Contact metamorphism and regional metamorphism have different proximate causes, affect areas of different sizes and produce different types of rock. Generally, contact metamorphism only occurs where hot magma has intruded on low-pressure surface rock and exposed the surrounding area to high temperature. Regional metamorphism is generally independent of igneous intrusions and tends to happen in places where tectonic forces have compressed the crust and put high pressure on the rocks.
Since contact metamorphism requires that the affected rocks exist within a local temperature gradient, it is necessarily limited to small areas. Regional metamorphism, as its name suggests, works over much larger areas. Typically, a regionally metamorphosed area is situated under a fold/thrust mountain range or along a boundary between tectonic plates.
Each type of metamorphism generates distinct rock types. Contact processes work by raising the local temperature and producing hornfels. Hornfels is a fine-grained rock that shows no signs of layering. Regional metamorphism works by compressing rock, which can often cause the rock to be layered, or foliated. Rock types produced under compression vary according to local conditions but are typically grouped as gneiss, slate and schist. These rocks are common in mountain ranges from the Alps to the Himalayas.