Exam 2 Flashcards
What kind of rocks can become metamorphic (look at rock cycle)
any!
last rock type in the story
Meta
Greek for after or beyond
Morph
change
Metamorphic
a parent rock is changed by heat and pressure to a new rock type
2 types of metamorphism
- contact
- regional
Where does contact metamorphism happen?
- Magma/lava touches rock
- Just heat, no pressure
Where does regional metamorphism happen?
- Compression due to plate collisions or burial
- Both heat and pressure
Where does metamorphism occur on Earth?
anywhere where heat and pressure can be applied to a rock without melting it
Accretionary Wedge
ocean sediments that were deposited in trench and get compressed/metamorphosed by converging plates
associated with regional metamorphism
Contact Metamorphism in Reality
a dark “layer” sandwiched between gray layers of rock
The black layer is actually an igneous intrusion…a body of magma that squeezed its way through the surrounding rocks. The name of this kind of intrusion is a sill.
The gray layers are limestones (sedimentary rocks).
The white halo here is marble, a metamorphic rock produced by heating limestones. The whiter appearance is the bake zone
One of the common locations of regional metamorphism
trench
Regional Metamorphism in Reality
Sedimentary rocks are folded and their texture changed by pressure (with some heat)
These are rocks from an ancient accretionary wedge
Regional Metamorphism
Metamorphism occurs at the core of large mountain ranges (like the Himalayas) where rocks get buried and squeezed, increasing their heat and pressure.
Also occurs where two large continental plates collide
Protoliths
Common Parent Rocks
Parent rocks
original rock before metamorphism
Common crust rocks that get metamorphosed…
limestone, shale, sandstone, granite
What Happens during Metamorphism?
Depending on the type of minerals present and the intensity and combination of heat and pressure…
In the order of increasing temp. and press.
1. Rocks become more dense.
2. Existing minerals grow larger (recrystallization).
3. Minerals become stretched (shear) and compressed and line up in one direction (foliation).
4. Minerals separate by composition (banding)
5. Brand new minerals may form (neo-crystallization)
a rock does not always go through all of these affects
The most common rock in the ocean
shale (sedimentary)
Minerals in a Shale
- clay-sized clay minerals and are deposited in a low energy environment (deeper ocean)
- Clasts in shale had a long journey from their original continental crust source (granite)
Quartz & clay is all that’s left after chemically-weathering granite
Where is shale commonly metamorphosed?
at an accretionary wedge, here, the shales are subject to low to high pressure with some heat
Steps - low grade, medium grade, high grade
- Apply Some Pressure and Some Heat
- Apply More Heat and More Pressure
- Apply Intense Heat and Pressure (not enough to melt)
Clay Minerals in Shale
microscopic sheet-silicate minerals
Micas (biotite, muscovite) are also sheet-silicates and are chemically related to clay.
Example of low grade metamorphism
Slate
{clay minerals compress, air pockets (pores) go away, and the platy clay minerals line up in one direction (foliation)}
Foliation
a metamorphic rock texture caused by directed pressure that causes minerals (typically platey and elongate clay and mica minerals) to align
{Equant minerals (square/round) do not show foliation as well (e.g., feldspar, quartz, calcite), but they do get squished!}
randomly oriented minerals: igneous or sedimentary
preferentially oriented material: metamorphic
Example of medium grade metamorphism
Schist
{- Clay minerals grow in size and become larger Mica (Biotite and Muscovite crystals.
- Mica becomes foliated.
- Note, the rock becomes shiny now because Mica is shiny}
Diffusion - a metamorphic process
- Atoms can migrate through the rock in a solid state (e.g., by diffusion) during temp and pressure changes
- Atoms re-combine to form unique, metamorphic minerals that are more stable at these high temps and pressure
Example of high grade metamorphism
Gneiss
{- Quartz grows and separates from the foliated mica.
- Some mica crystals change into feldspars and also separate from the mica (white/pink mineral).
- Separation of minerals here is called banding}
Banding
separation of minerals, shown like stripes
Compare the chemistry of gneiss, granite, and shale…
identical
(aside from the presence of water in clays in the shale)
What does limestone represent in metamorphism?
low grade
comprised of microscopic versions of the equant mineral calcite
–> chemical sedimentary rock of calcite and fossils + heat and pressure = marble, metamorphic rock
Characteristics of marble (after being metamorphosed from limestone)
Large calcite crystals, no cement, no fossils, no obvious foliation despite pressure, recrystallization occurred
What is the parthenon in Greece made out of
marble - beauty
however does not age well
What does sandstone represent in metamorphism?
comprised of sand-size quartz crystals. The crystals are often round
–> clastic sed. rock of quartz + heat and pressure = Quartzite
(metamorphic rock)
Characteristics of quartzite, after being metamorphosed
Larger quartz crystals (they grew) and no cement, lacks nay original pore spaces
Metamorphic Identification Chart (just 5 rocks!)
slate, schist, gneiss, quartzite, marble
What is Structural Geology?
The study of the 3 dimensional shape and distribution of large bodies of rock
What is the most basic structure that rocks beneath the surface can take?
Sedimentary rocks form originally in horizontal layers
ex. The Grand Canyon
Law of Original Horizontality
sedimentary rocks were originally deposited in horizontal layers (strata)
Why? water slows down as it enters a basin and sediment floating in suspension is deposited on the seafloor
Law of Superposition
the bottom layer in a stack of layers is the oldest layer, top layer is the youngest
Strata
rock layers
Structural Deformation
- Sedimentary layers stay horizontal until something happens.
- Deformation of sed. layers occurs due to stress (force) applied to a rock.
- Deformation: change in the orientation, pattern, width, length, thickness, (anything really) of a rock unit/layer
Different forms of stress, force
- compressional
- extensional
Compressional stress
At the convergence of two plates
ex. bunching up a carpet
Extensional stress
At the divergence of two plates
ex. pulling apart taffy/candy bar
Relative Tectonic Forces: Shear stress
Where rocks tear without vertical motion (Transform plate boundaries)
can cause masses of rock to slip
What structures form in sedimentary rocks due to stress?
deformation, also known as strain
Strain: the permanent result of stress in a rock
ex. - Tilted layers
- Folded layers (ductile behavior)
- Faults (brittle behavior)
- Metamorphic Foliation (pressure leads to mineral alignment and “squishing”)
Dip (tilting)
direction and angle at which the rocks tilt from original horizontal position
Tilted Sedimentary Layers
Tilting of layers can occur due to any relative tectonic motion (convergence or divergence).
Folding
- Typically caused by compression of sedimentary layers.
- Often associated with a more ductile-style of deformation.
- Ductile behavior = bending without breaking
What causes rocks to bend
Rocks may behave like a flexible plastic substance (meaning they are ductile and will permanently bend) when…
- Compressional stress is applied slowly and/or…
- When rocks are still warm/hot (buried at depth).
- Folding often occurs during mountain building at convergent margins
What causes rocks to break
When enough force is applied even something flexible can break.
We call this brittle behavior.
1. Rocks behave like a brittle solid when stress is…
- Applied quickly and/or
- When the rock is cold.
2. This behavior results in fracturing & faulting of rocks.
3. Faults are associated with any kind of plate boundary
Folds are classified into two main types:
anticline
syncline
Anticline
Fold where the strata dips away from the hinge
(makes an a frame)
striped pattern has the oldest rock layer in the middle of the fold/hinge
Syncline
Fold where the strata dips towards the hinge
(makes a v frame)
striped pattern has the youngest rock layer in the middle of the fold/hinge
Folds and Erosion
- Folding occurs during mountain building under compressional stress.
- Because the folds get pushed up into the air (uplift), they are subject to rapid erosion.
- Mountains erode very quickly (on a geologic scale = millions of years).
Appalachians
- 300 Myr Ago
Appalachian orogeny: continental-continental convergent
Orogeny = mountain building event
Mountainous area in PA with examples of folded rock {Notice the zig-zag pattern in the landscape (the dark patches are forested hills)}
Fault:
a fracture in rocks along which there has been movement
commonly associated with plate tectonics
{You can find a fault if you find offset layers of sedimentary rock.}
How Do Faults Form?
Three possibilities…
1. Compression (squeezing) of the rocks
- If rocks behave like a brittle solid you get a crack/fault during folding.
- Results in tall mountains.
2. Extension (pulling apart) of the rocks
- This forms cracks too.
- Cracking in this way forms a valley/rift instead of a mountain.
3. Transform (Shear) motion
- When rocks slide against each other.
- No compression (uplift) or extension (collapse).
How can we the the kind of relative motion (compression, extension, transform) that has occurred?
By the fault type
How do you determine the type of fault
must first identify how the land has moved around the fault
through the hanging and foot wall
Hanging wall
the block of rock that lies above the inclined fault
(overhanging)
Foot wall
the block of rock that lies below the inclined fault
(shape of a foot)
How do we identify fault types
by which relative direction the hanging wall moves (up, down, or side to side)
Normal fault
the hanging wall moves down relative to the footwall
caused by extension!
Extension
This kind of faulting occurs when the land spreads open (extension/divergence)
This creates a valley, not a mountain range
causes normal faults!
Reverse fault
the hanging wall moves up relative to the footwall
caused by compression!
Reverse fault
the hanging wall moves up relative to the footwall
caused by compression!
Compression
This kind of faulting occurs when the crust compresses/converges
Folded and faulted mountains are constructed by this process
causes reverse faults!
Thrust fault
type of low angle reverse fault with a dip of < 45°
Most common form of reverse fault associated with mountain building.
Steeper, reverse faults are rare
Which faults often occur together?
Folds and reverse (also known as thrust)
Mountain ranges that form due to plate convergence…
fold and thrust belts
Transform/Strike Slip (shear) fault
No vertical movement. Walls slide against each other (lateral movement)
Types of transform faults, two…
left lateral
right lateral
How to tell apart left from right lateral
If you are standing birds eye, looking across the fault the land appears to have moved to the left relative to where you are standing, and this will appear the same on the other side
vice versa for right
Famous transform fault example
San Andreas
The Geologic Timescale
divided into time blocks
The structure of the geologic time scale:
Eons – The largest subdivision of time (100s to 1000s Ma).
Eras – Subdivisions of an eon (65 to 100s Ma).
Periods – Subdivisions of an era (2 to 70 Ma).
Epochs – Subdivisions of a period.
Age – Subdivisions of epochs.
Goal of Today (relative age & geological age)
To be able to decipher the geologic history of Earth by interpreting a sequence of rocks (structures, layers, etc.).
Relative age
the comparative timing of events (oldest to youngest)
