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 think many of Earth’s hotspots (in Iceland, for example) are powered by mantle plumes. These plumes rise in columns from the very bottom of the mantle, some 1,800 miles down, and carry heat from near the core to the crust.