Plates also slide past each other, creating the faults that produce earthquakes. Lithosphere Cool, rigid, outermost layer of Earth that is divided into enormous pieces called tectonic plates consists of the crust and the rigid uppermost part of the mantle. If the converging plates have land masses on them, the boundaries crumple, forming mountains. 3) Third type of collision happens between two plated that are made of oceanic lithosphere. Plate Tectonic Theory Theory that the lithosphere is divided into tectonic plates that slowly move on top of the asthenosphere. Where they collide, one plate is submerged beneath the other, and material from the bottom one returns to Earth's mantle. Where plates pull apart, new crust is formed.

Along their margins, the independently moving plates interact in three main ways. Although the plates move only a few inches per year, over the hundreds of millions of years of geological time, the continents are carried thousands of miles. The plates' constant movement is powered by huge convection currents of molten rock in Earth's mantle, thought by many geologists to be heated by the decay of radioactive elements deep within Earth. According to the theory of plate tectonics, which was proposed in the 1960s, the continents (and ocean floors) ride atop about a dozen semirigid plates-huge slabs of Earth's lithosphere-that are much larger than the continents themselves.

A Closer Look Although German physicist, meteorologist, and explorer Alfred Wegener proposed the theory of continental drift in 1912, suggesting that the continents were once joined as one large landmass, the explanation for the movement of such large landmasses into their current positions was not developed for several more decades.

Plate tectonic vocab