4.3: The Lithosphere

The Lithosphere

The lithosphere Links to an external site., which is composed of the crust Links to an external site. and upper mantle Links to an external site., is about 100 km (~62 mi) thick and behaves as a brittle, rigid solid (Figure 4.2). This brittleness causes breaks (earthquakes) when stresses are applied. The crust is very thin relative to the radius of the planet; however, there are two distinct types: 1) oceanic crust Links to an external site., which is composed mainly of mafic igneous rocks like basalt Links to an external site. and gabbro Links to an external site. and 2) continental crust Links to an external site., which is composed mainly of the felsic igneous rock, granite Links to an external site.. The two crustal types are very different and their density differences are significant to the mechanics of plate tectonics. Granite is much less dense than basalt, which results in a buoyant continental crust that sits higher on the underlying mantle. The denser and less buoyant oceanic crust sinks further into the underlying mantle, forming basins, many of which are filled with the Earth’s oceans. Table 4.2 highlights crustal differences in more detail.