2.4.1 Temperature, pressure and metamorphism
- Metamorphism is the isochemical (no elements added or removed) process where rocks are changed by heat and/or pressure.
Metamorphism may result in:
- destruction of fossils, beds, sedimentary structures
- hardening of rock
- change in colour
- alignment of minerals
- growth of new minerals
- As temp increases, rate of metamorphic reactions also increases because many chemical reactions need heat to occur.
- Higher temp increases the rate at which ions diffuse between minerals.
- Pore pressure-pressure exerted by fluids between grains in a porous rock which speeds up reactions as water helps ion exchange.
- Load pressure-weight of overlying rocks, physically brings minerals into contact with each other.
- Tectonic pressure-rocks undergo folding or faulting and high pressures are exerted.
Contact metamorphism: occurs adjacent to igneous intrusions --> increase temp in surrounding country rock, produces metamorphic aureole, temperatures are high, pressure is low, minerals are not aligned.
Burial metamorphism: medium-high pressure, low temperature, affects rock buried by weight of overlying sediments, occurs at subduction zones (where sea floor sediments and basalts are buried)
Regional metamorphism: affects larger areas than contact metamorphism, low-high pressure, low-high pressure at convergent plate margins, results from subduction or continental collision, minerals have preferred alignment, occur in the cores of fold mountain belts.
2.4.2 Identifying regional metamorphic rocks
Foliation: the texture in metamorphic rocks formed by the preferred alignment of flat, platy minerals. Foliated rocks are produced by regional metamorphism.
Slaty cleavage: texture in fine grained rocks formed by low grade regional metamorphism. Platy minerals recrystalise perpendicular to the direction of stress apllied, so the rock splits into thin sheets.
FOLIATED ROCKS PRODUCED BY REGIONAL METAMORPHISM
2.4.3 Identifying metamorphic rocks
UNFOLIATED ROCKS PRODUCED BY CONTACT/REGIONAL METAMORPHISM
2.4.4 Metamorphic textures
Slaty cleavage- will split into thin sheets along cleavage planes, occurs in fine grained rocks formed by low grade regional metamorphism
- Can only form in rocks with platy minerals like clay minerals, chlorite, micas.
- Minerals become aligned at 90 degrees to the direction of maximum pressure.
- It may be at any angle to bedding, but is usually parallel to axial planes of folds.
- Cannot occur in rocks with rounded grains, such as quartz in sandstones.
Schistosity- found in schists (medium grained rocks formed by regional metamorphism), results from the alignment of flat and platy mineral at 90 degrees to direction of maximum pressure, light coloured muscovite mica is in thin paralled bands, shiny appearance.
Gneissose banding- found in gneisses (coars grained rocks formed by R.M), formed when light and dark minerals are separated into bands.
Porphyroblastic texture- occurs in regional and contact metamorpic rocks, porphyroblasts are large crystals that grow during metamorphism and are surrounded by a finer grained groundmass.
Granoblastic texture- an unfoliated texture formed by thermal metamorphism, randomly orientated, equidimensional crystals, eg marble, quartzite.
2.4.5 Contact metamorphism 1
Metamorpic aureole: region surrounding an igneous intrusion where country rocks have been recrystallised and changed by heat from the intrusion.
Shale- depends on distance (temp) from intrusion
- close to contact with intrusion, temps are high so high grade metamorphism occurs. Shale is recrystallised to form hornfels.
- further away, heat is less intense-medium grade metamorphism-->andalusite.
- outer part of aureole, temps are lower so there is some recrystallisation-low grade metamorphism-->spotted rock.
Factors controlling metamorphic aureoles:
- volume of magma-larger intrusion cool slower-->wide aureole
- temp of magma-higher temps cool slower-->wide aureole
2.4.6 Contact metamorphism 2
Metamorphic aureole-thermal gradient, index minerals:
- index mineral are metamorphic minerals. They indicate the metamorphic grade.
- In C.M biotite is the low grade mineral found in spotted rocks.
- the Al2SiO5 polymorph andalusite indicateds medium grade-found in andalusite rich rocks.
- sillimanite (Al2SiO5 polymorph) indicated high grade-found in hornfels.
- An increase in grade represents a thermal gradient.
Polymorph: mineral that has the same composition but occurs in different crystal forms.