Tectonic Plates

Figure 2. Cross section of the Earth showing its main geologic layers (Kious, Tilling, Lindeberg [ed], 2001)

What is a tectonic plate? To answer this question I must begin by addressing the geologic makeup of the Earth. As shown in Figure 2, the Earth is composed of three main layers: the crust, the mantle and the core (Kious, Tilling, Lindeberg [ed], 2001). The crust is the top layer and it is rigid, brittle and much thinner than any other layer. The next main layer is the mantle and it is denser and hotter than the crust because it contains more iron, magnesium and calcium. The uppermost solid layer of the mantle and the crust form the lithosphere. Below the lithosphere, as part of the mantle, is a layer known as the asthenosphere, which consists of partially molten rock. The core lies at the center of the Earth and it is considered to be nearly twice as dense as the mantle. The core is extremely hot as temperature increases with depth. It is subdivided into inner and outer core. The inner core is metallic and solid, consisting primarily of an iron-nickel alloy. The outer core is liquid (molten rock) and consists of materials less dense than the inner core. (Kious, Tilling, Lindeberg [ed], 2001).

The extreme heat of the Earth’s interior acts as an engine driving the geologic processes we see on the surface. This internal structure of the planet has a direct influence on plate tectonics (Kious, Tilling, Lindeberg [ed], 2001).

Figure 3. Assumed convection cells in the mantle driving plate tectonic motion (Kious, Tilling, Lindeberg [ed], 2001)

According to Kious, Tilling, Lindeberg [ed] (2001), a tectonic plate is a “massive, irregularly shaped slab of solid rock, generally composed of both continental and oceanic lithosphere.” The asthenosphere, which is made up of partially molten rock, provides a viscous, semi-liquid surface on which tectonic plates can ride. Tectonic plates can “float” on the Asthenosphere because of their rock composition. Continental crust is primarily made up of granitic rocks which are relatively light weight. Oceanic crust on the other hand, is made up primarily of basaltic rocks which are denser and heavier (Kious, Tilling, Lindeberg [ed], 2001).

As indicated by Stanley, 2005, Tectonic plates move because of four principal reasons: convection, ascent of magma, subduction and suction caused by the break up of a subducting slab. Figure 3 shows what scientists generally agree are convection cells in the mantle.

All of these processes are driven by heat: hot magma rises and when it cools, it sinks back down forming enormous underground convection currents. As the hot magma rises at divergent plate boundaries, new crust is formed pushing the plates apart.

Figure 4. Major tectonic plates of the world (Kious, Tilling, Lindeberg [ed], 2001)

The opposite ends of the plates are then subjected to great forces, depending on the type of plates they are moving against. In a process called subduction, high-density oceanic lithosphere descends into the asthenosphere when it is pushed against the lower-density continental lithosphere slab. This type of boundary is known as a subduction zone (Stanley, 2005). We will later see that the Cocos plate and the Caribbean plate have this type of boundary. Two continental lithosphere slabs will slide past another in horizontal movement, based on the angle at which they are being pushed. This type of plate boundary is known as a transform boundary. The San Andreas Fault in California is an example of this type of boundary. When two continental lithosphere slabs collide, none of them is subducted because of their low density composition. The resulting compression gives rise to mountain chains. An example of these is the Himalayas. Figure 4 shows the major tectonic plates of the world.