Idaho Bureau of Mines and Geology Bulletin Craters of the Moon National Monument, Idaho

Many kinds of lava have been poured out on the surface of the earth, but they all fall into three main classes or types. One type, known as rhyolite, is poor in iron but rich in silica and consequently light-colored. It is the common lava of Yellowstone Park, and this rock and its decomposition products give the magnificent colors in the Grand Canyon of Yellowstone River. An other type of lava, known as andesite, is of intermediate composition, containing moderate amounts of both iron and silica, and is usually gray. This rock forms a considerable part of the volcano of Lassen Peak, California, and the lava extruded from this volcano in 1915 was an andesite. The third type, which is rich in iron and poor in silica, is called basalt. This is a dark colored rock. It covers the Snake River Plains, forms the rim rock along Snake River, and is the rock commonly known as lava in the Northwest. It is the only type of lava known to occur in the Craters of the Moon.

PLATE VII—The ropy and wrinkled surfaces of the pahoehoe are due to the hardening of a thin crust or scum on the flow while the crust is being pushed forward by the flowing lava below. Photograph by H. T. Stearns.

Because of the numerous queries of visitors to the Craters of the Moon as to the composition of the lava, an analysis, made by the United States Geological Survey, of a specimen of basalt collected by I. C. Russell near Big Cinder Butte is given below:

ANALYSIS OF RECENT LAVA FROM THE BIG CINDER BUTTE FLOW, CRATERS OF THE MOON NATIONAL MONUMENT, IDAHO.
[Analyst, W. F. Hillebrand.]