Invertebrate Inn Fireplace
Igneous rocks are formed by the cooling of hot molten material, called magma when it intrudes below the surface of the Earth and lava when it extrudes out across the Earth’s surface.
Igneous rocks are classified according to grain size and composition. The fireplace rocks are all intrusive, and are coarse grained because crystals had more time to grow in magma that cooled slowly deep below the Earth’s surface. Extrusive rocks are fine grained – grains too small to see with the naked eye. The chemical composition varies from rocks rich in silica (SiO2) to those with a higher proportion of other elements, typically manganese and iron. Silica-rich rocks tend to be lighter in color.
Intrusive Extrusive Silica
Granite Rhyolite >69%
Granodiorite Dacite 62-69%
Diorite Andesite 54-62%
Gabbro Basalt 45-54%
When a lava flow cools enough to harden, it is still quite hot (~1000o C). With further cooling, it contracts and breaks, often forming hexagonal columns. Three pieces of basalt columns from Central Washington are arranged in front of the fireplace.