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.