The earth can be divided into four concentric zones (Figure ). The innermost is called the inner core and is thought to be a solid, spherical mass of iron. Its radius is about 1,216 kilometers (730 miles). The next zone, called the outer core, is believed to be a layer of molten liquid rich in nickel and iron that is about 2,270 kilometers (1,362 miles) thick. The outer core is overlain by the mantle, which is a solid yet puttylike rock that can actually flow. The mantle is about 2,900 kilometers (1,740 miles) thick. The crust, the outermost zone, is the hardened exterior of the earth and varies in thickness from about 5 to 50 kilometers (3‐‐30 miles).
The Earth's Structure
Continental crust is thicker than oceanic crust. The solid lithosphere is composed of the crust and the upper part of the mantle. The softer, more flexible part of the mantle underneath the lithosphere is the asthenosphere (Figure 2).
The Crust, Lithosphere, and Asthenosphere
As the earth cools, the intense heat being produced in the core creates convection currents in the mantle that bring hot mantle material up toward the crust, and colder mantle and crustal rocks sink downward. This heat engine drives plate tectonics, or the movements of large segments of the earth's crust (plates) that are separated along deep cracks called faults. The plates move over the asthenosphere, which is softer and less resistant. The crust breaks into these segments because of the upward movement of molten material below. The powerful internal tectonic forces squeeze and fold solid rock, creating massive changes in the earth's crust, such as rugged mountains and deep submarine canyons.
The fault boundaries between plates are either convergent, divergent, or transform. A divergent boundary is one marked by plates that move away from each other (Figure 3).
A Divergent Boundary
A convergent boundary is one at which plates come together (Figure 4).
A Convergent Boundary
Plates slide past each other in opposite directions along a transform boundary (Figure 5).
A Transform Boundary
New ocean crust is formed along the deep midoceanic ridges (divergent boundaries) by the outpouring of mantle lavas on the ocean floor. These ridges are also called spreading centers. The new crust pushes to the side the older oceanic crust, which eventually is subducted, or forced under another plate at a convergent boundary. The subducted crust moves down a dipping subduction zone toward the mantle.
The jostling or rubbing of plates results in high heat flows, volcanic activity, deformation, mountain‐building, and earthquakes— creating ideal places to melt rock into magma. Rocks in subduction zones are subjected to friction and higher geothermal gradients that contribute heat to the melting process.