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Information about Metaa2

Published on July 5, 2007

Author: john.payne

Source: slideshare.net

Description

geology - metamorphic rocks

METAMORPHISM AND METAMORPHIC ROCKS

WHAT IS METAMORPHISM? recrystallisation of pre-existing rocks, leaving larger crystals new minerals form, but composition usually stays the same metamorphic rocks form from igneous, sedimentary or other metamorphic rocks 3 types of metamorphism are recognised thermal/contact metamorphism regional metamorphism dynamic/dislocation metamorphism

recrystallisation of pre-existing rocks, leaving larger crystals

new minerals form, but composition usually stays the same

metamorphic rocks form from igneous, sedimentary or other metamorphic rocks

3 types of metamorphism are recognised

thermal/contact metamorphism

regional metamorphism

dynamic/dislocation metamorphism

 

WHAT ARE THE AGENTS OF METAMORPHISM? heat , usually from proximity to igneous intrusions, but also from depth and temperature increases with the geothermal gradient pressure , due to overlying rocks, but more especially in tectonically active zones hydothermal fluids , especially adjacent to igneous intrusions

heat , usually from proximity to igneous intrusions, but also from depth and temperature increases with the geothermal gradient

pressure , due to overlying rocks, but more especially in tectonically active zones

hydothermal fluids , especially adjacent to igneous intrusions

HEAT breaks chemical bonds mobilises hydothermal fluids of hot water and gases softens rocks to allow deformation Geothermal gradient

breaks chemical bonds

mobilises hydothermal fluids of hot water and gases

softens rocks to allow deformation

PRESSURE Can be lithostatic, equal in all directions, or compressive/ non uniform which gives a degree of foliation Compressive pressure

Can be lithostatic, equal in all directions, or compressive/ non uniform which gives a degree of foliation

HYDROTHERMAL FLUIDS Water comes from minerals, rocks, seawater or from magma fluids are water with dissolved gases and elements work most effectively at high temperatures and pressures when reaction rates are speeded up fluids move through the fissures and pore spaces in rocks

Water comes from minerals, rocks, seawater or from magma

fluids are water with dissolved gases and elements

work most effectively at high temperatures and pressures when reaction rates are speeded up

fluids move through the fissures and pore spaces in rocks

 

CONTACT/THERMAL METAMORPHISM

CONTACT/THERMAL METAMORPHISM forms around an igneous intrusion the zone of alteration is called a metamorphic aureole metamorphism is by heat and hydrothermal fluids pressure does not operate so the rocks are unfoliated metamorphism varies from HIGH GRADE to MEDIUM GRADE to LOW GRADE away from the intrusion high grade metamorphic rocks have undergone the most heat and alteration by hydrothermal fluids unaltered rock well away from the intrusion is called country rock a good example is the altered slates around the Skiddaw granite in the Lake District

forms around an igneous intrusion

the zone of alteration is called a metamorphic aureole

metamorphism is by heat and hydrothermal fluids

pressure does not operate so the rocks are unfoliated

metamorphism varies from HIGH GRADE to MEDIUM GRADE to LOW GRADE away from the intrusion

high grade metamorphic rocks have undergone the most heat and alteration by hydrothermal fluids

unaltered rock well away from the intrusion is called country rock

a good example is the altered slates around the Skiddaw granite in the Lake District

SIZE OF THE AUREOLE Determined by:- Size of the intrusion shape of the intrusion type of magma - lower temperature acidic intrusions may have larger aureole due to producing more hydrothermal fluids Type of country rock transfer of heat porosity permeability mudstone produces a larger aureole than sandstone

Size of the intrusion

shape of the intrusion

type of magma - lower temperature acidic intrusions may have larger aureole due to producing more hydrothermal fluids

Type of country rock

transfer of heat

porosity

permeability

mudstone produces a larger aureole than sandstone

ROCK TYPES Argillaceous country rock: shale/slate Away from the intrusion large crystal called porphyroblasts form in a generally unchanged rock this medium grade metamorphism may give large andalucite crystals in the form of chaistolite the loose patterns of porphyroblasts shows no orientation cordierite crystals may contain other minerals to give a poikiloblastic texture Near the intrusion high grade metamorphism gives hornfels this is completely recrystallised with an equi-sized granoblastic texture

Away from the intrusion large crystal called porphyroblasts form in a generally unchanged rock

this medium grade metamorphism may give large andalucite crystals in the form of chaistolite

the loose patterns of porphyroblasts shows no orientation

cordierite crystals may contain other minerals to give a poikiloblastic texture

Near the intrusion high grade metamorphism gives hornfels

this is completely recrystallised

with an equi-sized granoblastic texture

Spotted slate

andalusite

porphyroblasts Poikiloblast of cordierite (poikilitic texture)

hornfels

CONTACT METAMORPHISM OF OTHER ROCKS Sandstone metaquartzite results large interlocking crystals with 120 degree angles a granoblastic texture Limestone pure calcite produces marble with a large grained sugary or saccharoidal texture impure limestones have other elements eg dolomite, and magnesium oxide (periclase) remains

Sandstone

metaquartzite results

large interlocking crystals with 120 degree angles

a granoblastic texture

Limestone

pure calcite produces marble

with a large grained sugary or saccharoidal texture

impure limestones have other elements

eg dolomite, and magnesium oxide (periclase) remains

metaquartzite

marble

 

 

 

 

Table 2 Common Metamorphic Rocks Rock Parent Rock Key Minerals Metamorphic Environment Foliated Slate Shale Clay minerals, micas. chlorite. graphite Relatively low temperature and pressure Foliated Schist Shale, basalt, graywacke sandstone, impure limestone Mica, chlorite, garnet, talc, epdiodte, hornblende, graphite, staurite, kyanite Intermediate - to - high temperature and pressure Foliated Gneiss Shale, felsic igneous rocks, graywacke, sandstone, granite, impure limestone Garnet, mica, augite, hornblende, staurolite, kyanite, sillimanite High Temperature and pressure Nonfoliated Marble Pure limestone calcite, dolomite Contact with hot magma, or confining pressure from deep burial Nonfoliated Quartzite Pure sandstone, chert Quartz Contact with hot magma, or confining pressure from deep burial

 

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