When a Rock is Not a Rock

There is often some confusion about several terms used to describe volcanic rocks. Rocks that cool from a hot liquid to form crystals are known as igneous rocks. Igneous rocks are formally named according to their chemical composition, specifically on how much silicon (Si) and oxygen (O) they contain. These two elements are usually lumped together and reported as SiO₂ when chemical analyses are listed. This system results in these fine grained igneous rocks being named as follows:

Name	SiO2
Rhyolite	70% or more
Dacite	Approximately 65%
Andesite	Approximately 60%
Basaltic-Andesite	Approximately 55%
Basalt	50% or less

Those in bold are the predominant types found at Crater Lake.

These values are approximate. The entire classification scheme is artificial (established by humans to give values to the rocks). Volcanic rock composition forms a continuum from as low as 35% to as much as 75%. The range of rocks at Crater Lake is from a low of about 54% (the Union Peak lavas) to as high as 71% (staircase dike underneath Llao Rock).

Other terms such as pumice and scoria are commonly used as rock names but do not reflect the compositional meaning as noted above. According to the glossary of geology, these are established definitions of other materials:

Pumice: An excessively cellular, glassy lava. It has the same basic composition of rhyolite. It is best described as "volcanic froth".

Scoria: Pyroclastic ejecta (or hot, welded materials ejected from a volcano). It has the same basic composition of basalt. It is characterized by marked vesciularity (see definition for vesicle), a dark color, and is relatively heavy. Its texture is partly glassy and partly crystalline. Small fragments are called "cinders".

Vesicle: Small, circular, enclosed space. A small cavity in a glassy igneous rock that is formed when bubbles of gas or steam expand during the cooling and solidification of the rock itself.

Pumice may also occur with compositions other than rhyolite. As this term is defined, it could have any composition but because of the nature of magma/lava with varying SiO₂ content, pumice forms predominantly with certain SiO₂ rich magmas. The pumice found at Crater Lake is dacite rather than rhyolite. Scoria is similar to pumice but has a chemical composition at the basaltic end of the scale. It will be less porous and not as frothy in appearance as pumice.

Both pumice and scoria are pyroclastic ejecta (that hot, welded stuff again) that result when gases are released in solution. As magma approaches the earth's surface, confining pressure is released and gases come out in solution. Magma with high SiO₂ content contains more dissolved gas and is more viscous than those of basaltic composition. Because of these factors, high silicon-oxygen magmas will tend to be more explosive than those with a lower percentage.

One other term that is often used in referring to volcanic rocks is obsidian. A partial definition: Ancient name for volcanic glass. It tends to exhibit banding and have a glassy luster and is rhylotitic in composition.

The term "glass" refers to liquid rock material that has cooled so rapidly that no crystals are formed. Both pumice and scoria are usually glassy but commonly contain crystals of minerals that were formed earlier. Since there are no rocks in the park that are considered to have a rhyolitic composition, it may not be appropriate to refer to obsidian at Crater Lake. Some of the dacite rocks are glassy and resemble obsidian in appearance. Examples of this are found on the back side of Llao Rock in the road cuts just to the east of the north junction.

References:

American Geological Institute, 1960, Glossary of Geology and Related Sciences with Supplement

Williams, Howell, 1942, The Geology of Crater Lake National Park, Carnegie Institute Publication 540, 162 pages.

rev. 5/2001 rdp