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| BASALT
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Typical Minerals Minerals too small to identify (except phenocrysts). Composition will be from the top of Bowen's Reaction series (i.e. mafic) and we would expect Ca plagioclase, pyroxene, and perhaps some olivine. Note that these minerals are present in this specimen, just too small to see.
| Description
A mafic igneous rock from the top of Bowen's Reaction Series. Typically dark colored, although weathered specimens can appear quite light, or reddish. Olivine is sometime present as phenocrysts but is not essential. And although this specimen does not show it, vesicular (cellular) varieties are quite common, and these grade completely into scoria. |
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Tectonic Association
Basalt is one of the most common igneous rocks found. It is the major constituent of the upper layer of the ocean floors (usually as pillow lava), and hot spot volcanoes (such as the Hawaiian islands). Basalt commonly forms on the continents too, usually the result of hot spot activity. Here it may exist as intrusive dikes and sills, or extrusive cinder volcanoes and lava flows. In the western U.S. such occurances are common and usually quite visible since the volcanoes are relatively young. But basalt is also common in the east, if you can get through the vegetation to see it. In the east basalt shows up mostly as dikes, or more spectacularly in the 1000 foot thick Palasides sill, across the Hudson river from New York City.
 Tectonic Cross Section - pdf version
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| Last Update: 9/29/00 | e-mail: (Fichtels@jmu.edu) |