Basalt paves most of the Solar System's rocky surfaces, including Venus
More than nine in ten volcanic rocks on Earth are basalt, and the pattern repeats elsewhere. The dark seas on the Moon are old basalt floods, the plains covering about 80% of Venus are basaltic, and the rock is common on Mars. Geologists still watch it pour from roughly 20 volcanoes every year.
Basalt is lava that cooled fast, so its crystals stayed too small to see. It is rich in iron and magnesium and holds only 45 to 52% silica, which makes the molten form runny, roughly as thick as ketchup. That fluidity lets flows travel far before they freeze, and stacked sheets of them, called flood basalts, can blanket hundreds of thousands of square kilometres. The same melt, cooled slowly underground, becomes coarse-grained gabbro.
Most of it begins in the upper mantle through decompression. Mantle rock is solid only because immense pressure raises its melting point; when tectonic forces let hot rock creep upward, the pressure eases and part of it melts. This happens at mid-ocean ridges, in continental rifts, above hotspots and in subduction zones, where water released by sinking plates lowers the melting point further. Because each setting leaves its own chemical fingerprint, basalt is a window into processes deep inside the planet.
Its minerals carry other records too. Iron-titanium oxides such as magnetite lock in the direction of Earth's magnetic field as the rock cools, making basalt a mainstay of paleomagnetic studies. Gas bubbles escaping during eruption can freeze in place as vesicles, and when they riddle the rock it is called scoria. Density averages 2.9 grams per cubic centimetre, heavier than typical granite.
The name has a tangled history. Latin basanites meant very hard stone, borrowed from a Greek word for touchstone, and a later misspelling gave basaltes. Georgius Agricola fixed the modern sense in 1546, applying it to black volcanic rock beneath Stolpen castle, which he took to be the stone Pliny the Elder had described in AD 77.
Source: Basalt
