The term country rock refers to a body of rock that receives or hosts an intrusion of a viscous geologic material. Intrusions into country rock are most commonly magmatic, but may also consist of unconsolidated sediments or salt horizons. Country rock may consist any other kind of rock that was present before the intrusion: sedimentary, igneous, or metamorphic.
In most cases, country rock is intruded by an igneous body of rock that formed when magma was forced upward through fractures or melted its way up through the overlying rock. The magma then cooled into solid rock forming a mass distinct from the enveloping country rock. Occasionally, a fragment of country rock will break off and become incorporated into the intrusion, and is called a xenolith .
The country rock is usually altered by the heat of the intrusion. The change that takes place in country rock as a result of an intrusion cooling off is called contact metamorphism . The extent and intensity of contact metamorphism depends on the heat of the magma, the temperature of the country rock, the amount of fluids present, the permeability of the country rock, and the depth of intrusion (which determines to a great extent, the pressure). The metamorphism is strongest at the contact of the country rock and the intrusion and diminishes outward from the intrusion. A discernable halo of contact metamorphism that extends into the country rock is often produced and is called the contact aureole.
When the country rock has been contact metamorphosed, it often experiences mineralogical alterations that result in a rock quite different from the original. One common rock type produced by contact metamorphism is called hornfels . It is a very fine-grained rock with little recognizable texture. Another is called skarn, a rock rich in calc-silicate minerals and often the product of a limestone or dolomite country rock.
The other, less common form of intrusion into country rock consists of geologic material that is able to flow, but is not molten rock. This material can be unconsolidated sediment that has sufficient water content to act as a fluid. These are termed soft sediments and if there is sufficient pressure from the overlying rocks, they can be forced into fractures into country rock. The resulting intrusion is called a diatreme. Another type of material that forms diatremes is salt. Salt has a lower density than most other rocks and when buried, salt horizons can become viscous and will flow upward. In both soft sediment intrusions and salt diatremes, the county rock is not metamorphosed. However, diatremes do disrupt the country rock, sometimes producing visible bulges.
See also Intrusive cooling