Andesite pumices are usually yellow or brown. Basaltic pumices are black. Commercially useful pumices are found in the Canary Islands, Iceland, Italy, Hungary, Greece, New Zealand, and Germany. In the United States it is mined in the Rocky Mountains, on the West Coast, and in Hawaii. Andesites, which are named for the Andes Mountains, are a large family of rocks found in nearly all volcanic areas.
Basaltic eruptions can produce two different types of flow.
Aa lava moves slowly and is covered with tough, cindery rock called clinker. A flexible, glassy skin forms on the surface, insulating the lava that lies beneath and keeping it molten for weeks or even years. Volcanoes and cracks may pour forth lava under the sea as well as on land. Many islands have been formed by successive lava flows building a volcanic island higher and higher, until it rises above sea level. The Hawaiian Islands were formed in this manner.