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Physical
Igneous Rock
Igneous rock forms when molten rock, called magma underground and lava at the surface, cools and turns solid.
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The idea
Igneous rock is one of the three main types of rock, along with sedimentary and metamorphic rock. Its name comes from the Latin word igneus, which means "fiery." Igneous rock forms when magma or lava cools and becomes solid. Magma is molten rock below the ground, and lava is molten rock that has reached the surface.
Magma forms when existing rock deep in a planet's mantle or crust partly melts. Three things usually cause this melting: a rise in temperature, a drop in pressure, or a change in the rock's makeup. When magma cools and hardens underground, it becomes intrusive rock, such as granite. When lava cools on the surface, it becomes extrusive rock, such as basalt.
How fast the rock cools shapes how it looks. Slow cooling deep underground gives mineral crystals time to grow, so the rock turns out coarse and grainy. Fast cooling at the surface makes only tiny crystals, and very fast cooling can make natural glass with no crystals at all, such as obsidian. Igneous rocks are found in many geological settings, from the ocean floor to mountain belts and the ancient cores of continents.
Check yourself
Question 1 of 3
What does the Latin word igneus, the root of the name, mean?
Composition
what it's made of
- Magma or lava — The molten rock that cools to make igneous rock: magma below ground, lava at the surface.
- Intrusive rock — Igneous rock that solidifies below the surface, usually slowly, such as granite.
- Extrusive rock — Igneous rock that solidifies on the surface, usually quickly, such as basalt.
- Mineral crystals — Grains that grow as the melt hardens; slower cooling gives larger crystals.
- Natural glass — Rock that cooled without forming crystals, such as obsidian.
Connection
how it relates
Under Rock Cycle
Change
how and why it evolves
- Past
- Earth's earliest solid crust formed as molten rock at the surface cooled and hardened.
- Now
- New igneous rock still forms wherever magma rises, such as at volcanoes and along mid-ocean ridges, where fresh ocean floor is made.
- Next
- Over very long spans of time, igneous rock at the surface will probably be worn down into sediment or changed by heat and pressure, carrying on the rock cycle.