Igneous intrusions are masses of rock formed when magma cools beneath the surface. They rise to the surface because they are less dense
than the surrounding rock. When a magma within the crust loses its mobility and slowly cools and solidifies it forms a mass of igneous rock called an intrusion. Igneous intrusions become exposed on the Earth's surface when the previously overlying rock surface has been removed by erosion. They are classified according to their sizes, shapes, and
relationships to the older rocks that surround them. Some common intrusive rocks are batholiths, stocks, dikes, sills, and laccoliths.

Batholiths are large masses of coarsely crystalline rock (usually granitic composition) that cool in the magma chamber. They are found almost exclusively in continents and large island arcs, and do not occur in oceanic islands. They form in the deeper zones of mountain belts and
are exposed after considerable uplift and erosion. They are the largest rock bodies on Earth and can cover several thousand square kilometres. They appear to be huge, slablike bodies, with a horizontal extent much greater than their thickness. It is suggested (still unknown) that they
were emplaced at depths of more than 7km below the surface. Areas of
extensive exposure of batholiths are considered to be the roots ofancient mountain ranges, which have long since been eroded to lowlands.

Stocks are an intrusive rock mass with an outcrop (an exposed area atthe surface) of less than 10 kilometres squared. Some stocks are knownto be small protrusions rising from the main batholith, but much is unknown. A stock is generally composed of granitic rock, and many
are porphyritic (air spaces) with a fine grained groundmass. Many deposits of silver, gold, lead, zinc, and copper are found in fractures and form veins extending from a stock into the surrounding rock.