Basalt is a fine-grained, dark-gray, dark-green, or greenish-black rock which is hard enough to be scratched with difficulty by steel. It originated by the solidification of lava. Today, basalt rock is forming where lava at the Hawaiian volcanoes solidifies.
The relatively small amount of basalt in southeastern Missouri solidified mostly in cracks within other rocks through which it rose. Those occurrences—that is, fillings in nearly vertical cracks—are called dikes. The basalt dikes in southeastern Missouri have been exposed by the weathering and erosion of rocks which previously covered them.
A dark dike of basalt in granite near Silver Mines.
In northern Missouri, boulders of basalt may be found in deposits of glacial clay, sand, and gravel (glacial drift), where they were left after the melting of the great ice sheet which brought the basalt down from ancient dikes and igneous bodies cropping out in the northern United States and Canada. Many of the boulders have been rounded by weathering, and their shape, together with their dark color, has stimulated the local name “niggerhead” for them.
Basalt in hand specimen.
Basalt is a strong, tough, well-knit rock that will withstand heavy blows from a sledge hammer, which usually rebounds upon striking. Except for use as rubble stones, basalt has no commercial value. It weathers characteristically to a yellowish, brownish, or reddish surface coating of iron oxide and clay.
Gabbro and Diabase
Gabbro and Diabase are dark-colored, coarse-grained, hard igneous rocks, which may be found in the granite and porphyry regions of southeastern Missouri and as separate boulders in the glacial deposits north of the Missouri River. Both resemble basalt, which has been described in detail elsewhere, except that basalt is fine-grained, whereas gabbro and diabase are coarse-grained (separate grains easily distinguished without a magnifying glass). The layman is ordinarily not concerned with the technical differences between gabbro and diabase, which appear about the same. Both contain plagioclase feldspar (see [FELDSPAR]) and a dark green mineral of the pyroxene family.