Metamorphic rock
In 1795, the Scottish naturalist James Hutton wrote that some rock beds of the Scottish Highlands had originally been sedimentary rock but had been transformed by great heat. This observation marked a pivotal moment in geological history when the concept of metamorphism began to take shape. Hutton is often described as the father of modern geology for his pioneering work on these transformations.
Hutton also speculated that pressure was important in metamorphism. His friend James Hall tested this hypothesis by sealing chalk into a makeshift pressure vessel constructed from a cannon barrel and heating it in an iron foundry furnace. Hall found that this produced a material strongly resembling marble rather than the usual quicklime produced by heating chalk in open air.
French geologists subsequently added metasomatism, the circulation of fluids through buried rock, to the list of processes that help bring about metamorphism. However, metamorphism can take place without metasomatism or at depths of just a few hundred meters where pressures are relatively low. Contact metamorphism represents one such example where rocks experience localized heating.
Hot fluids circulating through pore space in the rock can dissolve existing minerals and precipitate new minerals. Dissolved substances are transported out of the rock by the fluids while new substances are brought in by fresh fluids. This process changes the mineral makeup of the rock significantly.
Changes in mineral composition can take place even when the bulk composition of the rock does not change. At atmospheric pressure, the mineral kyanite transforms to andalusite at a temperature of about 500 degrees Celsius. Andalusite, in turn, transforms to sillimanite when the temperature reaches about 600 degrees Celsius.
All three minerals have the identical chemical composition yet exist under different conditions. Forsterite is stable over a broad range of pressure and temperature in marble but converts to pyroxene at elevated pressure and temperature in more silicate-rich rock containing plagioclase. These complex high-temperature reactions occur between minerals without them melting due to rapid diffusion of atoms at elevated temperatures.
The small calcite crystals in the sedimentary rock limestone and chalk change into larger crystals in the metamorphic rock marble. In metamorphosed sandstone, recrystallization of the original quartz sand grains results in very compact quartzite with interlocked quartz crystals that are often much larger than their predecessors.
High temperatures allow the atoms and ions in solid crystals to migrate thus reorganizing the crystals while high pressures cause solution of the crystals within the rock at their point of contact. This process reduces surface energy by creating coarser crystals from finer ones.
Although grain coarsening is common, rock that is intensely deformed may eliminate strain energy by recrystallizing as fine-grained mylonite. Certain kinds of rock such as those rich in quartz carbonate minerals or olivine are particularly prone to form mylonites while feldspar and garnet resist this transformation.
If the protolith of a metamorphic rock can be determined, the rock is described by adding the prefix meta- to the protolith rock name. For example if the protolith is known to be basalt the rock will be described as metabasalt. A metamorphic rock whose protolith is known to be a conglomerate becomes metaconglomerate.
Under the British Geological Survey classification system if all that can be determined about the protolith is its general type the classification is based on mineral mode. Metasedimentary rocks divide into carbonate-rich rock or carbonate-poor rocks with further classification by mica abundance ranging from low-mica psammite through semipelite to high-mica pelite.
Where mineral mode cannot be determined classification must be based on texture. Schists show medium-grained strongly foliated characteristics with schistosity defined as platy minerals aligned in a single direction allowing easy splitting into plates less than one centimeter thick. Gneisses display coarser grain and thicker foliation over five millimeters thick.
Metamorphic rock forms extensively in orogenic belts produced by collision of tectonic plates at convergent boundaries. Here formerly deeply buried rock has been brought to surface by uplift and erosion. The lower continental crust contains mostly metamafic rock and pelite reaching granulite facies while middle continental crust holds amphibolite facies rock.
When subduction occurs basalt of the subducting slab transforms to high-pressure metamorphic facies initially undergoing low-grade metamorphism to metabasalt of zeolite and prehnite-pumpellyite facies. As basalt subducts deeper it becomes blueschist facies then eclogite facies releasing water vapor that drives volcanism in overlying volcanic arcs.
Eclogite is significantly denser than blueschist driving further subduction of slab deep into Earth mantle. Many samples of eclogite appear as xenoliths brought to surface by volcanic activity. The main islands of Japan show three distinct paired metamorphic belts corresponding to different episodes of subduction.
Slate tiles are used in construction particularly as roof shingle. Quartzite is sufficiently hard and dense that it is difficult to quarry yet some quartzite serves as dimension stone often as slabs for flooring walls or stairsteps. About six percent of crushed stone used mostly for road aggregate consists of quartzite.
Marble remains prized for building construction and as medium for sculpture. Schistose bedrock can pose challenge for civil engineering because of pronounced planes of weakness. On the 17th of August 1959 a magnitude 7.2 earthquake destabilized mountain slope near Hebgen Lake Montana composed of schist causing massive landslide killing twenty-six people camping in area.
Metamorphosed ultramafic rock contains serpentine group minerals including varieties of asbestos posing hazard to human health. These practical applications demonstrate how geological processes directly impact daily life and infrastructure development across the globe.
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Common questions
Who wrote about rock beds in the Scottish Highlands being transformed by heat in 1795?
The Scottish naturalist James Hutton wrote that some rock beds of the Scottish Highlands had originally been sedimentary rock but had been transformed by great heat. This observation marked a pivotal moment in geological history when the concept of metamorphism began to take shape.
What did James Hall find when he sealed chalk into a cannon barrel and heated it?
James Hall found that sealing chalk into a makeshift pressure vessel constructed from a cannon barrel and heating it produced a material strongly resembling marble rather than the usual quicklime produced by heating chalk in open air. His experiment confirmed that pressure was important in metamorphism.
How do minerals change composition during contact metamorphism without melting?
Hot fluids circulating through pore space dissolve existing minerals and precipitate new ones while dissolved substances are transported out of the rock by the fluids. Complex high-temperature reactions occur between minerals without them melting due to rapid diffusion of atoms at elevated temperatures.
When does kyanite transform to andalusite and then to sillimanite?
At atmospheric pressure, the mineral kyanite transforms to andalusite at a temperature of about 500 degrees Celsius. Andalusite transforms to sillimanite when the temperature reaches about 600 degrees Celsius.
Why did an earthquake near Hebgen Lake Montana cause a massive landslide on the 17th of August 1959?
A magnitude 7.2 earthquake destabilized a mountain slope composed of schistose bedrock which poses challenges for civil engineering because of pronounced planes of weakness. The event caused a massive landslide killing twenty-six people camping in the area.