What exactly causes huge plates to move and collide with such a colossal force? How do they not create extremely wide trenches in places where they move apart? Why isn’t the place where they move apart just molten magma? What is the force propelling magma and ash upwards during an eruption? I searched on the Internet, but I want a deeper explanation, which I couldn’t find. TIA!
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So, other answers have done a decent job of covering how plates move. The one small correction here is that the mantle is actually solid, but just a bit “soft”. Melting does occur beneath areas of volcanic activity.
But let’s turn to volcanoes. Magma underground usually has some gas in it, which stays dissolved because the magma is under pressure. This pressure mostly comes from the weight of the rock lying on top of it. As magma rises, the pressure on it is reduced. The gas can no longer stay dissolved and forms bubbles. As pressure decreases even more, the bubbles expand and force lava and/or ash out of the volcano’s vent. The process is actually rather like opening a can of soda after shaking it.
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