Geology Midterm #2

What conditions affect the deformation behavior of rock? (we discussed 4)

1) Temperature- warmer temps are more likely to have plastic behaviors 2) Pressure- higher pressure likely to have plastic behavior 3) Deformation rate- slower deformation favors plastic behavior 4) Composition of rock- softer rock types are more likely to have plastic behavior

Grain size definitions: clay, silt, sand, pebbles

1000 microns = 1 mm Pebbles > 2 mm Sand 62 microns - 2 mm - sandy Silt 4-62 microns - gritty Clay <4 microns - creamy

Within a convergent boundary, where would these metamorphic environments be found, and what rocks would be produced? 1) low-temperature, high-pressure 2) high-temperature, low-pressure 3) high-temperature, high-pressure

3 would be found in magma chamber, 2 would be found in the crust below volcanoes, and 1 would be found in the subducting slab.

How much does lithostatic pressure increase with depth?

?

Faults—what is a fault?

A fracture where sliding or slip has occured.

Which is the faster of the two body waves?

Body waves are faster than surface waves

Where would the oldest rocks be found in a dome or bowl?

Bowl = Syncline = Oldest rocks next to hinge Dome = Anticline = Youngest rocks next to hinge

Types of sedimentary rocks

Clastic: made of particles from other rocks Biochemical: made of shells Biochemical: Made of shells. Organic : made of carbon-rich relicts of plants or other organics. Chemical: minerals precipitated directly from water solutions

What are the 3 types of differential stress we considered?

Compression (shortening) Tension (stretching) Shear

Clastic sedimentary rocks

Conglomerate (large grain) Breccia (large angular clasts) (Course to very course, angular) ----- Sandstone - beach shore (Medium to coarse, quartz and feldspar sand) ------ Siltstone - shallow marine (Fine) ------- Shale - deep marine Mudstone - deep marine (Very fine)

Non-foliated:

Contact metamorphism Low grade → high grade Fine grained → coarse grained Hornfels (slicates), quartzite (quartz), marble (calcite) grade: higher grade, more severe conditions. Types of Non-foliated rock: Hornfels: Very fine grained, granular. crystals randomly oriented. silicates. Quartzite: medium grained, looks sugary. Marble: Medium grained, calcite crystals.

What does the moment depend upon?

Depends on energy released Which depends on length and distance of rupture

Typical organization of sedimentary units from shoreline beaches to deep ocean

Diagram on Slide 8 of "10.11.19 Sedimentary basins" lecture slide

How does the energy release of a Mw 7 compare with the energy release in a Mw 6 Earthquake?

Difference in 1 unit = 31 x more energy

How does the energy release of a Mw 7 compare with the energy release in a Mw 5 Earthquake?

Difference in 2 units = 1000 x more energy

How do we identify the location of an earthquake?

Difference in P Wave and S Wave arrival times at 3 different sources

Strike and dip: what does this measure?

Dip: the acute angle that a rock surface makes with a horizontal plane. Strike: the direction of the line formed by the intersection of a rock surface with a horizontal plane

What are the 3 deformation styles that occur in response to differential stress?

Displacement, change in orientation, change in shape.

Foliated:

Dominates Regional metamorphism Low grade → high grade Fine grained → coarse grained Slate, phyllite, schist, gneiss Rock usually break along the plates of rock, but gneiss can break thru grains Types of foliated rock: Slate. Very fine grained, hard and brittle. low grade Phyllite. fine grained, very small foliation. Schist: Medium coarse grained, Gneiss: Coarse grained, alternating felsic and mafic materials. high grade

What is the moment (M0) of an earthquake?

Energy Released

How are metamorphic rocks from deep in the crust ever exposed on the surface?

Exhumation. squeezed sideways like bread, getting pushed up. sun heats it up and it flows sideways, moved further by erosion.

Where is it fatter—around the equator or the poles?

Fatter around the equator Reference spheroid has radius 6378 around equator and 6357 around poles - (km?)

What are some hazards from earthquakes?

Ground shaking- building damage Landslides and rockfall Sediment liquefaction Fire Tsunamis We can assess hazard from history of earthquakes Monitoring of earthquakes Monitoring plate motion Understanding plate tectonics context and subsurface structure

What are the hanging wall and the footwall?

Hanging wall is the wall with the sharper edge, even if it is below the footwall.

What are the 4 processes involved in metamorphism?

Heat Pressure sheer - pressure/force with direction Hydrothermal fluids Steam or supercritical fluids

What are the 3 main causes (drivers) of metamorphism?

Heat, Pressure, and Hydrothermal fluids. Occurs deep in the crust.

What is a shield?

High-grade metamorphic rocks that form tectonically stable areas.

What is a metamorphic aureole?

Hot magma put pressure on surrounding sedimentary rocks and changed them to metamorphic. An area that surrounds rock that has been changed and undergone metamorphosis

What is the hypocenter (or focus) of an earthquake? the epicenter?

Hypocenter: point underground where earthquake occurred Epicenter: spot above hypocenter above ground

Oxidation/hydration cracking

In biotite (intermediate silica content) tetrahedral sheets iron oxidises, and destabilizes Potassium Ions, which causes them to leave. Water enters the structure, and the mineral expands as the biotite hydrates.

What is the moment magnitude (Mw) of an earthquake?

Index is called Moment Magnitude Mw Reported on logarithmic scale

Oxidation

Iron reacts with water to make iron oxides (rust).

What do we mean by geologic structure?

Large scale deformation of the Earth's crust = Plate tectonics Small scale deformation of the Earth's crust = structural geology

What is foliation?

Layers (visible in gneiss or schist) or alignment in Rock form by a preferred orientation (in metamorphic rocks) Alignment of platy minerals (alternating light and dark minerals). If minerals are oriented randomly, then the rock is non-foliated.

Exfoliation jointing

Like an onion skin, joints are fracture planes Unloaded is erosion taking load off rock. Will only form in strong, flawless rock. Joints are a result of erosion unloading, relieving stress.

What are the limbs, hinge line, and axis of a fold?

Limbs: sides or flanks of folds Hinge Line: where the limbs of the fold meet Axis: the intersection of the axial plane with one of the strata of which the fold is composed

How are sedimentary rocks made?

Materials are pressed together, compaction (lithification). Or minerals precipitated from water solutions on Earth's surface... just like evaporating water from salt water.

Regolith

Materials produced by weathering from bedrock.

Metamorphic rock definition

Metamorphic rock is rock formed from pre-existing rock (the protolith), by solid state change (no melting), in response to changes in its environment

Dissolutions

Minerals dissolve in water producing salt solutions and often clay minerals Clay minerals are sheets, crystals are microscopic, rich in Al and poor in cations Chemically very stable at earth's surface conditions

Chemical weathering processes: (Larger Category)

N/A

Metamorphic rocks: (Larger Category)

N/A

Physical weathering processes: (Larger Catagory)

N/A

Fault types: know the relative displacements, the stress regime in which these occur, and whether the fault is associated with shortening, extension or shearing of the crust

Note: Dipslip faults: displacement is up or down the dip direction. --- Normal fault: displacement is down the dip plane Reverse fault: displacement is up the dip plane Thrust fault: shallow dip (<30 degrees) on fault plane Strike slip fault: like a transform boundary. ships in the night Oblique slip fault: normal/reverse plus right or left lateral displacement.

In short, why do earthquakes occur repeatedly on the same failure surface?

Once a failure has occurred, that location is a plane weakness, makes it more likely for another failure to occur there, easiest place for a slip to be accommodated

What are the four kinds of seismic waves (two body waves and two surface waves)?

P-waves (primary wave, travels through all layers of earth) S-waves (secondary waves, slower, ) Love Waves () Rayleigh Waves (wave motion) --- Body waves are faster than surface waves

Weathering—physical vs. chemical processes

Physical breaks the rock apart, while chemical dissolves the less stable materials.

Folds

Places where rocks have been compressed into bends by colliding plates

Differentiate plastic deformation and brittle deformation

Plastic: Recoverable deformation Brittle: Unrecoverable deformation

Sedimentary rock definition

Rock that has formed through the deposition and solidification of sediment, especially sediment transported by water (rivers, lakes, and oceans), ice (glaciers), and wind. Sedimentary rocks are often deposited in layers, and frequently contain fossils.

What is a marine transgression?

Sea level rises in relation to the land... shorelines moves to higher ground and sediment is deposited.

What is the geoid?

Shape the earth's surface would have if only gravity and rotation were involved, and other forces were absent - mathematical ideal Potential energy is the same at all points on the geoid

How fast do seismic waves move through rock?

Speed of Sound

How fast do seismic waves move through rock?

Speed of sound

Thermal expansion cracking

Spikes in surface temperature (fire?) cause rock to expand Surface expanding, interior still cool, stress generates fractures Fractures are parallel to surface Can also happen from small temp fluctuations over long time Thermal expansion caused by fire, solar radiation, air temp. Just the top 10's cm.

What is stress, and how does it differ from force?

Stress = Force acting over an area. has magnitude and direction.

what are the components of a strike and dip measurement?

Strike: Orientation of the plane on a compass (a compass bearing with an imaginary horizontal plane interfering) Dip: Inclination of the plane (an angle) between the rock and the imaginary flat plane Direction of the dip (a compass quadrant NSEW)

What kind of fold is a syncline? anticline? dome? bowl? Monocline?

Syncline- Down Fold, limbs dip toward the hinge Anticline- Up Fold, limbs dip away from the hinge Dome- Anticline with center point Bowl- Syncline with center point Monocline- SImple bend makes it not horizontal

Where would the oldest rocks be found relative to the fold axis in an anticline or a Syncline?

Syncline- Oldest rocks next to hinge Anticline - Youngest rocks next to hinge

How is it possible to accumulate thousands of meters of sediments in an area?

Tectonic shifts that cause rifts, basins, and any other thinning of the crust tend to shift some of the mantle out of the way, creating space for sedimentation.

Why doesn't the low spot fill with sediment?

The asthenosphere that is below the sediment, deforms from the weight of the sediments above.

How do we know about the Cretaceous Interior Seaway?

The bottom of the seaway had very little oxygen, which meant that any of the unique sea creatures that died were fossilized. Fine grains (shallow to deep): sandstone---> siltstone (gritty but not course)--->limestone (small shells) (calcium carbonate) (calcite: mineral)---> shale Large, shallow sea Recorded in sedimentary rocks (sedimentary units)

What is permanent brittle deformation?

The rock cannot return to it's original shape

What is elastic deformation?

The rock returns to its original shape when the stress is removed.

What is an earthquake?

The shaking and trembling that results from the movement of rock beneath Earth's surface.

What is a marine regression?

The water retreats, leaving exposed seafloor.

What is the pattern of sedimentary bedding that shows transgression? regression?

Transgression: depositional environments moving upwards towards shore sandstone->limestone-> shale -deep depositional environments on top of sedimentary column -shallow underneath Regression: falling sea level Stratigraphic column: sandstone on top of limestone on top of shale Shallow depositional environments on top of deep

Biological processes

Tree roots, fungal hyphae growing towards minerals

How common are great earthquakes (Mw > 8)? How frequent are lower magnitude Earthquakes?

Uncommon. But how frequent?

Tectonic jointing

Uplift and displacement of the crust breaks rock, forming faults and pervasive sets of fractures called joints Regularly spaced fractures Signs of stress at great depths Lithosphere doesn't bend, it breaks Whole series of fractures that are typically not faults

Frost-cracking

Water in cracks might not freeze at exactly 0 degrees C Water can migrate through porous material to freezing area, against gravity.

What is Colorado's state rock?

Yule Marble

What is metamorphic grade?

a scale denoting the level of pressure and temperature involved in forming a particular metamorphic rock

How deep in Earth do earthquakes occur?

at divergent boundaries: Shallow transform boundaries: Shallow convergent boundaries: Shallow Intermediate Deep collision zones: Shallow Intermediate

What can we do to protect ourselves from earthquake hazards?

build buildings with reinforced corners.

You should be able to read a map and sketch a cross-section from the map, and you should be able to draw the strike and dip symbols on a map based on the cross section (as we did in ICE 8)

cool

What does geothermal gradient mean, and what is a typical geothermal gradient?

geothermal gradient = change in temperature with depth dt/dz (t is temp in C, z is depth in km) Usually about 25 C/km, but it varies from place to place Typically, western US has higher geothermal gradients Where is there a better potential for geothermal energy? Differences in tectonic settings Plate boundary on the west, but the east coast is passive margin

Limestone Chert Learning about depositional environments from sedimentary rocks:

how do Grain size, particle shape (angularity), and sorting vary with energy and or transport distance? Size decrease with more transport distance and lower energy Shape rounder with more transport distance Sorting will increase with more transport distance and lower energy Conglomerate (larger grain): close to sediment source Sediments generated→ uplands Mobilized by→ landslides, rock fall, debris flows and creep Transported by flowing water Deposited Color of rocks: red = on land (oxidizing), black/blue = organic rich, no oxygen, often water logged Sedimentary structures Surface features: mudcracks, ripple marks Internal features: cross-bedding indicates deposited in flowing water or air in dunes or ripples -With Fossils

Is the Earth a perfect sphere?

no, spinning as it formed caused it to "bulge" along the equator. Smoother than a billiards ball tho.

What does "protolith" mean?

original before change occured. separate grains, while metamorphic rock is large, interlocking crystals.

What pressure/temperature conditions and metamorphic rock types are associated with the following: 1) regional metamorphism in a mountain belt 2) contact metamorphism around a magma chamber 3) subduction zone metamorphism

subduction zone: high pressure, low temperature. Rock produced is blueshist.