From Ohio History Central
Stich einer Basaltformation in der Gegend von Verona (1778).jpg
Basalt is a common extrusive volcanic rock.

Igneous, or primary, rocks crystallized from melted rock called magma. They are divided into two main categories based on composition. Felsic igneous rocks are light-colored and dominated by quartz and sodium-rich plagioclase and potassium feldspars. They form continental plates. Granite is an example. Mafic igneous rocks are dense, dark-colored igneous rocks dominated by iron and magnesium. They make up the oceanic crust. Basalt is an example. Igneous rocks can be emplaced deep in the crust (intrusive) where they cool slowly and form large crystals or be extruded (extrusive) at the surface where they cool quickly and are fine grained.

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