Metamorphism
Marble: nonfoliated
The change in solid rock as a result of chemical and physical conditions imposed at depth below levels of weathering and lithification.
There are three agents of metamorphism:
1. temperature increase: caused by residual heat of formation( Earth formed at high temperatures and rocks are good insulators); radioactive decay heat (natural radioactive elements emit heat when they decay) and the geothermal gradient (increase in temperature with increase in depth 25C/km)
Temperature increase weakens the bonds and allow for chemical and mineral changes.
2. pressure increase: contracts atoms and prevents melting
Lithostatic pressure: qual dimensional stress resulting from the weight of overlying rocks.
Geobaric gradient: increase in lithostatic pressure with increase in depth (4,401 psi)
Directed pressure: pressure applied along certain directions resulting from plate tectonic conditions
3. chemical fluids: pore fluids and water contained in minerals can carry ions allowing more stable metamorphic minerals to form
There are four types of metamorphism
1.contact metamorphism: results from rocks being adjacent to igneous intrusions. Characterized by high temperatures and active chemical fluids (divergent plate boundary)
2. dynamic metamorphism: results from directed pressure along faults. Characterized by strong directed pressure and increased temperature due to depth. transform plate boundary)
3. burial metamorphism: results from weight of overlying rocks in thick accumulations of sedimentary and volcanic rocks. Charaterized by strong lithostatic pressure and increased temperature (geothermal gradient); occurs at depth greater than 10km and results in nonfoliated rocks.
4. regional metamorphism:results from mountain building and plate tectonic collisions. Characterized by strong directed pressure and increased temperature due to increased burial. Results in foliated rocks (convergent plate boundary)
Metamorphic rocks are classified basesd on their texture and composition. The different textures and compositions provide information about the conditions of metamorphism and the protolith (parent rock)
Texture
1. foliated
a.slaty cleavage: planar parallel alignment of fine-grained clay size minerals that form planes of weakness in the rock
Example: Slate
b. schistosity: planar alignment of platy grains
Example: talc schist, chlorite schist, garnet mica schist, staurolite schist
c. gneissic layering: distinct bands or layers of dark and light colored minerals
Example: gneiss
2. nonfoliated
no preferred alignment of minerals that result from lithostatic pressure or absence of platy grains
Examples: marble, quartzite, amphibolite
Composition
1.protolith composition
parent rock determine what minerals can form
Examples: clay-rich can only form slate or schist
calcite can only form limestone
quartz can only form quartzite
2.conditions of metamorphism
a. metamorphic grade- low or high tempertures and pressures
b. index minerals- form under specific and limited temperature and pressure conditions Ex. garnet, kyanite, staurolite, chlorite
c. metamorphic facies- rocks with similar metamorphic minerals because they formed under similar conditions Ex. green schist and blue schist
There are three agents of metamorphism:
1. temperature increase: caused by residual heat of formation( Earth formed at high temperatures and rocks are good insulators); radioactive decay heat (natural radioactive elements emit heat when they decay) and the geothermal gradient (increase in temperature with increase in depth 25C/km)
Temperature increase weakens the bonds and allow for chemical and mineral changes.
2. pressure increase: contracts atoms and prevents melting
Lithostatic pressure: qual dimensional stress resulting from the weight of overlying rocks.
Geobaric gradient: increase in lithostatic pressure with increase in depth (4,401 psi)
Directed pressure: pressure applied along certain directions resulting from plate tectonic conditions
3. chemical fluids: pore fluids and water contained in minerals can carry ions allowing more stable metamorphic minerals to form
There are four types of metamorphism
1.contact metamorphism: results from rocks being adjacent to igneous intrusions. Characterized by high temperatures and active chemical fluids (divergent plate boundary)
2. dynamic metamorphism: results from directed pressure along faults. Characterized by strong directed pressure and increased temperature due to depth. transform plate boundary)
3. burial metamorphism: results from weight of overlying rocks in thick accumulations of sedimentary and volcanic rocks. Charaterized by strong lithostatic pressure and increased temperature (geothermal gradient); occurs at depth greater than 10km and results in nonfoliated rocks.
4. regional metamorphism:results from mountain building and plate tectonic collisions. Characterized by strong directed pressure and increased temperature due to increased burial. Results in foliated rocks (convergent plate boundary)
Metamorphic rocks are classified basesd on their texture and composition. The different textures and compositions provide information about the conditions of metamorphism and the protolith (parent rock)
Texture
1. foliated
a.slaty cleavage: planar parallel alignment of fine-grained clay size minerals that form planes of weakness in the rock
Example: Slate
b. schistosity: planar alignment of platy grains
Example: talc schist, chlorite schist, garnet mica schist, staurolite schist
c. gneissic layering: distinct bands or layers of dark and light colored minerals
Example: gneiss
2. nonfoliated
no preferred alignment of minerals that result from lithostatic pressure or absence of platy grains
Examples: marble, quartzite, amphibolite
Composition
1.protolith composition
parent rock determine what minerals can form
Examples: clay-rich can only form slate or schist
calcite can only form limestone
quartz can only form quartzite
2.conditions of metamorphism
a. metamorphic grade- low or high tempertures and pressures
b. index minerals- form under specific and limited temperature and pressure conditions Ex. garnet, kyanite, staurolite, chlorite
c. metamorphic facies- rocks with similar metamorphic minerals because they formed under similar conditions Ex. green schist and blue schist
Metamorphic Rock Parent Rock
Foliated Slate shale/mudstone Schist shale/clay-rich rocks Gneiss granite Nonfoliated Quartzite quartz Marble limestone |
For more pictures please visit:
http://geology.com/rocks/metamorphic-rocks.shtml For more information please visit: http://itc.gsw.edu/faculty/tweiland/metamor.html Video: http://www.videojug.com/film/geology-metamorphic-rocks |