If you ever roasted a marshmallow over a hot campfire and ended up with a lump of something black and charred, rather than something gooey and delicious, what you actually did was performed a mini-experiment in contact metamorphism. Of course with true contact metamorphism we are using heat to change rocks, not marshmallows. we will learn more about contact metamorphism and how it differs from regional metamorphism, which relies more on pressure to cause rock minerals to change. Hydrothermal Metamorphism
Now, before we go too far into our terminology, it's important to understand that when geologists are talking about something going through a process of metamorphism, they are most likely talking about metamorphic rocks.
These rocks are kind of neat because they were previously-formed rocks that have been transformed by exposure to heat and/or pressure into new rocks. So, at one time these rocks may have been igneous, sedimentary or even other metamorphic rocks, but due to forces placed upon them they were transformed, or metamorphosed, into different rocks.
(Read about Metamorphic Rock Textures)
There are two types of metamorphism: contact metamorphism and regional metamorphism.
Let's start by discussing contact metamorphism, which is the one we introduced with the marshmallow experiment. Contact metamorphism is a type of metamorphism where rock minerals and texture are changed, mainly by heat, due to contact with magma. This is an easy name to recall if you remember that these rocks change by actually coming in contact with something very hot, like magma.
Image you were looking at a cross-section of the Earth where you can see the Earth's surface, as well as some deep layers of rock underground. Now imagine that an intrusion of hot liquid magma forces its way up through these deep layers of rock.
The magma bakes the surrounding rocks causing them to change, or metamorphose. So all around the outer boundary of the intrusion of magma you will have the formation of metamorphic rocks. Now, keep in mind that because there is a somewhat direct contact with the heat source, contact metamorphism takes place over a relatively small area.
Regional MetamorphismRegional or Barrovian metamorphism covers large areas of continental crust typically associated with mountain ranges, particularly those associated with convergent tectonic plates or the roots of previously eroded mountains. Conditions producing widespread regionally metamorphosed rocks occur during an orogenic event.
The collision of two continental plates or island arcs with continental plates produce the extreme compressional forces required for the metamorphic changes typical of regional metamorphism.
These orogenic mountains are later eroded, exposing the intensely deformed rocks typical of their cores. The conditions within the subducting slab as it plunges toward the mantle in a subduction zone also produce regional metamorphic effects, characterised by paired metamorphic belts. The techniques of structural geology are used to unravel the collisional history and determine the forces involved.
Regional metamorphism can be described and classified into metamorphic facies or metamorphic zones of temperature/pressure conditions throughout the orogenic terrane.