Settings in geology when the term
country rock is used include:
Igneous intrusions '' (dark) intrudes into the country rock (light),
Baranof Island,
Alaska, United States When describing a
pluton or
dike, the igneous rock can be described as intruding the surrounding
country rock, the rock into which the pluton has intruded. When country rock is intruded by a dike, perpendicular to the
bedding plane, it is called
discordant intrusion, while a parallel intrusion by a
sill indicates a
sub-parallel or
concordant intrusion. Most intrusions into country rock are via
magma. Usually, country rock is intruded by an
igneous body of rock which formed when magma forced upward through
fractures, or melted through
overlying rock. Magma then cooled into solid rock, different from the surrounding country rock. Sometimes, a fragment of country rock will break off and become incorporated into the intrusion, and is called a
xenolith, from
Greek, ,
xenos, "strange,", and ,
lithos, the ancient Greek word for "stone." The heat of the intrusions usually changes the country rock, often to
contact metamorphic rock. Commonly,
hornfels is produced, or
skarn.
Alluvial settings When describing recent
alluvium, the material that has arrived through
volcanic,
glacial or
fluvial action can be described as a veneer on the (older) country rock. ==References==