How It Works: The Inner Earth
Plate tectonics—the theory that explains the sinking, spreading, and slip-sliding of big chunks of Earth’s surface—is a bedrock of geology. But it can’t explain what happens to plates once they sink, or account for the forces that drive many of the planet’s volcanic hotspots. Today, advances in seismology, geochemical analysis, and computer modeling have enabled researchers to collect a wealth of new geological data about our planet and form a complementary theory of what’s going on beneath its surface. Andrzej Wojcicki/Science Photo Library/Corbis THE PROCESS SLABS When one tectonic plate is forced beneath another, forming a subduction zone—the cause of many earthquakes—its leading edge sinks deeper into the mantle. The slab descends slowly, mixing molten rock as it goes, and as it nears the core, it partially melts. PLUMES Most volcanoes begin in the relatively cool upper mantle and shoot up along the rims of tectonic plates. But geologists now...