rocks that due to increases in temperature and/or pressure, have undergone changes in mineralogy, texture, chemical composition or all three.
3 Principle Factors that drive metamorphism
driven by heat and pressure from : internal heat, weight of overlying rock (vertical pressure), and horizontal pressure.
Rates that temperature increases with depth
20-60 C / km
Low Grade Rocks
metamorphic rocks formed under the lower temperatures and pressures of shallower crustal regions
High Grade Rocks
metamorphic rocks formed at the higher temperatures and pressures of deeper zones
the increasing of temperature with depth
As the rock adjust to its new temperature what happens to its atoms and ions?
they recrystallize, linking up in new arrangements and creating new mineral assemblages.
What happens to the size of the new crystals?
they will grow larger than the crystals in the original rock
Average Temperature of kilometer per depth
Example of location where geothermal gradient is steep
Nevada's Basin 50 C per km
Example of an area where the continental lithosphere is old and think, and the geothermal gradient is shallow
beneath central North America 20 C per km
The role of heat on met rocks
can break chemical bonds and change existing crystal structures
given a specific assemblage of mineral in metamorphic rock, the geologist can infer the temperature at which the rock formed.
Primary mechanisms that form most metamorphic rocks
plate tectonic processes such as subduction and continental collision, which transport rocks and sediments into the hot depths of the crust
kinds of pressure
general force applied equally in all directions, like a pressure a swimmer feels when submerged in a pool.
Directed Pressure/ Differential stress
force exerted in a particular direction, such as when a ball of clay is squeezed between thumbs and forefinger.
location where directed pressure is usually concentrated
within zones or along discrete planes
Example of directed pressure
where plates converge, results in deformation of rocks near the plate boundary
What reduced the strength of rock?
What is directed pressure likely to cause?
severe folding and deformation of met rocks in mtn belts where temperature are high
What happens to rocks subjected to differential stress?
can cause met minerals to be compressed, elongated, rotated to line up in particular direction
during deformation how can rocks become banded?
when minerals of different compositions are segregated into separate planes
What kind of minerals/phases do confining pressure generally favor?
favors denser states, which is what inhibits melting at depth
one kilo bar is equivalent to..
the air pressure of the surface on Earth
given a specific assemblage of minerals in a met rock, the geologist can determine the range of pressures and therefore depths at where the rocks formed
How can metamorphism alter a rocks mineralogy?
by introducing or removing chemical components that dissolve in water.
a kind of change in a rock's bulk composition by fluid transport of chemical substances into or out of the rock
What do hydrothermal fluids produced during metamorphism carry?
dissolved carbon dioxide as well as chemical substances-- such as sodium or potassium --that have the capability to dissolve in hot water under pressure.
Most widespread type of met, takes place where both high temperature and high pressure are imposed over large parts of the crust.
Where does regional metamorphism occur?
volcanic arcs ( Andes), mountain chains (Himalayas), ocean trenches, converging continental plates
changes in the mineralogy and texture of rock resulting from the heat and pressure in a small area, such as the rocks near and in contact with an igneous intrusion.
What type of region does contact metamorphism affect?
a thin region of country rock along the contact.
Where are pressure effects only important during contact metamorphism?
where the magma was intruded (thrusted into preexisting rock) at great depths.
Why is contact met by extrusives limited to very thin zones?
because lavas cool quickly at the surface and their heat has little time to penetrate deep into the surroundings rocks and cause metamorphic changes.
associated with mid-ocean ridges, in which changes in a rock's bulk chemical composition are produced by fluid transport of chemical components into our out of the rock
Low Grade/ Burial Metamorphism
Metamorphism in which buried sedimentary rocks are altered by the progressive increase in pressure exerted by overlying sediments and sedimentary rocks and by the increase in heat associated with increased depth of burial in the Earth.
At what depths does low grade metamorphism typically begin?
6 - 10 km
At what temperature does low grade metamorphism range from?
100- 200 C
How many kilobars does low grade metamorphism consist of?
less than 3 kbars
What is low grade met defined as to those in the oil and gas industry?
economic basement. oil and gas wells are rarely drilled below this depth because temperatures as this depth convert organic matter into carbon dioxide instead of crude oil.
High Pressure and Ultra High Pressure Metamorphism
occurring at high pressure (8 to 12 kbars) and ultra high pressures (greater than 28 kbars)
Why are Hp and Uhp met rarely studied?
these rocks are unusual because they form such great depths that it takes a very long time for them to be recycled back to the surface.
Where are most high pressure rocks formed?
in subduction zones where sediments scraped from subducting oceanic plates are plunged to depths of over 30 km experiencing pressures of up to 12 kbars
was once located at the base of the crust and can sometimes be located at the surface. a metamorphic rock formed under very high pressure and moderate to high temperature, typically containing minerals such as coesite ( a dense high pressured form or quartz)
Sometime eclogites can contain what type of organic material?
What do geologist that that High and Ultra High met rocks that have appeared at the surface represent?
pieces of the leading edges of continents that were subducted during collision and subsequently rebounded back to the surface before they had time to recrystallize at lower pressures.
occurs when a meteorite collides with Earth.
country rock can be shattered and even partially melted. look like droplets of glass.
What are the highest pressure forms that quartz can be transformed into?
coesite and stichovite
How is the texture of met rock determined?
sizes, shapes, and arrangements of crystals. some met textures depend on the minerals formed or to original attributes of parent rock.
What kind of met occurs at divergent plate margins?
seafloor and contact around intruding plutons inthe ocean crust
What type of met occurs at convergent plate margins?
regional, high and ultra high, and contact around intruding plutons
what type of met can occur at transform plate margins?
in oceanic setting, seafloor met can occur. in both oceanic and continental settings, more extensive shearing along the plate boundary.
most prominent textural feature of regionally met rocks. a set of flat or wavy parallel planes produced by deformation.
In what way can foliation planes cut across the bedding?
at any angle of it can be parallel to the bedding.
Major cause of Foliation
the presence of platy minerals, mainly micas and chlorite.
the planes of all the platy crystals are aligned parallel to the foliation. As platy minerals crystallize the P.O is usually perpendicular to the main direction of the deformation forces squeezing the rock during metamorphism.
Most familiar type of foliation
classified according to: size of crystals, nature of foliation, degree to which their minerals are segregated into lighter and darker bands, metamorphic grade
lowest grade of foliated rocks. finely grained. commonly produced by the metamorphism of shale or volcanic ashes. splits in thin sheets.
slightly higher grade than slate. more or less glossy sheen resulting from crystals of mica and chlorite. tends to split in thin sheets, though not as perfect as slates
intermediate grade rocks. is among the most abundant met rock. higher met grades causes platy crystals to grow large enough to be visible to the naked eye. minerals may tend to segregate into lighter and darker bands. contain more than 50% platy minerals. especially micas.
produced by parallel arrangement of sheet minerals. pervasive coarse, wavy foliation.
coarser foliation, coarse bands of segregated light and dark minerals, dominated by quartz and feldspars, less mica and chlorite, can be formed from range of rock types
mixture of igneous and met rock. high temperature causes partial melt of country rock,the melt may migrate only a short distance before freezing again. badly deformed and contorted. penetrated by many veins. lenses of melted rock
composed mainly of crystals that grow in equant shapes, such as cubes and spheres rather than in platy, elongated shapes.
hard, non foliated white rocks derived from quartz rich sandstones
nf. met products of heat and pressure (contact or regional met) acting on limestones and dolomites.
nf. low grade rocks. form when mafic lavas and ash deposits react with percolating seawater or other solution. large area of seafloor covered with basalts. abundance of chlorite.
non foliated. product of medium to high grade metamorphism of mafic volcanoes. consists mainly of amphibole and plagioclase feldspar. produced when deformation occurs.
predominant minerals that define the zones. formed under a limited range of pressures and temperatures.
drawn lines that plot the transition from one zone to the next. based on index minerals.
produced when the pressure is high and the temperature is moderate. (name comes from the abundance of glaucophane, a blue amphibiole)
groupings of rocks of various mineral compositions formed under different grades of metamorphism from different parent rocks.
what does it mean when the concept of met grade is completely static?
the grade of metamorphism can inform us of the maximum press or temp to which a rock was subjected, but it says nothing about where the rock encountered these conditions or how it was transported back into the surface.
Metamorphic P-T Path
history of changes in P and T that occurred during metamorphism. The P-T path can be a sensitive recorder of many important factors that influence metamorphism and thus may provide insight into the plate tectonic settings responsible for metamorphism.
How to obtain a P-T Path
must analyze specific metamorphic minerals in the laboratory
segment indicates increasing pressure and temperature
segment indicates decreasing pressure and temperature