Basalt

• Donor / collector : Jean Guyader

• Rock type : volcanic rock

• Age : unknown

• Origin : Volvic, Auvergne

• Dimensions : 25 cm x 23 cm

During volcanic eruptions, high-temperature lava (1100-1200°C) is expelled and sometimes sprays over several thousands of square miles. Once it cools down, it solidifies (at a temperature of approximately 1000°C). In 95% of cases, lava cools into basalt rock. It is therefore classified as an extrusive igneous rock.

When you look at a basalt sample, you will notice it is black and there are little or no crystallised minerals. This is due to the fact that lava cools down quickly when exposed to air. Because of that, minerals do not have the time to develop.

Basalt is composed of pyroxene, hornblende and olivine. Submarine lava flows are called pillow lavas.