the amalgamation of many smaller continental cores and island arcs was broken up approximately 500 million years ago into at least three major continents: Gondwana (or Gondwanaland), Laurentia, and Baltica. These three landmasses were widely separated by the so-called Iapetus Ocean (a precursor to the Atlantic). By about 250 million years ago, the continued drifting of these continents resulted in their fusion into a single supercontinental landmass called Pangaea. Some 70 million years later, Pangaea began to fragment, gradually giving rise to today's continental configuration. The distribution is still asymmetric, with continents predominantly located in the Northern Hemisphere opposite the Pacific basin.

The entire conceptual framework in which geologists and geophysicists now understand the evolution of the Earth's lithosphere is termed plate tectonics (see the article plate tectonics). Analogies from plate tectonics have been applied to understanding surface features on Venus and Mars, as well as to some of the icy satellites of the outer solar system, but with only moderate success.

Читайте також:

1. Asthenosphere and lithosphere

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