<span>the amount of silica in the magma</span>
When a liquid is heated, the molecules gain kinetic energy. As the liquid begins to boil they have enough energy to break the intermolecular attractions between their neighbors. This happens first at the surface before the volume below has enough energy to boil. Thus you see high energy water molecules escaping from the surface as mist.

where:
F - force
m - mass
a - acceleration
We transform this formula to get a:

I think the correct answer is C. Volcanoes are formed in subduction zones or in rift valleys. They are formed when hot magma within the upper mantle of the Earth would work way up the surface. As it reaches the surface, an eruption would happen forming lava flows and deposits of ash. As it continues to erupt, it would grow bigger and bigger. These would form commonly in divergence or convergence tectonic plates. On Earth, there are about 1500 volcanoes that are active. About 80 of these can be found under the oceans. The active ones are mostly found in washington, oregon, alaska, california and hawaii.