Plate Tectonics

Tectonics

Differences in internal planetary structure manifests itself on the surface of the planet. Rocky planets, like Earth, Mars and Venus, have similar structure, with a core, mantle and crust.

Inside rocky planets the hot mantle convects, bringing heat to the surface of the planet, and the nature of the convection is manifested in the appear- ance of the lithosphere. The lithosphere can be a single, solid rigid layer as on Mars (which results in stagnant lid convection), or it can be divided into mobile segments, as on Earth, in which case mantle convection manifests as plate tectonics (a class of mobile-lid convection). There are also transi- tional states between stagnant-lid and mobile surface convection, referred to as sluggish-lid and episodic mantle convection, where in the latter, the litho- sphere is comprised of a single rigid surface for long periods before foundering and being overturned and replaced with newer/younger mantle material.

Plate Tectonics

Evidence of long-lived, ongoing plate motion on the Earth appears in the magnetic field of the seafloor, seafloor sediment thickness, in the occurrence of earthquakes and even in the appearance of volcano locations. This mobile plate motion is referred to as plate tectonics.

The Earth’s lithosphere, is divided up into different regions that com- prise the tectonic plates. The plates are defined by rigid, non-deforming sections of lithosphere moving at nearly uniform velocity with weak regions between them. The low strain-rate in the interiors of the plates allow them to transmit stress over large distances, which helps to drive the plates. The plate boundaries are marked by a large occurrence of earthquakes and volca- noes. The ability to show these earthquakes and volcanoes occur along the plate boundaries helped to support the development of the theory of plate tectonics.

J. Tuzo Wilson was an early supporter of seafloor spreading and conti- nental drift. His work helped unify the theory of plate tectonics (Wilson, 1963, 1965). The theory evolved to include the definition of different types of plate boundary motion.

The motion of the rigid plates on the surface of Earth are determined in part by the type of boundaries between the plates. As shown in Figure 4, there are three types of plate boundaries. Divergent boundaries occur where new lithosphere is created as the plates move away from each other, as in mid- ocean ridges. Convergent boundaries exist where lithosphere is destroyed, as one plate sinks below the other (e.g., at oceanic trenches). Transform boundaries do not produce or destroy lithosphere, rather one block of the lithosphere slides horizontally past each other (Wilson, 1965).