basalt [key], fine-grained rock of volcanic origin, dark gray, dark green, brown, reddish, or black in color. Basalt is an igneous rock, i.e., one that has congealed from a molten state. Basaltic magma is derived by partial melting of the peridotite that is found in the asthenosphere which reaches the mid-ocean ridges, such as the Mid-Atlantic Ridge, and forms the new oceanic crust, the uppermost layer of the lithosphere. 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 dominates the mid-ocean islands and surrounding regions of the Hawaiian Islands and Iceland, as found by samples of lava flows found in drill cores recovered by vessels of the Deep Sea Drilling Project and the now defunct Project Mohole (see Mohole, Project). 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). Phenocrysts are usually formed in the molten lava before eruption and are often composed of the minerals olivine and pyroxene. 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. Basalt may form as columns of rock, such as the Devil's Tower in Wyoming; or it may form as twisted coils of rope, or cinders of jagged rock, called “pahoehoe” and “aa,” respectively. Gabbros are similar in composition to basalt, but gabbros are coarse-grained rocks formed by slow cooling in large underground masses, common in New York's Adirondack Mts. When subjected to metamorphism, i.e., high temperatures and great pressures, basalt is transformed into various kinds of schists including hornblende schist. Fine and coarse-grained crystalline rocks returned from various regions of the moon by Apollo astronauts were similar in many respects to terrestrial basalts. Fine-grained basaltic lunar rocks are vesicular, with glass-lined pits on exposed surfaces that have been interpreted as micrometeorite impact scars. Lunar rocks differed from terrestrial basalts in lacking water and organic compounds, and were higher in titanium, magnesium, and iron.
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