For an explanation, see the main splats page
SPLATS about igneous rocks
The principles of igneous rocks
- Igneous rocks form when magma cools: granite cools slowly, deep down and forms large crystals, basalt cools faster near the earth's surface and has no crystals.
- There are many kinds of igneous intrusions: a sill is a horizontal intrusion between beds of rock, a dike is a vertical intrusion by way of a joint plane.
- In 1785, James Hutton predicted and discovered a number of pink veins of granite, pushing their way up into the dark schist above, the first record of dikes.
- When a basalt flow cools fast, this can produce columnar joints, resulting in columns with 6, 7 or 8 sides. These may be seen all over the world.
- Crystals form in igneous rocks in accordance with Bowen's reaction series, and this can lead to different types of rock forming from one batch of magma.
- Pumice forms when dissolved gas expands in molten rock, which then cools and solidifies before the gas has time to escape, leaving a rock that floats.
- When igneous rocks push their way through other rocks, or flow over them, they cause local changes in those other rocks and this is called contact metamorphism.
- If a sheet of basalt has traces of contact metamorphism both above and below, then it formed originally as a sill, pushing between two other layers of rock.
- If a sheet of basalt between two other rocks only has contact metamorphism below it, it was originally a flow that was later covered over by other material.
- Some igneous rocks undergo weathering faster than others, but fast or slow, igneous rocks usually form a soil which is rich in the minerals plants need.
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