Some geologists think the Cenozoic flood basalts in eastern Washington and southern Idaho, and Yellowstone s volcanism, all owe their origins to one hot spot over which the North American plate has been moving. Obsidian The glass produced if lava forms very quickly from water-poor magma and no crystals have time to form. The rock is silica rich but, due to impurities, it is usually black in color. It breaks in a way similar to breakage of a Coke bottle. Any volcanic rock containing so many vesicles that the rock will float.
Pumice is usually felsic.
Resurgent Dome A protrusion created within a crater after an eruption, as new magma forces its way up from underneath. You may have seen this term in the newspapers concerning Mt. St. Helens. A light-colored volcanic rock with abundant silica. It is a felsic rock, the extrusive equivalent of granite. Scoria Volcanic rock that has many vesicles but does not qualify as pumice.