Rocks
Basalt
"A dark, fine-grained volcanic rock rich in iron and magnesium, formed from the rapid cooling of low-viscosity lava."
Basalt is the most common volcanic rock on Earth (and the Moon/Mars). It makes up most of the ocean floor and forms the massive shield volcanoes of Hawaii and Iceland.
Characteristics
- Color: Usually black or dark grey.
- Viscosity: Low (runny). Basaltic lava flows like syrup or water, allowing it to travel great distances.
- Temperature: Erupts at very high temperatures (1100°C - 1250°C).
- Silica Content: Low (< 50%). This low silica content means it doesn’t trap gas well, so eruptions are usually effusive (flowing) rather than explosive.
Where to See It
- Giant’s Causeway (Ireland): Famous for its hexagonal basalt columns, formed as the thick lava pool cooled and cracked.
- Iceland: The entire island is essentially a pile of basalt. The black sand beaches (like Reynisfjara) are made of pulverized basalt.
- Hawaii: The active flows from Kilauea turn into basalt rock within minutes of cooling.
Basalt is the building block of our planet’s crust, constantly being created at mid-ocean ridges and destroyed at subduction zones.