A batholith is a very large mass of igneous rock that was originally magma. Batholiths are commonly formed deep in the roots of mountain chains and may be many tens of miles in diameter.
In Britain, a batholith exists under much of Cornwall and reaches out under the sea to the Scilly Isles. This covers an area of 40 mi (65 km) by 25 mi (40 km). Many batholiths are larger than this.
Before the giant mass of magma in the batholith was intruded (forced in), there was other rock in that part of the Earth's crust. Geologists have puzzled over what happened to the original rock. It is believed that some of it was melted and incorporated into the magma.
Granite magma itself may be the result of the melting of other rock at great depth.
Sills and dykes may stretch upward from some batholiths.
Most batholiths are made of granite. Some contain other coarse-grained igneous rocks like syenite and gabbro.
Even when the rock in a batholith has been crystalline for millions of years, heat will still rise through it from a great depth. This heat may be trapped if clay and other rock have formed later.
Scientists have discovered that this heat could be used to produce electricity in a safe, clean way.