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Physical
Basalt Formation
Basalt forms when runny lava rich in magnesium and iron cools quickly at or near a planet's surface, making fine-grained volcanic rock.
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The idea
Basalt is a dark, fine-grained volcanic rock. It forms when lava rich in magnesium and iron cools quickly at or near the surface of a rocky planet or moon. Because it cools so fast, its mineral crystals stay too small to see without magnification, a texture geologists call aphanitic. More than 90% of all volcanic rock on Earth is basalt.
Basalt lava is runny because it holds relatively little silica, between 45% and 52%. Runny, or low-viscosity, lava flows quickly and can spread over great areas before it cools and hardens. When many such flows pile up, they make flood basalts, which can cover hundreds of thousands of square kilometres. The magma that feeds basalt is thought to come from Earth's upper mantle.
As a lava flow cools, it shrinks and cracks. The cracks can form a regular pattern of tall columns, a structure called columnar jointing. Columns most often have six sides, although they can have anywhere from three to eight. They range from a few centimeters to 3 meters across and can stand as much as 30 meters tall. If magma with the same chemistry as basalt cools slowly instead, it forms gabbro, a rock with much bigger crystals.
Check yourself
Question 1 of 3
Why are the crystals in basalt so small?
Composition
what it's made of
- Mafic lava — Low-viscosity lava rich in magnesium and iron.
- Rapid cooling — Fast cooling at or near the surface keeps the crystals tiny.
- Low silica — Between 45% and 52% silica, which makes the lava runny.
- Lava flows — Fast-moving flows that can spread over great areas before hardening.
- Columnar joints — Cracks that form as cooling rock contracts, often making six-sided columns.
Connection
how it relates
Under Igneous Rock
Change
how and why it evolves
- Past
- Long before people studied it, eruption after eruption of runny basalt lava piled up into flood basalts, the most voluminous of all volcanic formations.
- Now
- Geologists see basalt lava erupt at volcanoes every year, and basalt also forms the Moon's maria and is a common rock on the surface of Mars.
- Next
- Because basalt carries chemical clues from the upper mantle, studying new eruptions will likely keep improving what scientists know about Earth's interior.