If large masses of this igneous rock cool slowly, it can form a polygonal joint pattern, such as at the Giant's Causeway in Northern Ireland. When this volcanic rock erupts underwater, it can form into pillow shapes, and this type of igneous rock forms most of the material of tectonic plates. This hard, dense igneous rock is estimated to make up over 90% of all volcanic rock on Earth, and its low viscosity allows for extensive lava flows over a wide area. For 10 points, name this extrusive igneous rock formed from the rapid cooling of magnesium and iron-rich lava.
A) Granite
B) Basalt
C) Obsidian
D) Pumice