Plate Tectonics

Plate tectonics embraces the concepts of seafloor spreading (the creation of new oceanic crust at midoceanic ridges) and continental drift (the gradual movement of Earth's continents over geologic timescales) to provide a theory for the large-scale movements of Earth's lithosphere. This theory, together with the advent of remote imagery from satellite and aerial platforms, has led to a greater understanding of the large-scale geomorphology of Earth's crust. It has also helped us understand other aspects of physical geography, such as biogeography (which helps explain the distribution of biota across the globe), and emerging areas such as planetary geomorphology (which aids the identification of plate tectonic features on remote imagery of Mars). This entry outlines the theory of plate tectonics and its relevance to other physical geography phenomena.

Composition of the Earth

A section through Earth would reveal a rigid lithosphere that is 0 to 100 km (kilometers) thick. Although this layer behaves in a brittle-elastic manner, it can, under high loads and long time spans, deform as a plastic. The lithosphere is broken up into eight major and several minor plates, which are in motion with respect to each other and the underlying asthenosphere. The asthenosphere is a weaker mechanical layer, whose outermost portions contain some (less than 1%) partial [Page 2196]melt (material at >1000 °C). This behaves as a high-viscosity fluid. It is the interactions between the upper part of Earth's mantle (the asthenosphere) and the lithospheric plates that are significant for plate tectonics. Interaction between the plate boundaries and the resultant stresses within the plates determines to a large extent the distribution of seismic and volcanic activity, which is mainly concentrated at plate margins (Figure 1).

Figure 1 The major lithospheric plates, plate boundaries, and directions and rates (in millimeters per annum) of plate movement

Source: Adapted from Summerfield, M. A. (2001). Global geomorphology. New York: Longman Scientific & Technical.

Notes: Ca: Caribbean Plate; Co: Cocos Plate; Ph: Philippine Plate; JF: Juan de Fuca Plate.

A tectonic or lithospheric plate is generally composed of continental or oceanic lithosphere from a few hundred to thousands of kilometers across. Plate thicknesses range from less than 15 km for young oceanic lithosphere to ∼200 km for ancient continental lithosphere. The continental crust is composed predominantly of granitic rocks, which are made up of relatively lightweight minerals such as quartz and feldspar. Oceanic crust, however, is dominantly [Page 2197]composed of basaltic rock, which is denser and heavier.

Figure 2 A schematic cross section through examples of convergent and divergent plate boundaries and their associated major landforms

Source: Author.

Notes: Blue represents oceanic lithosphere; brown represents continental lithosphere. Red arrows indicate the relative movement of convection cells in the asthenosphere (upper mantle). White arrows indicate movement of the lithosphere.