There are two kinds of igneous rock – intrusive and extrusive.
Intrusive igneous rocks, also known as plutonic, form when magma cools deep beneath Earth’s surface. They are coarse textured with large interlocking crystal grains.
This is because the magma cools slowly, allowing time for big crystals to grow.
Extrusive igneous rocks form when magma flows onto the Earth’s surface as lava. They are fine grained with small or no visible crystals. This is because it is relatively cold on the Earth’s surface and lava cools quickly, allowing little time for crystals to grow. These rocks often contain holes called vesicles which are formed by gas bubbles.
Volcanic eruptions can be explosive and result in a pyroclastic flow – a mixture of hot semi-molten rock fragments and searingly hot expanding gases that flow down the volcano flank at immense speeds. Pyroclastic flows can be caused by collapse of the eruption column above the volcano or by a sideways blast from the flank of the volcano due to a build-up of pressure inside the volcano wall. Eventually the flow material settles and hardens into rock.
