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How do igneous rocks form from magma or lava?
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geology
A rock formation called “faux amphibolite,” from the Nuvvuagittuq greenstone belt in northern Quebec, dated to 4.28 billion years ago.
Igneous rocks form through the cooling and solidification of molten earth material known as magma or lava. Magma is generated deep within the Earth, typically below 60 kilometers, and rises toward the surface because it is less dense than the surrounding solid rock.12
The environment in which this material cools determines the resulting rock's characteristics:
- Intrusive (Plutonic) Rocks: When magma cools slowly deep within the Earth's crust, the high temperatures and pressures allow minerals to grow into large, visible crystals, resulting in a phaneritic texture.23
- Extrusive (Volcanic) Rocks: When magma erupts at the surface as lava, it is chilled rapidly. This quick cooling prevents large crystals from forming, resulting in an aphanitic texture (fine-grained) or a glassy, hyaline appearance.23
During the cooling process, minerals form in a systematic manner often described by Bowen’s reaction series, where high-temperature crystals react with the remaining liquid to form new minerals as the temperature drops.4