Geoscience Reference
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sisting of iron and magnesium-rich rocks like basalt. Continental crust is formed of granite and other igneous
and metamorphic rocks with a higher silica content and is lighter than its oceanic counterpart. This difference
is important in understanding the dynamic process of plate tectonics.
The outer layers of the Earth are divided into twenty tectonic plates, which, like colossal barges, are in constant mo-
tion relative to one another. Seven of the major plates are identified here.
This traditional view of the Earth's inner structure was based largely on composition. More recently, geo-
physicists have devised a descriptive system based on the strength and fluidity of the layers. In this system, the
outer layers of the Earth have been divided into lithosphere and asthenosphere. The lithosphere is defined as
the cooler, more rigid outer layer, which includes the crust, both oceanic and continental, and the upper mantle.
The asthenosphere is the weaker, more plastic layer underlying it.
According to plate tectonics, the lithosphere is divided into a jigsaw puzzle of twenty separate pieces or
plates that are propelled, like colossal barges, by convection currents within the plastic asthenosphere. Result-
ing from the temperature differences in the molten rock, these currents have been compared by science writer
and geologist Simon Winchester, in Krakatoa: The Day the World Exploded: August 27, 1883, to “a vat of ve-
getable soup simmering on the stovetop.” Convection currents rise up from hot regions, as deep as 1,500 kilo-
meters (930 miles), at the slower-than-molasses rate of millimeters per year. They pass through the pliable as-
thenosphere, where they encounter the brittle crust and then turn back down again. In places, however, this
