Editing Metamorphism (section)

====Dynamothermal====
[[File:The stones of the Dutch - Lleida Pyrenees 04.JPG|thumb|A metamorphic rock, deformed during the [[Variscan orogeny]], at [[Vall de Cardós]], [[Lérida]], [[Spain]]]]
To many geologists, regional metamorphism is practically synonymous with dynamothermal metamorphism.{{sfn|Jackson|1997|loc="regional metamorphism"}} This form of metamorphism takes place at [[convergent plate boundaries]], where two [[Tectonic plate|continental plates]] or a continental plate and an [[island arc]] collide. The collision zone becomes a belt of [[mountain formation]] called an ''[[orogeny]]''. The orogenic belt is characterized by thickening of the Earth's crust, during which the deeply buried crustal rock is subjected to high temperatures and pressures and is intensely deformed.{{sfn|Yardley|1989|p=12}}{{sfn|Kearey|Klepeis|Vine|2009|pp=275–279}} Subsequent [[erosion]] of the mountains exposes the roots of the orogenic belt as extensive outcrops of metamorphic rock,{{sfn|Levin|2010|pp=76–77, 82–83}} characteristic of mountain chains.{{sfn|Yardley|1989|p=12}}

Metamorphic rock formed in these settings tends to shown well-developed [[Foliation (geology)|foliation]].{{sfn|Yardley|1989|p=12}} Foliation develops when a rock is being shortened along one axis during metamorphism. This causes crystals of platy minerals, such as [[mica]] and [[Chlorite group|chlorite]], to become rotated such that their short axes are parallel to the direction of shortening. This results in a banded, or foliated, rock, with the bands showing the colors of the minerals that formed them. Foliated rock often develops planes of [[Cleavage (geology)|cleavage]].  [[Slate]] is an example of a foliated metamorphic rock, originating from [[shale]], and it typically shows well-developed cleavage that allows slate to be split into thin plates.{{sfn|Yardley|1989|p=22, 168–170}}

The type of foliation that develops depends on the metamorphic grade. For instance, starting with a [[mudstone]], the following sequence develops with increasing temperature: The mudstone is first converted to slate, which is a very fine-grained, foliated metamorphic rock, characteristic of very low grade metamorphism. Slate in turn is converted to [[phyllite]], which is fine-grained and found in areas of low grade metamorphism. [[Schist]] is medium to coarse-grained and found in areas of medium grade metamorphism. High-grade metamorphism transforms the rock to [[gneiss]], which is coarse to very coarse-grained.{{sfn|Wicander|Munroe |2005|pp=174–77}}

Rocks that were subjected to uniform pressure from all sides, or those that lack minerals with distinctive growth habits, will not be foliated. Marble lacks platy minerals and is generally not foliated, which allows its use as a material for sculpture and architecture.

Collisional orogenies are preceded by [[subduction]] of oceanic crust.{{sfn|Yuan|Pan|Wang |Jiang|2009|pp=31–48}} The conditions within the subducting slab as it plunges toward the [[Mantle (geology)|mantle]] in a subduction zone produce [[Subduction zone metamorphism|their own distinctive regional metamorphic effects]], characterized by [[paired metamorphic belts]].{{sfn|Miyashiro|1973|pp=368–369}}

The pioneering work of [[George Barrow (geologist)|George Barrow]] on regional metamorphism in the Scottish Highlands showed that some regional metamorphism produces well-defined, mappable zones of increasing metamorphic grade. This ''[[Barrovian metamorphism]]'' is the most recognized [[metamorphic series]] in the world. However, Barrovian metamorphism is specific to [[pelitic]] rock, formed from [[mudstone]] or [[siltstone]], and it is not unique even in pelitic rock. A different sequence in the northeast of Scotland defines ''[[Buchan metamorphism]]'', which took place at lower pressure than the Barrovian.{{sfn|Philpotts|Ague|2009|p=417}}