Midterm 2 Metamorphic Rocks

So specific grades of metamorphic minerals form under specific ___________________________ and ______________________ ranges.

temperature and pressure

Continental crust is _____________ and more __________________ than oceanic crust. A plate with oceanic crust will ______________ beneath one capped by continental crust

thicker and more buoyant subduct

migmatite

under certain conditions (high temps and in the presence of water) gneiss may BEGIN to melt, producing felsic magma and leaving behind still solid relatively mafic metamorphic rock. If the melt freezes again a mixture of igneous rock and relict metamorphic rock forms called migmatite -in effect a migmatite is part metamorphic AND part igneous.

plastic deformation

which happens when rock becomes warm enough to behave like soft plastic, so that minerals within it can change shape without breaking .

metamorphic facies

-A set of metamorphic mineral assemblages indicative of a certain range of pressure and temperature -Each specific assemblage in a facies reflects the chemical composition of the metamorphic rock in which it occurs.

plastic deformation

-At higher temperatures and pressures -rocks can change shape without breaking -stress can flatten grains into panake-like shapes that lie parallel to one another -or can stretch them into elongate)sausage shaped)shapes that align to point in the same direction

Please note as the intensity of metamorphism increases what happens?

-Crystal size gets larger - Rock gets more coarseness of foliation

Thermal or Contact Metamorphism (They are the same)

-Involves Heat...think of the marshmallow -usually involves hydrotheral fluids as well. -highest grade rocks will form closest to the pluton , while lower grade rocks will form farther away from the heat source. -there will be a metamorphic aureole (also called contact aureole) -this metamorphism is a result of HEAT and NOT a change in pressure or compression or shear. -Aureoles typically contain HORNFELS-non foliated met rock -this kind of metamorphism occurs anywhere that plutons intrude into the crust which in plate tectonic theory plutons intrude at CONVERGENT boundaries, RIFTS, and during MOUNTAIN building Thus, thermal or contact met can occur in these settings

What does the lack of foliation mean when referring to non foliated rocks?

-Means either that the metamorphism occurred in the absence of compression or shear -OR it resulted in the growth of only equant crystals Equant means a term for a grain that has the same dimensions in all directions

Change agent-Metamorphism due to changing both pressure AND temperature

-On Earth, changes in temp and pressure increase together with increasing depth Why? rocks deeper int he crust lie beneath the weight of more overburden, so the pressure is greater ad because rocks deeper in the crust endure higher temperatures due to the GEOTHERMAL GRADIENT -The stability of a mineral to form and survive depends on BOTH temp and pressure. As depth increases the original mineral assemblage in a rock becomes unstable and reactions take place that produce a new, stable assemblage. As a consequence, minerals that grow in a metamorphic rock at 15km differ from those that grown in metamorphic rock at 30 km.

Metamorphism in Subduction Zones Blueschist

-Rare Rock -requires very high pressure but relatively low temperatures -contains an unusual blue colored amphibole -these conditions (high pressure, low temp) CANNOT develop in continental crust for the high pressure needed to produce blue amphibole the temp in the continental crust IS TOO HIGH. -SO to figure out where this can form plate tectonics determine that blue schist develops ONLY in the ACCRETIONARY PRISMS that form at subduction zones. -prisms grow to be over 20km thick -rock at the base of the prism has lot of pressure put on it. but because sub ducted oceanic lithosphere beneath the prism is cool, temperatures at the base of the prism remain relatively low. -rocks develop a foliation (schistosicty) because of the compression and shearing that takes place in an accretionary prism

Processes that take place during metamorphism

-Recrystallizaition -phase change -noecrystallzation (also known as metamorphic reaction) -pressure solution -plastic deformation (3P'sRN)

Common types of foliated metamorphic rocks

-Slate:finest grained LOW GRADE low temp/pressure -Phyllite -Schist (metaconglomerate) -Gniess (nice) HIGH GRADE high temp/pressure (migmatite) SPSG

Where does metamorphism occur? What geological settings? From the perspective of plate tectonics....you will find that the conditions under which metamorphism occurs are NOTthe same in all geologic settings

-Thermal or Contact metamorphism -Burial Metamorphism -Dynamic -Dynothermal or Regional -Hydrothermal at Mid Ocean Ridges -At subduction Zones -Shock Metamorphism

Change agent: Metamorphism due to Heating (nor pressure changes)

-When you heat a rock, its "ingredients" transform into new material- metamorphic rock -250 degrees-850 degrees -heat causes the atoms to vibrate rapidly, stretching and bending chemical bonds that lock atoms to their neighbors. -repetition of this process leads to "solid-state diffusion"

How do you distinguish foliated rocks from one another?

-according to their composition -grain size -nature of their foliation

inequant grains

-both platy ( I THINK is the flattened pancake shape) and elongated (sausage shape) are these and means that their dimensions are NOT the same in all directions Eequant grains means they do have same dimensions in all directions.

"modifications to its environment"

-change in temp and/or pressure -application of stress(directed push, pull, shear) -and/or exposure to hydrothermal fluids (very hot water solutions)

Foilation

-comes from the word folium, meaning leaf -refers to the parallel surfaces and/or layers that can occur in metamorphic rock -can give rock a striped or streaked appearance in an outcrop -can give it the ability to split into thin sheets (slate)

Hornfels

-fine grained -has a variety of minerals -mineral assemblage depends on the composition of the protolith and the temp and pressure of metamorphism -Inequant mineral grains in hornfels are randomly orientated (unlike those of schist)

Phyllite

-fine grained met rock -contains a foliation defined by preferred orientation of very fine grained white mica flakes -forms when SLATE is metamorphiszied at temps high enough to cause neocrstallization and the growth of white mica causing the rock to have a sheen

Gneiss (pronounced nice)

-forms at high temps and pressure -HIGH grade -compostionally layered metamorphic rock -alternating dark layers with high concentrate of mafic minerals (dark mafic) -with light colored layers have higher concentrations of FELSIC minerals (light felsic) -layers range in thickness to millimeters to meters -compostional layering also called gneissic banding (hence gneiss) gives gneiss a striped appearance

Marble

-from the metamorphism of limestone (protolith) -during formation, calcite of the protolith recrystallizes so fossil shells, pores space and the distinction between grains and cement disappear -thus marble has a fairly uniform mass of interlocking calcite crystals. -soft and has uniform texture -variety of colors based on impurities ***NOT all marble is non foliated. if the original protolity had layers of different impurities and shear caused the marble to flow plastically, the resulting marble has a color banding that makes it a prized decorative stone.

pressure solution

-happens where the surface of mineral grain pushes against the surface of another, under conditions that allow a water film to exist between the grains.

Change agent-Hydrothermal fluids

-hydrothermal fluids are hot water solutions -can accelerate metamorphic reactions because atoms involved in the reactions can migrate faster through a fluid than they can through a solid. -HF can also provide water that can be absorbed by minerals during metamorphic reactions and thus can influence which minerals form. -HF passing through a rock may pick up some dissolved ions and drop off others (like a bus)and can change the overall CHEMICAL composition of a rock during metamorphism (called metasomatism)

Slate

-low grade -forms under relatively low temp/pressure -foilation is called slaty cleavage -splits into long thin sheets along slaty cleavage planes -shale or mudstone are the original rock that has been metamorphisized

Schist

-med to coarse grained -foliation is a call schistosity (schist....makes sense) it has a preferred orientation of large mica flakes -forms at higher temperatures where mica crystals can grow larger -Forms from PHYLLITE (see the pattern?)

What is metamorphic differentiation?

-not a completely understood process -where minerals grow preferentially in differenty layers where chemical reactions cause felsic (lighter) and magic (dark) minerals to grow in distinct separate layers

you will see what types of foliation?

-platy mineral grains lie parallel with one another -elongated mineral grains align with one another -AND/OR the rock displays alternating light colored and dark colored layers,

A particular metamorphic mineral assemblage in a rock depends on what?

-pressure and temperature -composition of the protolith-which determines what chemicals are available for the growth of new metamorphic minerals!!!

Quartzite

-protolith is pure quartz sandstone -preexisting quartz grains recrystallize yielding new larger grains -distinction between cement and grains disappears, open pore space disappears and the grains become interlocking. -when quartzite fractures, cracks cut ACROSS grain boundaries. in contrast, cracks in sandstone (protolith) curve AROUND grains. -Quartzite is glassier and does not have the grainy sandpaper like surface of sandstone Quartzite can vary in color-white gray maroon and green depending the not the impurities it contains

Thermal or contact metamoprhism

-referred to as LOCAL metamorphism caused by the igneous intrusion. Also called CONTACT to emphasize that it develops ADJACENT to the contact between an intrusion with its wall rock

High grade

-relatively high temperatures over 600 degrees gneiss

Low grade

-relatively low temperatures(250-400degrees) slate

Shock metamorphism

-the process of changing rock in response to the passage of a shock wave (due to meteorites) is called this -quartz in rocks undergoes a phase change and becomes a more compact mineral called coesite

Formation of gneiss and why the gneissic banding?

-very high temp and pressure -sometimes the protolith itself has alternating layers say of sandstone and shale that could produce metamor gneiss with alternating bands of quartzite and mice (metamorphic rocks) -sometimes involves extreme SHEAR....contrasting rock types are smeared into parallel layers shearing stretches and smears out any pre existing compositional contrast in the rock -banding my also develop by metamorphic differentiation

Recrystallization in a metamorphic rock occurs when?

-when rock is subjected to increased temperature and/or pressure BUT while the rock is still SOLID

What factors determine what type of metamorphic rock forms? Classification of metamorphic rocks is extensive and technical.

1. original parent rock 2.the heat and pressure the rock endured 3. the presence or absence of fluids within the rock

How many facies are there?

8 recognized facies know the extremes -boundries are not precisely determined (gradational and approximate) -transitions between facies are gradual

What is a shield?

A broad region of long lived stable continental crust where sedimentary "cover" either was not deposited or has been eroded away so that the Precambrian "basement" rocks are exposed. These basement rocks wet metamorphosed during a succession of Preambrian mountain building events that led to the growth of continents in the first place.

Dynamic Metamorphism

A consequence ONLY of shearing (along a fault line) under metamorphic conditions WITHOUT requiring a CHANGE in pressure and temperature. there is still some pressure though...just not a change. Rocks become plastic like Resulting rock is MYLONITES-has foliation roughly parallels the fault. Mylonites are FINE GRAINED due to processes during dynamic metamorphism that replace larger crystals with a mass of very tiny ones. Because dynamic met takes place where faulting occurs at depth in the crust mylonites can be found at all plate boundaries, rifts and in collision zones.

During metamorphism, a metamorphic rock can differ in appearance from its prolith a lot or a little?

A lot....the example given is similar to the difference between a butterfly and a caterpillar.

Environmental conditions that drive metamorphism can be called what?

Agents of metamorphism. Rocks undergo metamorphism when they are subjected to agents of metamorphism. Temp, pressure, stress and fluids

Burial Metamorphism

As sediment undergoes burial in a subsiding sedimentary basin the pressure increases due to the weight of overburden. The temp also increases due to eh geothermal gradient. At depths greater than 8 to 15 km temps may be great enough for metamorphic reactions to begin and LOW grade metamorphic rocks form -metamorphism due ONLY to the consequences of very deep burial is called this.

Why does a rock have foliation?

Because its minerals have a preferred gender identity (ha ha...want to see if your reading!) preferred orientation AND/OR because it contains alternating dark-colored and light -colored layers.

Geologist discovered that metamorphic rock DOES NOT contain a hodgepodge of minerals formed at different times and in different places but rather......

Contain a DISTINCT set of minerals that grew in association with each other at a certain pressure and temperature. THis is called mineral assemblage ...and more or less represents a condition of CHEMICAL EQUILIBRIUM ...meaning the chemicals making up the rock had organized into a group of minerals that were "comfortable" with each other and their surroundings and did not feel the need to change further.

solid state diffusion

During diffusion, atoms either rearrange within grains or migrate into and out of grains-Diffusion allows RECRYSTALLIZATION and/or NEOCRYSTALLIZATION

Dynamothermal or Regional Metamorphism

During mountain building, convergent margin boundaries and continental collision, rocks end of at great depth beneath mountain ranges. In this environment, the Protolith: 1. heats up because of the geothermal gradient and because of igneous activity 2. endures great pressure because of the weight of overburden 3.undergoes compression and shearing As a result of these changes the protolith metamophs into foliated rock that depends on the grade of metamorphism. SLATE forms at shallow depths Schist and GNEISS form at greater depths. This type of metamorphism involves HEAT AND COMPRESSION AND SHEARING. IT AFFECTS A LARGE AREA, HENCE WHY WE CALL IT REGIONAL METAMORPHISM.

How do you classify metamorphic rocks?

Foilated-which we distinguish by their component minerals and partly by the nature of their foliation Nonfoilated-distinguished only by their component minerals

Finally at great depth where the temperature and pressure are high, LARGER CRYSTALS of BLOCKY minerals such a quartz and feldspar form resulting in a HIGHLY metamorphosed rock called ______________ .

Gniess (pronounced nice)

Name the most common nonfoliated metamorphic rocks

Hornfels Quartzite Marble

Change agent- Metamorphism due to Pressure (not temp)

If you subject minerals to extreme pressure , the atoms in them pack together more closely, and denser minerals tend to form. Such transformations take place during phase change and/or neocrstallization

If the rock is buried deeper, it becomes so hot it melts, forming

Magma

Quick RECAP

Metamorphism takes place in response to: -changes in temp -changes in pressure -application of compression and shear -and/or interaction with hydrothermal fluids The process involves reactions that take place WITHOUT melting and can produce NEW TEXTURES and NEW MINERALS

Recrystallization

Mineral grains recrystallize to form new interlocking grains of the same mineral. Typically, grains get larger

When inequant minerals in a rock are parallel or aligned, geologist say the minerals have a _____________________________.

Preferred orientation -preferred orientation can develop when plastic deformation reshapes preexisting grains AND more commonly, it develops when NEOcrystallization produces new inequant crystals that become parallel or aligned as they grow.

Change agent- Stress and the development of preferred orientation

Rocks in the crust may be subjected to- -compression (squeezing)-horizontal and vertical -tension(pulling apart) -shearing-when one part of the material is pushed sideways past another part.

Example: As the sedimentary rock SHALE is INITIALLY buried it begins to compress and heat up forming what LOW grade metamorphic rock?

Slate

Compression, tension and shear are all types of.....

Stress-formally defined as the force applied per unit area of a materials surface

metamorphic aureole or contact aureole

The distinct belt of metamorphic rock that forms around and igneous intrusion. The width of the aureole depends on the size of the intrusion. larger intrusions the wider aureole

metasomatism

The process of changing a rocks chemical composition by interactions with hydrothermal fluids

Hydrothermal Metamorphism at Mid Ocean Ridges

The rising hot magma at a ridge heats water, which then convects (hot water rises, cold water sinks into crust) the hot water reacts with the crust and forms metamorphic minerals. Causes hydro(water)thermal metamorphism of ocean-floor basalt.

What are metamorphic minerals?

These are NEW minerals that grow within the solid rock only UNDER metamorphic conditions...commonly this yields a group of metamorphic minerals that geologists refer to as metamorphic mineral assemblage.

Phase Change

Transforms one mineral into another mineral (called polymorph) with the SAME composition BUT a DIFFERENT crystal structure. on an atomic scale phase change involves the rearrangement of atoms.

Metaconglomerate

Under the same conditions that produce slate, phyllite or schist, a conglomerate metamorphoses into metaconglomerate. Typically pressure solutions and plastic deformation flatten pebbles and cobbles of such rock into pancake like shapes or tension stretches them into cigar like shapes. The alignment of these inequant coasts defines a foliation.

metamorphic texture

a distinctive arrangement of mineral grains that form during metamorphism in some cases the grains that are BIGGER than those that existed in the prolith and /or by grains that interlock with each other, even if the grains in the prolith did NOT!

Intermediate

between these two extremes

"solid-state"

by this we mean that the met (now term for metamorphic) rock does NOT form by freezing magma (that's igneous rock)

"change"

by this we mean that the met rock contains NEW MINERALS that did not occur in the original prolith (starter rock) AND/OR a new TEXTURE that differs from the prolith

The higher the metamorphiced grade a rock is the more it has ______________ from its original composition.

changed -but don't be confused a higher grade means its gone lower down into the earth!

As grade increases, recrystallization and neorecrystallization tend to produce ___________ grains and new __________________ assemblages that are _______________ at _________________ temperatures AND pressure

coarser mineral stable higher

Nonfoliated metamorphic rocks

contain minerals that recrstallized or grew during metamorphism but do NOT have a preferred orientation....therefore it does NOT possess foliation

Metamorphism can be understood on a simple level if one imagines what happens to a specific parent rock as it is buried ______________ and ______________ in the earth.

deeper and deeper (thus encountering increased temperature and pressure.

What is the overall process by which deeply buried rocks end up back at the surface?

exhumation

How does metamorphic rock formed at great depth return to the Earths surface?

from a collisional mountain range, as two continents push together, the rock in the middle is squeezed upward (think of the dough picture).this upward movement takes place by slip on fautls and by plastic like flow of rock. -Next as mountain range grows the crust at depth beneath it warms up and becomes softer and weaker and able to flow like soft plastic (think cheese). Eventually the range starts to collapse under its own weight, much like the melting cheese. As a result of this collapse, the upper part of the crust spreads out laterally. Horizontal stretching of the upper part of the crust causes it to become thinner in the VERTICAL (up/down) basically its thickness. The deeper part of the crust ends op closer to the surface. Then EROSION, takes place at the surface -weathering -landslides -river flow -glacial flow all strip away rock at the surface and exposing rock that was once below the surface.

Neocrystallization (metamorphic reaction)

greek neo= new Results in growth of new minerals that differ from those of the protolith. During neocrystallization CHEMICAL reaction effectively "digest" minerals of the protolith, so the chemicals can go into production of new minerals.

Where a plate sub ducts beneath another, you'll find what kind of activity?

high mountain peaks, explosive volcanoes and violent earthquakes.

metamorphic grade

informal way to indicate the intensity of the metamorphism....meaning the amount or degree of metamorphic change -depends primarily on the TEMPERATURE of metamorphism-which plays the dominant role in determining recrystallizations and neocrystallization that takes place -TEMP determine how easy or difficult it is for chemical bonds to break, or form and therefore for diffusion to take place. Another term I believe is metamorphic FACIES-showes the intensity of the metamorphism

As it goes deeper, _______________ crystals develop (especially the flaky mineral mica) forming intermediate grade____________ then ____________.

larger crystals, phylitte and schist

Notice, as the deeper the rock goes into the Earth, the _______________ the crystals become and the more it has ____________ from the original rock

larger, changed

texture

means the arrangement of the mineral grains

Commonly, metamorphic texture causes rock to have a type of layering called?

metamorphic foilation

Difference between pressure and compression

pressure is a special condition of stress where compression is the same in all directions

Metamorphic rock is one that forms when a pre-existing rock, known as a _______________ undergoes a "solid-state" "change" in response to the "modifications of its ___________________. " This process is known as _____________________.

prolith (the "starter" rock- before it undergoes metamorphism environment metamorphism

A metamorphic rock forms by _____________________ of some or all of the minerals in an ____________________ rock. 33

recrystallization , existing

A given metamorphic facies includes _____________________ different ________________ of rocks that vary in terms of __________________ composition and THEREFORE _________________ content BUT ALL the rocks of a given ____________ formed under roughly the SAME __________________ and __________________.

several kinds chemical mineral facies temp and pressure