Igneous rocks are formed by crystallization upon the cooling of molten rock, or magma. If the magma is trapped inside the earth’s crust at this point and is unable to come to the surface, the resulting slow cooling allows the formation of large crystals, giving rise to coarse-grained plutonic rocks, whereas if the magma reaches the surface, e.g. through a volcanic eruption, the lava cools rapidly and fine-grained supracrustal rocks are formed. The crystallization of magma in cracks in the earth’s crust also yields fairly fine-grained hypabyssal rocks.
The principal igneous rocks used for building are the plutonic granites, granodiorites, diorites and gabbros and the hypabyssal rock diabase. There are no true supracrustal rocks in Finland, because of the absence of volcanic activity, but volcanic eruptions were common in this area many millions of years ago, as a result of which there are large numbers of metamorphic variants of such rocks.
These acidic or intermediate vulcanites are not suitable as sources of building stone, but many of them belong to the prime group for the production of crushed rock aggregates.