Volcanoes & Causes of Plate Crunching

Plate Tectonic Theory

• Plate tectonic theory, developed in the 1960s and 1970s, seeks to explain how the Earth is composed of different layers: the crust, mantle and core. The crust is broken up into several plates, which are moving in different directions. The plates move due to convection currents, driven by heat from radioactivity deep in the Earth's mantle, as explained by University of California Museum of Paleontology.

Different Plate Boundaries

• The various tectonic plates meet at three different types of plate boundaries, determined by the direction in which the plates are moving. Convergent boundaries occur when two plates meet head on, and crash together. Divergent boundaries occur when two plates are moving apart, an example of which is the mid-Atlantic ridge. Two plates moving side by side form a transform boundary, such as the San Andreas Fault in California.

Volcanoes

• Volcanoes, unlike earthquakes, only feature two of the three types of plate boundaries: divergent and convergent. At divergent boundaries, plates moving in opposite directions cause the crust to break apart. The gap left behind is filled by magma, forming new crust, with volcanoes occurring when the magma reaches the surface. In contrast, at convergent boundaries, huge forces are exerted as two plates meet, with one plate forced below the other, forming a ridge along which volcanoes and mountains develop. The forces exerted as the plates meet cause cracks to occur in the crust, allowing magma to escape, forming a volcano. When pressure within volcanoes reaches a certain level, they erupt, spewing molten magma and volcanic ash into the surrounding area.

Types of Volcanoes

• The U.S. Geological Survey identifies four types of volcanoes. Composite volcanoes are typically large, steep-sided, symmetrical cones built on alternating layers of lava flows. Shield volcanoes are composed almost entirely of lava flows, forming a broad, gently sloping cone of a flat, domical shape. The third type are lava domes, formed by relatively small, bulbous masses of lava, too viscous to flow any great distance. Cinder cone volcanoes are simple volcanoes that form when cinders spew out of the vent and settle around it. They are numerous in western North America in particular.