Igneous rock
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Igneous rocks are formed when molten rock is either ejected directly on to the earth's surface (extrusive rock) or pushed up near the surface and cools beneath it (intrusive rock). Molten rock beneath the earth's crust is called magma; when magma is ejected throught the crust it is called lava.
Most igneous rocks are composed of an interlocking mosaic of crystals. Lava rocks are often riddled with air holes.
Extrusive Igneous
Crystals are generally small, often microscopic.
Intrusive Igneous
Crystals are generally large and visible to the naked eye. Intrusive igneous rocks are the source of many gemstones.
Kavanaugh, Pocket Naturalist
Sources
- Kavanaugh, James. Pocket Naturalist - Geology - An Introduction to familiar Rocks, Minerals, Gemstones & Fossils, Waterford Press: Arizona (2000)
