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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Because the tectonic plates are a thin layer (relative to the size of the planet overall) of mostly solid rock floating on a massive layer of viscous magma (the mantle).
Think of it like thin ice on a puddle. If you step on the puddle, water wells up through the cracks in the ice.
Now imagine that water is both thick, like jam, and boiling hot. Because it’s being heated, it rolls and boils (like water boiling on a stove). Those rolling currents are “convection.”
Those convection currents are what is pushing the plates around on the surface.
Where the plates slide apart, magma wells up to fill the hole (this is what happens at the Mid-Atlantic Ridge, where two plates are moving apart).
Bonus: In some places, there’s an upwelling of extra hot magma and it partially melts the crust above it (kind of like a hot spring under a lake would melt the ice in one spot in the wintertime). Those are “hot spots” and they account for areas of volcanic activity that are far from plate boundaries, like Yellowstone, or Hawaii.
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