Hornfels is a fine-textured metamorphic rock formed by contact metamorphism . Contact metamorphism occurs when a mass of hot magma intrudes into preexisting rock , whether by injecting itself into a crack or by ascending in a large body (e.g., pluton ). Rock in close proximity to the magma is temporarily softened or melted and recrystallizes with an altered texture, producing a hornfels. The term hornfels is often restricted to rocks produced by contact metamorphism of shale, slate , or mudstone.
In contrast to schists and gneisses, hornfelses show little or no foliation or layering. They form under conditions of approximately anisotropic (directionless) stress, so there is no tendency for the crystals to align in any particular direction. Traces of bedding present in the parent rock may remain in a hornfels but are not caused by metamorphosis.
Because they form by contact metamorphosis, hornfelses occur in shells or layers around bodies of intrusive magmatic rock. When seen in cross-section, as at Earth's surface, these shells or layers appear as rings or bands surrounding areas of magmatic rock. These rings are termed contact aureoles. A contact aureole may be only a few centimeters thick or several kilometers thick, depending on the size of the magmatic intrusion. An aureole of less-metamorphosed rocks, often spotted slates and semihornfels, frequently surrounds the hornfels aureole and blends smoothly with it. As is generally the rule with metamorphic rocks, coarser texture in a hornfels indicates more thorough melting and slower recrystallization.
Hornfels may be chemically altered by the magma that metamorphoses them, but generally reflect the chemical composition of their parent rocks; thus, quartz , feldspar , biotite, muscovite, pyroxenes, garnet, and calcite are common ingredients of hornfelses. However, as hornfelses are defined by process of origin (contact metamorphism), not by composition, one must establish that a rock has originated in a contact aureole to classify it as a hornfels.
See also Country rock; Intrusive cooling; Metamorphic rock