Their composition is rhyolite and pumice, with subordinate obsidian. Obsidian is volcanic glass. It is a volcanic rock that forms when the lava is cooled very rapidly. Mineral crystals do not have time to form in the molten lava, and the noncrystalline mass becomes a glass when it is subject to sudden cooling. Obsidian is often found associated with pumice, often in layers as at Obsidian Butte.
Pumice is the hardened residue of volcanic froth, or very liquid rock highly charged with gases.
It is commonly thought that pumice rock floats on water. Pumice boulders, because they are light and easily handled, are used extensively for landscaping. The marked lack of erosion of the cone attests to its youthful age. Young volcanoes at Cerro Prieto are apparently part of the same suite of volcanic activity, all being associated with the East Pacific Rise. Carbon Dioxide From 1933 to 1954, carbon dioxide was produced from a small field near Niland.