What are the properties of basalt?
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Because molten basalt is lighter than peridotite, it rises more rapidly. Basaltic magmas contain around 50% silica; they are the most common extrusive rocks and comprise more than 90% of all volcanic rock. It forms mostly lava flows, including present-day Hawaiian flows, and the ancient Columbia River plateau of the NW United States. Basalt contains a high percentage of iron and magnesium. Some basalts are porphyritic, i.e., they contain large crystalline structures called phenocrysts embedded in a matrix called a groundmass (see porphyry ). Where molten basalt cools rapidly, as at the earth's surface, fine-grained rocks are formed. Basalt may be compact or vesicular, i.e., porous because of gas bubbles contained in the lava while it is solidifying. If the vesicles become subsequently filled with secondary minerals, e.g., quartz or calcite, the rock is called amygdaloidal basalt... When subjected to metamorphism, i.e., high temperatures and great pressures, basalt is transformed into various kinds of schists including hornblende schist.
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