Basalt, Granite, Obsidian, and Pumice?

Basalt is magma that rises from the mantle into the deep high pressure water of the ocean and cools quickly into rock-the sea bed is mostly basalt. Granite started out as basalt, but, was chemically altered in the distant past into the rock that makes up most of the continental rock-it is lighter than basalt so it kind of floats on top of the basalt. Pumice is magma that is cooled in the low pressure air of the atmosphere and is very light compared to basalt and granite.