Ch. 8 - Metamorphism and Metamorphic Rocks

8.1 What is Metamorphism

Compare and contrast the environments that produce metamorphic, sedimentary, and igneous rocks.

Metamorphism

Presentation content:

• Metamorphism means to “change form”

  • The transition of one rock into another by temperatures and/or pressures unlike those in which it formed
  • Changes in mineralogy and sometimes chemical composition

• Every metamorphic rock has a parent rock (the rock from which it formed)

  • Parent rocks can be igneous, sedimentary, or other metamorphic rocks

Metamorphic grade

Presentation content:

• Metamorphic grade is the degree to which the parent rock changes during metamorphism
  • Progresses from low grade (low temperatures and pressures) to high grade (high temperatures and pressures)
• During metamorphism, the rock must remain essentially solid

8.2 What Drives Metamorphism?

List and distinguish among the four agents that drive metamorphism.

Heat

• Most important agent
  • Provides the energy needed for chemical reactions
  • Recrystallization is the process of forming new, stable minerals larger than the original
• Two sources of heat:
  • Geothermal gradient: an increase in temperature with depth (about 25ºC per kilometer)
  • Contact metamorphism: rising mantle plumes

Confining Pressure and Stress

Presentation content:

• Confining Pressure

  • Forces are applied equally in all directions
    • Analogous to water pressure
  • Causes the spaces between mineral grains to close

• Differential Stress

  • Forces are unequal in different directions
  • Stresses are greater in one direction

• Compressional stress

  • Rocks are squeezed as if in a vice
  • Shortened in one direction and elongated in the other direction
  • In high pressure and temperature environments rocks are ductile and will stretch, flatten, or fold

Chemically Active Fluids

Presentation content:

• Chemically Active Fluids

  • Water becomes a hot ion-rich fluid
    • Hydrothermal solution
  • Enhances migration of ions
    • Aids in recrystallization of existing minerals
  • Can change overall chemical composition
    • In some environments, fluids can transport mineral matter over considerable distances

• The Importance of Parent Rock

  • Most metamorphic rocks have the same overall chemical composition as the original parent rock
    • Except for loss/gain of volatiles (H2O, CO2)
  • Mineral makeup determines the degree to which each metamorphic agent will cause change

Notes from the lecture:

• We can also see divergent plate boundaries where there is rifting and the bottom of the ocean is in tremendous amount of pressure.
• This can drive metarmophism at that divergent plate
• Parent Rock (what is the chemical composition) alters metamorphism greatly
  • Example:
    • Limestone (Calcium Carbonate) you alter it into a marble
    • Igneous rocks can generate new minerals from that alteration

8.3 Metamorphic Textures

Explain how foliated and nonfoliated textures develop.

Texture and Foliations

Presentation content:

• Texture describes the size, shape, and arrangement of mineral grains

• Foliations

  • Metamorphic rocks can display preferred orientation of minerals, where the platy mineral grains exhibit parallel to sub-parallel alignment
  • Describes any planar arrangement of mineral grains or structural features within a rock
  • A strong, parallel alignment of coarse mica flakes and/or of different mineral bands in a metamorphic rock.

Notes from the lecture:

• Texture: descriptor that we use to characterize metamorphic rocks
• Certain rocks will exhibit foliation
  • If you take a shale, it presents a significant amount of metamorphism

Foliations

Presentation content:

• Foliation can form in various ways, including:

  • Rotation of platy minerals
  • Recrystallization that produces new minerals perpendicular to the direction of maximum stress
  • Flattening spherically shaped grains

• Examples of foliation

  • Parallel alignment of platy and/or elongated minerals
  • Parallel alignment of flattened mineral grains or pebbles
  • Compositional banding of dark and light minerals
  • Cleavage where rocks can be easily split into slabs

Notes from the lecture:

• The first metamorphic rock form is slate
• Less heating = less pressurization

Foliated Textures

Presentation content:

• Rock or Slaty Cleavage
  • Rocks split into thin slabs
  • Develops in beds of shale with low-grade metamorphism
• Schistosity
  • Platy minerals are discernible with the unaided eye
    • Mica and chlorite flakes begin to recrystallize into large muscovite and biotite crystals
  • Exhibit a planar or layered structure
  • Rocks having this texture are referred to as schist
• Gneissic texture
  • During high-grade metamorphism, ion migration results in segregation of minerals into light and dark bands
  • Metamorphic rocks with this texture are called gneiss
  • Although foliated, gneisses do not split as easily as slates and schists

Notes from the lecture:

• Slaty CLeavage:
  • Used for roofs in places with a lot of snow
  • Slate breaks of on slices
• Schistosity:
  • Often exhibit sharp edges (there is a word for this)
  • Schist -> metamorphic rock (several different types)
    • Typically name by the dominant element in the schist
• Gneissic:
  • You can actually see the crystals

Other Metamorphic Textures

Presentation content:

• Nonfoliated metamorphic rocks are composed of minerals that exhibit equidimensional crystals and lack foliation
  • Develop in environments where deformation is minimal, and from parent rocks with equidimensional minerals (e.g., quartz and calcite)
• Porphyroblastic textures
  • Unusually large grains, called porphyroblasts, are surrounded by a fine-grained matrix of other minerals

Notes from the lecture:

• You do not get any foliation with calcite
• Porphyroblasts
  • Minerals that grow from the impurities within the rock
  • Squeeze out to generate different minerals

8.4 Common Metamorphic Rocks

List and describe the most common metamorphic rocks.

• Limestones form marbles
• Calcite forms Quartzites

Foliated Rocks

Presentation content:

• Schist
  • Medium- to coarse-grained
  • Parent rock is shale that has undergone medium- to high-grade metamorphism
  • The term schist describes the texture
  • Platy minerals (mainly micas) predominate
  • Can also contain porphyroblasts
• Gneiss
  • Medium- to coarse-grained metamorphic rock with a banded appearance
  • The result of high-grade metamorphism
  • Composed of light-colored, feldspar-rich layers with bands of dark ferromagnesian minerals

Notes from the lecture:

• Schist
  • Easily visible
• Gneiss
  • high-grade metamosrphism
  • Bands of dark ferramagnesian minerals are a characteristic of them

Nonfoliated Rocks

Presentation content:

• Marble
  • Crystalline rock from limestone or dolostone parent
  • Main mineral is calcite
    • Calcite is relatively soft (3 on the Mohs scale)
    • Used as a decorative and monument stone
    • But… weathers easily in acid rain
  • Impurities in the parent rocks provide a variety of colors of marble
• Quartzite
  • Formed from a parent rock of quartz sandstone
  • Quartz grains are fused together
  • Pure quartzite is white
    • Iron oxide may produce reddish or pink stains
    • Dark minerals may produce green or gray stains
    • Sedimentary structures can be preserved in quartzite
• Hornfels
  • Parent rock is shale or clay-rich rocks
  • Fine-grained with variable mineral composition
  • “Baked” by an intruding magma body

Notes from the lecture:

• Calcite is very suscpetible to weatehring
  • Example: TajMahal made of Marble
• Quartz:
  • As it is pressurized those crystals fuse together
  • There is basically no cross...
  • Green and grey stains -> things like iron and magnesium
• Hornfels:
  • Often the result of contact metamorphism
  • Undergoing a different process of presurization

8.5 Metamorphic Environments

Write a description for each of the following environments: contact metamorphism, hydrothermal metamorphism, subduction zone metamorphism, and regional metamorphism.

Metamorphism occurs in a variety of environments

Presentation content:

• In the vicinity of plate margins
• Associated with igneous activity
  • Contact or thermal metamorphism
  • Hydrothermal metamorphism
  • Burial metamorphism
  • Subduction zone metamorphism
  • Regional metamorphism

Notes from the lecture:

• Related to interacting with hot materials
• Contact:
  • Rock on rock (basically heating the rock)
• Regional:
  • At a collision plate boundary
• Subducting
  • As descending with depth is heated and presurized

Contact, or Thermal, Metamorphism

Presentation content:

• Results from a rise in temperature when magma invades a host rock
• Occurs in the upper crust (low pressure, high temperature)
• The zone of alteration (aureole) forms in the rock immediately surrounding the magma
• Aureoles consist of distinct zones of metamorphism

Notes from the lecture:

• Often times we refer to this zones of alteration as Aureoles

Hydrothermal Metamorphism

Presentation content:

• Chemical alteration caused by hot, ion-rich water circulating through pore spaces and rock fractures
• Typically occurs along the axes of mid-ocean ridges
  • Black smokers are the result of the fluids gushing from the seafloor
• Also occurs associated with hot springs

Notes from the lecture:

• divergent plate boundary
• Magam upwelling
• Water is heated, it is dense but it is raising
• Example: Serpentinite
  • Iron and Magnesium rich
  • Recently: Hydrogen as a main energy source? Where does it come from?
  • Ideal hydrogen comes from serpentine, by injecting carbon dioxide into the rock.
  • It is a source for hydrogen.

Burial Metamorphism

Presentation content:

• Associated with very thick sedimentary strata in a subsiding basin
• Confining pressure and heat drive recrystallization

Notes from the lecture:

• Over time most of it gets subducted away
• Some of those metamorphic rocks get stuck in the middle

Subduction Zone Metamorphism

Presentation content:

• Sediments and oceanic crust are subducted fast enough that pressure increases before temperature
  • Differential stress drives metamorphism

Regional Metamorphism

Presentation content:

• Common, widespread type of metamorphism
• Produces the greatest quantity of metamorphic rock
• Associated with mountain building and the collision of continental blocks
• Crust is shortened, thickened, folded, and faulted
  • Most likely at great depths in the crust where two continents are colliding

Notes from the lecture:

• Degree of alteration is very dictated by the temperature

Metamorphism Along Fault Zones

Presentation content:

• Occurs at depth and high temperatures
• Pre-existing minerals deform by ductile flow
  • Minerals form a foliated or lineated appearance
  • Rocks formed in these regions are called mylonites

Notes from the lecture:

• A lot of temperature (heat) but not a lot of pressure (still relatively high)
• Fault zones generates a lot of friction and a lot of heating between boundaries

• Ductile Zone: starts to act like plastic
  • Zone of mylonite

Impact Metamorphism

Presentation content:

• Also called shock metamorphism
• Occurs when meteoroids strike Earth’s surface
• Product of these impacts (called impactiles) are fused fragmented rock plus glass-rich ejecta that resemble volcanic bombs

Notes from the lecture:

• Impact generates a lot of heat -> a large explosion
• We call this shock metamorphism
• All those materials are super heated immediately
• As they cool they fuse together to form these impactiles

8.6 Determining Metamorphic Environments

Explain how index minerals are used to establish the metamorphic grade of a rock body.

Textural Variations

Presentation content:

• In areas where regional metamorphism has occurred, rock texture varies based on intensity of metamorphism

Index Minerals and Metamorphic Grade

Presentation content:

• Changes in mineralogy occur from regions of low-grade metamorphism to regions of high-grade metamorphism

• Index minerals are good indicators of metamorphic grades, and thus zones of metamorphism

• Migmatites are rocks that have been partially melted

  • Represent the highest grades of metamorphism
  • Transitional to igneous rocks

Notes from the lecture:

• Use metamorphic rocks to determine tectonic conditions, things like schist
• Example: Garnets form at certain temperature
• If some minerals are present we can know some things about the conditions
• Migmatites actually undergo partial melting

• Why would you care about this?
  • Example: Spogemine -> critical mineral for electronic
  • We can use metamorphic rocks to determine where some kind of rocks will appear

Common Metamorphic Facies

Presentation content:

• Metamorphic rocks that contain the same mineral assemblage and formed in similar metamorphic environments
• Mineral assemblages can be used to determine the pressure and temperature conditions the rock formed under

Notes from the lecture:

• We want to map out minerals for exploration purposes (facies)

Metamorphic Facies and Plate Tectonics

Presentation content:

• High-pressure, low-temperature metamorphism is associated with the upper section of subduction zones
• Regional metamorphism is associated with colliding continental blocks
• Low pressure, low- to high-temperature metamorphism is associated with divergent plate boundaries

Notes from the lecture:

• They all exhibit different temperature relations and rocks
• Now we can actually delineate temperature zones
  • Exploration/Mining purposes: how far do we need to drill?
• Most US minerals are at West US
  • Rocky Mountains, etc.

Mineral Stability and Metamorphic Environments

Presentation content:

• Some minerals are only stable at certain temperature and pressure regimes
  • Examples include andalusite, kyanite, and sillimanite
• Temperatures and pressures associated with mineral formation can aid in interpreting the metamorphic environment

• Minerals Used to Predict Metamorphic Environments

End of Chapter 8 - Concept Checks

8.1 What is Metamorphism?

1. Metamorphism means “a change in form.” Describe how a rock may change during metamorphism.
2. What is meant by the statement “Every metamorphic rock has a parent rock”?
3. Define metamorphic grade.

8.2 What drives Metamorphism

1. List four agents that drive metamorphism.
2. Which agent of metamorphism is most important, and why?
3. What characteristic of a metamorphic rock is determined primarily by its parent rock?

8.3 Metamorphic Textures

1. Define foliation.
2. Distinguish among slaty cleavage, schistosity, and gneissic textures.
3. What is meant by nonfoliated texture? Name one rock that exhibits this texture.

8.4 Common Metamorphic Rocks

1. How can slate and phyllite, which resemble each other, be differentiated?
2. Briefly describe the appearance of the metamorphic rock gneiss.
3. Compare and contrast marble and quartzite.

8.5 Metamorphic Environments

1. Name three rocks that are produced by contact metamorphism.
2. What is the agent of hydrothermal metamorphism?
3. Which type of plate boundary is associated with regional metamorphism?

8.6 Determining Metamorphic Environments

1. Briefly describe the different grades of metamorphism that might be encountered moving west to east across the Appalachians.
2. How are index materials used to determine metamorphic grade?
3. What is a metamorphic facies? What two physical conditions vary within Earth to produce different metamorphic mineral groupings?