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Igneous Rocks

How are they formed?Rock-Cycle
Igneous rocks are formed when hot, molten rock (magma) cools and solidifies.  There are two main types of igneous rock, depending on where the magma cooled:

  • Intrusive igneous rocks form when the magma comes from deep underground and is forced into the upper layers of the Earth’s crust, where it cools slowly.
  • Extrusive igneous rocks form when the magma erupts from a volcano and cools quickly on the ground.

What are they like?
Igneous rocks contain randomly-arranged crystals.
If the magma cools quickly, the crystals will be large (coarse grained).
If the magma cools slowly, the crystals will be small (fine grained).

Because igneous rocks are formed from molten rock, they do not contain fossils.

Granite is an example of an intrusive igneous rock with large crystals.
Basalt is an example of an extrusive igneous rock with small crystals.

Igneous Rock:  formed by cooling & hardening of hot molten rock

Felsic: light colored rocks that are rich in elements such as aluminum, potassium, silicon, and sodium

Mafic: dark colored rocks that are rich in calcium, iron, and magnesium, poor in silicon

Coarse-grained:  cools SLOWLY, giving mineral crystals more time to grow (rough)

Fine-grained: cools QUICKLY with little to no crystals (smooth)

Intrusive Igneous Rocks: magma pushes into surrounding rock below the Earth’s surface

Extrusive Rocks: forms when magma erupts onto the Earth’s surface (lava),  cools quickly with very small or no crystals formed

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