Like dikes, sills are tabular or sheetlike in their shapes. Enlarge picture 3 to see a very thick, mafic sill. Volcanic necks are relatively uncommon igneous intrusions. They form when magma crystallizes within the neck of a volcano, and are exposed for us to see as weathering and erosion remove the rock comprising the volcano. A beautiful example of a volcanic neck is Shiprock Mountain, New Mexico shown in picture 4.
Batholiths are the largest of all igneous intrusions.
They are formed by the massive invasion of hot magma into country rock. Such a large intrusion can melt or metamorphose most rocks. Cooling of the magma can take thousands of years. Like dikes, batholiths cut through pre-eexisting rocks, so they are discordant intrusions. Many of the rocks exposed at the surface in California are granite batholiths.