Geology notes 2

Where do the most catastrophic earthquakes occur?

-At convergent boundaries, or along continental transforms.

Ductile deformation

-Folds -Foliation

Brittle structures

-Joints, veins, and faults

Reflection

The bouncing back of a wave when it hits a surface through which it cannot pass.

How does faulting cause earthquakes? The two are causes for vibrations during an earthquake:

-**When a new fault forms by rupturing previously intact rock.** When a fault forms and starts to slip, it eventually slows to a stop due to friction (the force that resists sliding on a surface). -**When a pre-existing fault suddenly slips again. **

Seismic-velocity discontinuities

-A boundary in the Earth at which seismic velocity changes abruptly. -These occur at depths where pressure causes atoms in minerals to rearrange and pack together more tightly. -Moho: seismic-velocity discontinuity that defines the boundary between the Earth's crust and mantle.

Fault

-A break in the earth's crust; a fracture plane on which sliding takes place. -Once a fault has formed, it remains weaker than surrounding, intact crust. Even though it's weaker, however, friction across a pre-existing fault resists sliding, so slip is not constant.

Fold

-A curve in the shape of a rock layer. -Limbs: Sides of the fold w/ less curvature. -Hinge: Line along which the curvature of the fold is greatest. -Axial surface: Imaginary plane that contains hinge lines of successive layers and divides the fold into two halves.

Gravity anomaly.

-A departure from the expected force of gravity; may be positive or negative -Positive anomalies mean higher gravitational field; negative anomalies mean lower field, with respect to reference geoid. -Can occur due to variations in mass, such as density differences between adjacent rock types or removal of lower crust by subduction. -A positive gravity anomaly within a surveyed section of the Earth's crust could be caused by rocks that are locally denser than the surrounding rocks.

Seismic tomography

-A method that uses the seismic waves from earthquakes recorded on thousands of seismographs all over the world to sweep Earth's interior in many different directions and construct a three-dimensional image of what's inside. -Done by placing a large array of seismic recording stations over a relatively small area. Sometimes involves inducing small earthquakes. -Generates 3D images of the earth.

Vein

-A mineral-filled crack in rock.

Active fault vs. inactive fault

-Active fault: Has moved recently or may move in the future. -Inactive fault: Last moved in distant past and likely won't move again in the near future.

Earthquake magnitude

-Amount of energy released from seismic source. -Indicated by the amplitude of ground shaking recorded by seismometer.

Types of folds / geometric characteristics of folds

-Anticlines -Synclines -Monoclines -Plunging/nonplunging -Domes -Basins

Mountains related to continental collision

-At convergent plate boundary. -Major stress: compression. -Once the oceanic lithosphere between two relatively buoyant crustal blocks completely subducts, the blocks themselves collide with each other. The buoyant blocks may be large or small continents, island arcs, or large oceanic plateaus. -Suture: Boundary between blocks that had been separate before collision. -Created the Himalayas, the Alps, and the Paleozoic Appalachian Mountains.

Mountains related to subduction

-At convergent plate boundary. -Major stress: compression. -Structures: Fold-thrust belt: Produced when compressional stress develops and drives crustal shortening in the overriding plate. -In such belts, the sedimentary strata of the upper several kilometers of crust shorten as if being pushed toward the interior of the continent by an imaginary giant bulldozer. Thrust faults transport rock away from the push, and folds develop as strata move up the thrusts. -Example: Andes Mountains.

Mountains related to continental rifting

-At divergent plate boundary. -Major stress: tension. -A continental rift is a place where a continent undergoes stretching, and may eventually split in two. -During rifting, tensional stress causes normal faulting in upper crust; movement on normal faults drops down blocks of crust, which tilt over as they move. As a result, rifts contain several elongate mountain ranges: tilted blocks of crust. -Basin and Range Province in the southwestern United States

Causes of Wadati-Benioff zone earthquakes (in convergent boundaries)

-Belt of seismicity. Earthquakes occur here during subduction: --in response to stresses caused by shear between sinking lithosphere plate and surrounding asthenosphere. -In response to the pull exerted by the deeper part of the plate on the shallower part as the plate sinks. -Due to stress generated by resistance that the subducting plate encounters as it pushes into the mantle below. -Due to volume changes taking place when olivine in the plate undergoes a phase change, collapsing to form denser materials under the extreme pressure in deeplh subducted lithosphere.

Explain how geologists use seismic waves to determine the composition and structure of Earth's interior.

-Boundaries between Earth's layers are based on changes in P-wave velocities. -P-Waves slow down at core/mantle boundary and refract, causing a 'shadow' where seismic stations can't detect these waves. -S-Waves stop at core/mantle boundary because the outer core is liquid, and s-waves cannot travel through liquid.

Low-velocity zone (LVZ)

-Boundary between lithosphere and asthenosphere that has been identified as a zone of lower velocity for seismic waves traveling through mantle. -A weak layer on which oceanic lithosphere plates move. -Is 100-200 km in the mantle. -Caused by partial melting of mantle rock.

Moho

-Boundary between the crust and the mantle. -This is a seismic-velocity discontinuity

Types of strain

-Brittle deformation: occurs when rocks break and shatter, much like glass shattering when hit with a hammer. -Ductile deformation: occurs when rocks behave like a plastic; that is, they can deform without breaking. You can mold clay without breaking it, and you can squash a plastic bottle without cracking it. They are both solids, but they behave ductilely. -Elastic deformation: occurs when a rock undergoes reversible deformation. As stress is applied, the rock deforms but will return to its original shape once the stress is removed. -As the amount of stress applied to a rock increases, the style of deformation (i.e., its rheology) typically changes from elastic to ductile to brittle

Types of stress

-Compression: Rock is squeezed together on two sides, causing shortening/contraction. Happens at convergent plate boundaries. -Tension: Rock is pulled apart on two sides, causing stretching/elongation. Happens at divergent plate boundaries. -Shear stress: One part of the rock moves sideways past another; stress acting in one direction isn't same magnitude as stress acting in other direction, so they're moving in equal but opposite directions. Causes smearing. Happens at transform plate boundaries. -Pressure: Same amount of stress applied equally from all directions onto an object.

Earthquakes within continents

-Continental rifts: Stretching of crust generates normal faults. -Collision zones: Two continents collide when oceanic lithosphere once separating them has been completely subducted. These form mountains, such as the Alps and Himalayas. These earthquakes involve movement on thrust faults that accommodate crustal shortening.

The ability of a seismic wave to travel through a material, as well as the velocity at which it travels, depends on

-Density: waves travel faster through denser rock. -Rigidity: waves travel faster in more rigid material. -Compressibility: How easily a material's volume changes in response to squashing. Waves travel faster in more compressible material. -Liquid vs. solid: P- and S-waves both travel through solids, but only P-waves can travel through liquids.

Ways strain occurs

-Displacement: Change in location. -Rotation: Change in orientation. -Distortion: Change in shape. -Shear strain: Change in angular relationships.

Seismicity at each plate boundary

-Divergent boundary seismicity: Along spreading seafloor segments, stretching generates normal faults. -Transform boundary seismicity: Strike-slip faults/motion. -Convergent boundary seismicity: Normal faults develop during subduction. Large thrust faults form along base of overriding plate. Wadatii-Benioff zone earthquakes.

Domes and Basins

-Dome: fold with the shape of an overturned bowl. Sedimentary strata are oldest in the center of the dome. -Basin: Fold with the shape of an upright bowl. Sedimentary strata are youngest in the center of a basin. -Both display circular patterns that look like bull's-eyes on a geologic map.

Explain what geologists have learned about Earth's magnetic field from geophysical studies.

-Earth's liquid iron outer core generates the magnetic field via convection. -Magnetic field reverses, as proven by paleomagnetism in seafloor spreading.

Preventing earthquake damage

-Earthquake engineering: Designing buildings that can withstand shaking. -Earthquake zoning: Determining where land is stable and where it isn't. Avoid land underlain by weak mud/clay/sand that could liquefy, steep environments, and downstream of dams. -Avoid certain kinds of construction in regions with significant seismic risk; use seismic retrofitting to strengthen existing buildings in seismically hazardous areas.

Intraplate earthquakes

-Earthquakes occurring in the interiors of plates and aren't associated with plate boundaries, active rifts, or collision zones. -Account for only 5% of earthquake energy released per year. -Caused by stress applied to the boundary of a plate causing the plate's interior to break suddenly at weak, pre-existing fault zones.

Foreshock, mainshock, aftershock

-Foreshock: Smaller earthquakes preceding larger ones. Possibly result from development/propagation of smaller cracks in the vicinity of what will be the major fault. -Mainshock: The major earthquake. -Aftershocks: Occur because slip during the mainshock doesn't leave the fault in a perfectly stable position. Largest tends to be 10 times smaller than main shock; most are smaller than that.

Why do folds form?

-In response to end-on compression; if a layer is shortened along its length, it buckles. -Where shear stress gradually shifts one part of a layer up and over another part. -When new slip on on a fault causes a block of basement to move up so that the overlying sedimentary layers must warp; faulting at depth may push up a block of crust, causing overlying beds to bend into a monocline. -Movement of rock layers up/over step-like bends in an underlying fault; when layers move up and over step-shaped faults, they must bend into folds to conform to the fault's shape as they move.

Brittle-plastic transition

-In typical continental crust, rocks generally behave brittlely above a depth of about 10 to 15 km and plastically below this depth. -Earthquakes in continental crust mostly occur at this depth because earthquake generation involves brittle breaking.

Can we predict earthquakes?

-Long-term prediction estimates the probability that an earthquake will happen within a specified time range. Researchers can determine regions where earthquakes are likely, but not precisely when/where it will occur. These are believed to be fairly accurate. -Short-term predictions . These are difficult to make, and are unreliable. -However, earthquake early warning system can detect an earthquake before the seismic waves have had time to reach populated areas far from epicenter. Can provide time needed to shut down gas pipelines, trains, power, etc. to reduce damages.

L-waves

-Love waves -Shake the surface from side to side horizontally like the movement of a snake.

Magnitude

-Measure of the energy released during an earthquake. -Measured using the Richter scale, which is logarithmic, meaning that magnitude 6 is 10 times stronger than magnitude 5, and magnitude 7 is 100 times stronger than magnitude 5.

Where do earthquakes occur?

-Most earthquakes occur on faults along plate boundaries. This is because relative motion between plates causes slips on faults.

Orogeny

-Mountain building event. -Has occurred in cratons of continents. -Orogeny is commonly associated with igneous rock formation because both mountain ranges and magmas commonly form at convergent and divergent tectonic plate boundaries. -Orogeny is commonly associated with abundant sedimentary rock formation because the high topographic relief of mountains caused increased weathering and the production of sediments. -Orogeny is commonly associated with abundant sedimentary rock formation because the weight of mountain belts may depress the surface of the lithosphere, creating more space for sediments to accumulate. -Orogeny is commonly associated with abundant metamorphic rock formation from local heating by magmas and from increased burial of rock as a result of thrust faulting. Orogen: Linear range of mountains.

Joints

-Natural cracks in rocks. -Develop in response to tensile stress in brittle rock. -Can form in response to decrease in pressure or when rock cools.

Foliation

-New layering developed when inequant grains align in response to deformation, specifically to applied stress. -An arrangement of minerals in flat or wavy parallel bands. -Lineation: lines of foliation that usually form perpendicular to the direction of greatest stress.

Plunging folds and nonplunging folds

-Plunging fold: Folds with a tilted hinge, aka a "plunging" hinge. -Nonplunging folds: Folds with a horizontal hinge. Layers in a plunging fold have a U-shape on the ground surface.

Hypocenter/focus

-Point where earthquake is generated. -Usually this is somewhere deep in the crust.

Craton

-Portions of continents that consist of long-lived blocks of durable continental crust commonly found in the stable interior of a continent. -Hasn't been affected by orogeny for at least the last 1 billion years. -Have cooled and are relatively strong/stable.

R-waves

-Rayleigh waves. -Shake the surface up and down and move similar to ocean waves.

Effect of earth layer on seismic wave speed / Discovery of the Moho

-Reach a *nearby* seismometer before seismic waves that travel through the mantle. -Seismic waves traveling for most of their path in the mantle reach a *distant* seismometer first.

Ways folds develop

-Rock layers accommodate layer parallel shortening by folding. -Flexural-slip folds: A stack of layers bends, and slip occurs between the layers to accommodate the bending without producing gaps between layers. The same phenomenon happens when you bend a deck of cards - to accommodate the change in shape, the cards slide with respect to one another. -Passive-flow: When the rock, overall, behaves like weak plastic and slowly flows; these folds develop simply because different parts of the rock body flow at different rates

S-waves

-Secondary waves -Second fastest type of waves and move by vibrating rock up and down perpendicular to the direction of the wave's propagation. -Shear body waves: waves in which particles of material move back and forth perpendicular to the direction in which the wave itself moves. -Cannot travel through liquid.

Two provinces of cratons

-Shields: Where Precambrian metamorphic and igneous rocks crop out at the ground surface. -Cratonic platforms: Where a relatively thing layer of Phanerozoic sediment covers Precambrian rocks.

Modified Mercalli Intensity Scale (MMI Scale)

-Specifying earthquake intensity depends on SUBJECTIVE observations, like how the shaking felt, NOT a direct measurement of an instrument. -Uses Roman numerals, ranging from I (not destructive) to XII (highly destructive). -Intensity tends to be greatest near epicenter and decreases the further one gets from epicenter. This is because energy carried by seismic waves decreases with increasing distance from source, as rocks/sediment act as shock absorbers.

Stress vs. strain

-Stress refers to the amount of force applied per unit area of a rock, whereas strain refers to a change in shape of a rock. Strain is aka deformation. -Stress causes strain—compression causes shortening, tension leads to stretching, and shear stress produces shear strain.

Fault movement

-Strike-slip faults: Tend to be vertical breaks in crust where blocks slide past one another horizontally. -Dip-Slip faults: One rock block moves up and the other moves down; there is no horizontal movement. In this fault system, the two blocks are referred to as hanging wall and footwall Three types: ---Normal faults: The hanging wall moves down with respect to the footwall. ---Reverse faults: The hanging wall move up relative to the footwall. Cause one side of the fault to be raised relative to the other. ---Thrust faults: Similar to a reverse fault, but the angle of the fault is very low (gentle slope). -Oblique-slip fault: Sliding occurs diagonally on the fault plane. Movement along these faults is a combination of strike-slip and dip-slip; that is, there is both vertical and horizontal motion along the fault. -Stress: Force that acts on an area. Can be a result of plate motion or other forces that take place inside the Earth. -Displacement along a fault: How much net movement occurs. -Elastic rebound theory. Rock on either side of the fault behaves elastically, bending as stress is applied and energy is stored, and once the energy is released it will snap back to its original shape much like a rubber band

Joint orientations

-Strike: angle between horizontal line on the plane and the direction to true north. -Dip: Angle of plane's slope; between a horizontal plane and an imaginary line parallel to the steepest slope on the structure.

Describe what geophysics is and discuss why it is studied.

-Subdiscipline of geology focused on the quantitative analysis and modeling of physical characteristics of the Earth; it includes the study of earthquakes, gravity, and magnetism. -It's studied to learn more about inside of earth we can't observe.

Causes of mountain building

-Subduction: -Continental collision -Continental rifting

What causes earthquakes?

-Sudden fracture of the Earth (creating a fault) -Sudden slip along an existing fault (most common) -Underground movement of magma or eruptions of a volcano -Large landslide -Meteorite impact -Human activity including underground bomb tests, hydraulic fracturing, and induced earthquakes

What causes rocks to deform bitterly or plastically?

-Temperature: Warmer rocks tend to deform plastically; colder rocks tend to deform bitterly. Heat makes materials softer. Temperature increases deeper in the earth. -Pressure: Rocks under great pressure behave more plastically than under low pressures. Pressure prevents rock from separating as it deforms. More pressure deep in the Earth than near surface. -Deformation rate: A sudden change in shape causes brittle deformation, whereas a slow change in shape can cause plastic deformation. -Composition: Some rock types are softer than others.

Seismic ray

-The changing position of an imaginary point on a wave front as the front moves through rock.

Focus

-The location where seismic waves of an earthquake first begin to be generated. -For earthquakes associated with faulting: the focus represents the point where the slip on the fault initiates.

Stress

-The push, pull, or shear that a material feels when subjected to a force; formally, the force applied per unit area over which the force acts. -If the same force is applied to a small area, then a large area, the small area will experience less stress than the large area, i.e. you stand on one can vs. you stand on a board atop 100 cans. The can only crushes in the former case.

Seismogram

-The record of an earthquake's seismic waves produced by a seismograph -Horizontal axis represents time; vertical axis represents amplitude (size) of the seismic waves. -The instant at which a seismic wave appears at a seismometer station is the arrival time of the wave. -The first squiggles on the record represent P-waves, because P-waves travel the fastest. Next come the S-waves, and finally the surface waves (L-waves and R-waves). -Typically, the surface waves have the largest amplitude and arrive over a relatively long interval of time.

What happens during an earthquake?

-The stress on the rock exceeds the strength of the rock and rapid or sudden failure of the rock occurs. Seismic wave energy radiates out from point of rupture. -Before an earthquake, rock bends elastically, like a stick that you arch between your hands. -Eventually the rock breaks and sliding suddenly occurs on a fault. This break generates vibrations. -Due to elastic rebound, the rock layers straighten out.

Why do earthquakes occur?

-When masses of rock suddenly vibrate. The energy released by this process travels away from the source in the form of as seismic waves, that can reach and shake the surface of the Earth. -They occur as stress built up on opposite sides of a fault become greater than the friction between the two -fault surfaces.

Fault scarp

A small step on the ground surface where one side of a fault has moved vertically with respect to the other.

Types of seismic waves

Body Waves: Pass through the interior of the earth. Examples: -P-waves -S-waves Surface waves: Travel along the Earth's surface. Examples: -L-waves -R-waves

How do earthquakes cause damage?

By damaging land and structures, collapsing buildings. -Landslides, sinkholes, etc. caused by sediment liquefication: -**Sediment liquefication:** Shaking causes grains to settle together, causing the pressure in water filling the pores between grains to increase. The water pushes grains apart so that they become surrounded by water and no longer rest against each other, making quicksand (which is incapable of supporting weight). displacement of land -fires: The shaking can make lamps, stoves, candles, etc. fall over and cause fires. -Broken gas lines, which can feed into fires. -Tsunamis: Wave produced by sudden displacement of seafloor. Near-field tsunami travel toward shores; far-field tsunami head out across ocean. -Disease: Cut water/sewer lines, destroying clean-water supplies and exposing public to bacteria. Cut transportation lines, preventing food/medicine from reaching area.

Why are magnetic anomalies of continental crust so much more complicated than those of oceanic crust?

Continental crust is lithologically much more heterogeneous, containing a complex distribution of igneous intrusions, lava flows, and iron-rich sediments.

Strain

Deformation of materials in response to stress. -Stretching: A layer becomes longer. -Shortening: A layer becomes shorter.

Anticlines

Folds that have an archlike shape in which the limbs dip away from the hinge.

Monocline

Folds that looks like the shape of a carpet draped over a stair.

Synclines

Folds with a trough-like shape in which the limbs dip toward the hinge.

P-waves

Primary waves are the fastest waves and move like a spring, compressing and dilating rock as they propagate. -Compressional wave: Waves in which particles of material move back and forth parallel to the direction in which the wave itself moves.

Displacement

The amount of movement or slip across a fault plane.

Refraction

The bending of a wave as it passes at an angle from one medium to another

Finding the epicenter

The difference between the time that the P-wave arrives and the time that the S-wave arrives at multiple seismometer stations

Transition zone

The lower portion of the upper mantle, from 400 to 670 km deep, in which there are several jumps in seismic velocity. -It's in-between the lower mantle and asthenosphere.

Epicenter

The point on earth's surface that lies directly above the focus of an earthquake.

Elastic-rebound theory

The theory that continuing stress along a fault results in a buildup of elastic energy in the rocks, which is abruptly released when an earthquake occurs. -Earthquakes happen because stresses build up, causing rock to develop elastic strain until either intact rock breaks or a pre-existing fault reactivates. When slip takes place, the once-bent rocks adjacent to the fault rebound and vibrate back and forth until they regain their relaxed shape, thereby relieving the elastic strain.

S-wave shadow zone

Those areas more than 103 degrees from an earthquake focus where no S-waves are recorded. -S-waves cannot pass through the core at all, otherwise an S-wave headed straight down through the Earth should reach the ground surface on the other side. S-waves are shear waves, thus they can only pass through solids; the fact that S-waves cannot pass through core means that part of the core is liquid (specifically the outer core).

Where do the formation of sets of narrow mountain ranges separated by basins typically occur?

in magmatic arches

From the center of the Earth to the surface of the Earth, the correct order of Earth's physical layers is:

inner core → outer core → lower mantle → transition zone → asthenosphere → lithosphere COMPOSITIONAL LAYERS: core --> mantle --> crust

the stress necessary to reactivate a fault tends to be __ than the stress necessary to break intact rock

less than the stress necessary to break intact rock, so most earthquakes probably represent slip on pre-existing faults

Earthquake intensity

the degree of overall shaking; measured in terms of its effect on people and structures using the Modified Mercalli Scale.

As a rule, if a ray enters a material through which it will travel more slowly . . .

the ray bends down and away from the interface between two materials.

Recognizing faults

‐ fault scarp: small step on the ground surface where one slide of a fault has moved vertically with respect to the other. ‐ slickenslide: polished surface of a fault caused by a slip on the fault; lineated slickenslides have grooves that indicate direction of fault movement. ‐ sag ponds ‐ water channels ‐ breccia/damage zones ‐ striations ‐ displaced features