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. 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. 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. 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. Along their margins, the independently moving plates interact in three main ways. Where plates pull apart, new crust is formed. Where they collide, one plate is submerged beneath the other, and material from the bottom one returns to Earth's mantle. If the converging plates have land masses on them, the boundaries crumple, forming mountains. Plates also slide past each other, creating the faults that produce earthquakes. The six major plates are the Eurasian, American, African, Pacific, Indian, and Antarctic.