Mod 3 Geo

Identify the three agents of metamorphism

Heat
Pressure

Fluids

Differentiate between the effects of confining pressure and directed pressure during metamorphism

Confining Pressure:

• Equal Pressure on all sides

• Generated by burial

• Responsible for causing chemical reactions to occur, recrystallizing minerals (just like heat can do)

• Because forces on all sides are balanced, it does not deform rock (change its shape)

Directed Pressure:

• Different amounts of pressure in each

• direction Aka. Differential or tectonic stress

• Generated by movement of tectonic plates

• Responsible for deformation (change in shape) of rock directed stress modifies the parent rock at a mechanical level, changing the arrangement, size, and/or shape of the mineral crystals. This creates unique TEXTURES

Identify the tectonic environments (i.e. plate boundary types) where metamorphism occurs

Contact Metamorphism

Regional metamorphism

Predict or describe the types of areas where contact metamorphism occurs. Regional metamorphism? Explain the difference between the processes that cause these different styles of metamorphism

Contact Metamorphism→ high T only Rocks around igneous intrusions are subjected to high T but not high P This is also commonly associated with hydrothermal metamorphism

Regional metamorphism→ high T and P Due to deep burial—all rocks in large a region are metamorphosed

Explain what rock textural features may show/create a foliation in a metamorphic rock

Foliation forms from the alignment of platy minerals, recrystallization into parallel orientations, compositional banding, and deformation of mineral grains under directed pressure.

Link the following metamorphic rock types with their protolith: Marble, quartzite, slate, phyllite, schist, gneiss

Marble → Limestone
Quartzite → Quartz sandstone
Slate → Shale
Phyllite → Shale
Schist → Shale
Gneiss → Shale

Describe the two factors that TOGETHER determine what mineral assemblage a metamorphic rock will have

Mineral assemblage is determined by the composition of the protolith and the metamorphic grade (pressure and temperature conditions)

Explain what is meant by the terms low, medium and high grade

• Low-grade metamorphism

  • Low temperature and pressure

  • Rock is only slightly changed

  • Original features may still be visible

  • Example: shale → slate

• Medium-grade metamorphism

  • Moderate temperature and pressure

  • Minerals start to grow and become visible

  • More noticeable foliation develops

  • Example: phyllite → schist

• High-grade metamorphism

  • High temperature and pressure

  • Rock is heavily altered

  • Minerals recrystallize and separate into bands

  • Example: schist → gneiss

Visually distinguish between slate, phyllite, schist and gneiss. List these in order of increasing metamorphic grade.

• Slate (lowest grade)

  • Very fine-grained, dull

  • Breaks into flat sheets (slaty cleavage)

  • Looks a lot like shale, just tougher

• Phyllite

  • Slightly coarser than slate

  • Shiny/pearly appearance (tiny mica crystals)

  • Surfaces may look wavy

• Schist

  • Visible, sparkly mineral crystals (especially mica)

  • Medium to coarse-grained

  • Strong foliation, looks glittery

• Gneiss (highest grade)

  • Distinct light and dark bands

  • Coarse-grained

  • Less shiny, more “striped” than sparkly

If given the metamorphic facies of a rock, determine the geologic setting the rock was formed in (i.e. subduction zone, continental collision, or contact metamorphism)

• Contact metamorphism

  • High temperature, low pressure

  • Happens near igneous intrusions

• Continental collision

  • High temperature and high pressure

  • Large regional metamorphism from crustal thickening

• Subduction zone

  • High pressure, relatively low temperature

  • Rock is pushed deep before it gets very hot

If given a chart showing what minerals are stable under different facies conditions, identify the facies of a given mineral assemblage

• Step 1: List the minerals present
  Look at the rock and note all minerals in the assemblage.

• Step 2: Find those minerals on the chart
  Each mineral is only stable under certain pressure–temperature (P–T) conditions.

• Step 3: Look for overlap
  The correct metamorphic facies is where the stability ranges of those minerals overlap.

• Step 4: Match to the facies name
  The overlapping region corresponds to a specific facies (e.g., blueschist, greenschist, amphibolite, etc.).

Differentiate between the following pairs of terms: Stress and Strain, Elastic Strain and Permanent Strain, Brittle Permanent Strain and Ductile Permanent Strain

• Stress & Strain:

  • Stress → A force applied over an area of rock

  • Strain → Change in shape that occurs when stress exceeds strength of rock

• Elastic Strain & Permanent Strain:

  • Elastic → Material will return to original shape when stress is removed

  • Permanent → material remains deformed when stress is removed can be ductile or brittle

• Brittle Permanent Strain & Ductile Permanent Strain

  • Brittle → Material is broken into pieces

  • Ductile → Material is stretched or bent
    

Predict which of the three types of stress would be most felt at each of the three plate boundary types

• Divergent boundaries → Tensional stress

  • Plates move apart

  • Rock is pulled and stretched

  • Think mid-ocean ridges or rifts

• Convergent boundaries → Compressional stress

  • Plates move toward each other

  • Rock is squeezed

  • Leads to folding and thrust faults

• Transform boundaries → Shear stress

  • Plates slide past each other

  • Rock is twisted sideways

  • Classic example: strike-slip faults

If given certain parameters experienced by rock (e.g. high temperature, low pressure and low strain rate), predict whether that rock is likely to deform in a brittle or ductile fashion

• High temperatures favor DUCTILE deformation

• High pressures favor DUCTILE deformation

• High strain rates favor BRITTLE deformation

• Strength of the rock you are deforming matters too

Differentiate between normal, thrust, and left-lateral and right-lateral strike-slip faults

Dip-slip faults (vertical movement):

• Normal fault

  • Caused by tensional stress

  • Hanging wall moves DOWN relative to footwall

  • Crust is being stretched

• Thrust fault (reverse fault)

  • Caused by compressional stress

  • Hanging wall moves UP relative to footwall

  • Crust is being shortened/squeezed

Strike-slip faults (horizontal movement):

• Right-lateral strike-slip fault

  • Block across the fault moves to the right

  • Imagine standing on one side looking across

• Left-lateral strike-slip fault

  • Block across the fault moves to the left

Link the fault type with the plate tectonic setting in which they are most likely to form

• Normal faults → Divergent boundaries

  • Caused by tensional stress

  • Plates pull apart → crust stretches → hanging wall drops

• Thrust (reverse) faults → Convergent boundaries

  • Caused by compressional stress

  • Plates collide → crust shortens → hanging wall moves up

• Strike-slip faults (left- or right-lateral) → Transform boundaries

  • Caused by shear stress

  • Plates slide past each other horizontally

Differentiate between anticline and syncline folds

Anticline → The oldest layers are the center of the fold
Syncline → The youngest layers are in the center of the fold

Identify the plate tectonic setting in which folds are most likely to form

Convergent plate boundaries (continental collision)

• Caused by compressional stress

• Rocks are squeezed and bent, forming folds instead of breaking