When magma intrudes the crust, most often it cools out of our reach. We infer the processes that operate using observations of old, now exposed igneous intrusions. Intrusions occur in a variety of sizes and shapes.
Enormous volumes of igneous magma can intrude the crust. The largest are called batholiths (bathos means deep). Batholiths can be hundreds of kilometers in length and are themselves composites of plutons (from Plutos), which are kilometer-scale intrusives.
Several large batholiths are exposed in the western United States: The Sierra Nevada, the Idaho, the Coast Range, and the southern California batholiths.
Smaller volumes of magma sometimes intrude into the upper crust in near-vertical sheets called dikes or near-horizontal sheets called sills. Volcanic plugs or necks are also common and easily noticed due to their erosion-resistant composition.