GEO 401 Exam 2

Importance of Deformation

earthquakes release energy and oil/gas stored in geological formations 

Shearing Forces

tears a rock by pushing one portion in one direction and the other portion in another - transform-fault boundaries

Compressive Forces

pushes rocks together (shortening, squeezing) - convergent boundaries

Tensional Forces

pulls rocks apart through stretching - divergent boundaries (gravitational force + mantle upwelling)

Ductile vs. Brittle Deformations

Brittle: sudden breaking, faulting (e.g. earthquakes)

Ductile: smooth, continuous plastic deformation, folding

Conditions that Vary in the Crust

temperature, confining pressure, strain rate (length/time), water availability, rock type(s)

Pressure-Temperature in Shallow Crust

low confining pressure, low temperature and water content

Brittle deformation dominates; fractures and brittle faults more likely to occur

Pressure-Temperature in Deeper Crust

high confining pressure, high temperature and water content

Ductile deformation dominates; folding of rock strata and ductile layers are formed

San Andreas Fault

Brittle behaviors (most earthquakes) along the San Andreas Fault are relatively shallow (<20 km depth)

Brittle vs. Ductile at San Andreas Fault

• Fault Breccia found at < 20 km depth (cataclastic deformation) VS. Mylonite found at > 20 km depth (continuous slow deformation)

  • Fine grained laminated rock formed by the shifting of rock layers along faults

  • fault zone characterized by cataclastic textures  

Fault Plane: Strike and Dip

Strike: direction of the intersection of a rock layer with a horizontal surface 

• Expressed as a compass direction 

Dip: measured at right angles to strike is the angle at which the bed inclined from horizontal (lifting)

• Expressed as both an angle and dip direction

Strike-Slip Fault

relative displacement (offset) of two opposing blocks of rock horizontal to the surface

• Displacement parallel to strike of fracture plane

• Dip is vertical 

• Right lateral fault: right moves DOWN from point of view

Dip-Slip Faults

involve relative movement of the formation up or down the dip of the fault plane (>45°)

Dip-Slip Faults (Normal)

rocks above fault plane move down in relation to the rocks below the fault plane (extensional; gravitational)

Dip-Slip Faults (Reverse)

the rocks above the fault plane move up in relation to the rocks below the fault plane (compression)

Oblique-Slip Fault

involves both strike-slip and dip-slip movement

Divergent Boundary with Normal faults

African and Arabian plate are drifting apart to form Rift Valley with extensive normal faults (tensional forces)

Thrust Faults

low-angle reverse fault, < 45°

• Large lateral displacement, repeated sequences 

• Compressive forces create a fault, old layers now overlie younger layers, erosion reveals the view we see today 

Convergent Margins (continental-continental collisions)

Compressional forces → reverse and thrust faults at shallow depths, folds at greater depths

Also in Cascade Range (Cascades) in western North America

Joints

fractures with no offsets – due to tectonics, expansion, contraction 

Folding

a result of compression

occurs when the structure is subjected to a compressive force that contracts layering (shortening at convergent plate boundaries)

Axial Planes

imaginary surface that divides a fold as symmetrically as possible

Fold Axis

line made by a length-wise intersection of the axial plane with the beds

Plunging Fold

fold with a non-horizontal (plunging ) fold axis

Anticline Fold

convex-upward fold whose core contains stratigraphically older rocks

two sides of the folds have limbs

Syncline Fold

concave-upward fold whose core contains stratigraphically younger rocks

two sides of the folds have limbs

Symmetrical Limbs

limbs dipping symmetrically away from axial plane (vertical)

Asymmetrical limbs

beds in one limb dip more steeply than those in others

Overturned limbs

bent so badly such that one limb has been tilted beyond vertical

Geologic Maps and Cross Sections

show spatial relationships of different formations; strike and dip are recorded as symbols and different rock types are assigned different patterns

shows a vertical slice along particular plane through crust

Relative Age

the occurrence of one event/rock relative to another – which one is older or younger geologically

• principle of original horizontality is in place (younger on top, older on bottom)

Absolute age

age of an event in years in the past (Ka, Ma,Ga) – when did an event happen?

Principle of Superposition

in a sequence of undisturbed layered rocks, the oldest rocks are on the bottom while the younger rocks are on the top

Principle of Stratigraphy

study of rock strata in distribution, disposition, and age of sedimentary rocks

Cross Cutting Relationships

faults and igneous intrusions must be younger than the rocks they cut 

Principle of Faunal (group of animals in a specific region) Succession (series of rock strata)

sedimentary strata in an outcrop contain fossils in a “definite” time sequence from older to younger strata. Sequences can be found in outcrops among other locations that can be matched to each other.

• Used to establish Continental Drift and Plate Tectonics Theory

Time Scale for Relative Ages

distinguished by assemblages of fossils in sedimentary rock formations were used to establish geological time scales before absolute age dating was invented (paleozoic → mesozoic → cenozoic). Some of the geologic time scale is related to mass extinction of life.

Gaps in Geological Records

not all geological strata were preserved. Some were disturbed and eroded away (missing records)

Unconformity

a surface between two layers that were laid down in broken sequence (a gap in the record)

Angular Unconformity

younger sediments rest upon the eroded surface of tilted or folded older rocks

Deposition and lithification, uplift and deformation, erosion (missing strata), and subsidence and further sedimentation

Disconformity

occurs between beds that are parallel

Deposition and lithification, uplift, erosion (missing strata), and subsidence and further sedimentation

Nonconformity

occurs between stratified rocks above and unstratified igneous or metamorphic rocks below

Relative Timing by events

Lithification → uplifting and deformation → magma intrusion → faulting

Geological Time Scale (oldest to youngest)

hadean, archean, proterozoic, phanerozoic

Radioactive Isotope Dating

radioactive isotopes are not stable and decay to become daughter isotopes over time. Used as geological “clocks”

Radioactive Decay of 14C to Stable 14N isotope

goes from 6 protons/8 neutrons to 7 protons/7 neutrons 

A neutron decays, ejecting an electron, and producing a proton, which changes the atom 

Radioactive Decay of 87RB to 87Sr

goes from 37 protons/50 neutrons to 38 protons/49 neutrons 

A neutron decays, ejecting an electron, and producing a proton, which changes the atom 

Half-lives of Radioactive Decays

at a constant half life, the parent isotope decays into the daughter isotope at a constant rate exponentially (N → N/2)

Exponential Decay Function

ratio of number of daughter to parent isotope is exponentially proportional to time elapsed 

# daughter isotope/# parent isotope = (e^yr-1t-1)

• T is time passed since formation of mineral 

• Decay constant reflects rate of radioactive decay for a given isotope

Zircon for Isotope Dating

Certain minerals when formed contain large amounts of uranium isotope with respect to the daughter element that can be used for uranium isotope dating 

Zircon commonly used for geological isotope dating because it preserves uranium/lead isotopes well in crystal lattice (very stable and resilient)

Earthquake

caused by the sudden breaking (faulting) that occur when one mass of rock slides past another, setting off seismic waves; duration is relatively short and is around an hour

frequency depends on earthquake size and rate of strain accumulation

Focus

site of initial rupture along a fault plane

seismic wave fronts travel outwards concentrically from the focus

Epicenter

the location on Earth’s surface above the focus where an earthquake occurs 

Elastic Rebound: Fault Rupture

Rocks accumulate strain (ductile deformation) and elastic rebound when fault ruptures (brittle)

What happens during an Earthquake?

Stress builds as tectonic forces deform rocks on either side of a locked fault. When the stress exceeds the strength of rocks along the fault, the fault slips, releasing the stress suddenly and causing an earthquake.

Foreshocks

occur before the earthquake (near the focus of future earthquake)

Aftershocks

occur after the earthquake (more spread out and can even occur on subsidiary faults)

Studying Earthquakes

Seismic waves are recorded by seismographs – now mostly digital recordings; records both horizontal and vertical earth movements

P waves (primary or compressional waves)

the fastest of the two body types (~6-8 km/s). First to arrive at a station

travel as a series of contractions (push) and expansions (pull)

directions are parallel to the direction of movement (similar to sound waves) 

S waves (secondary or shear waves)

body waves travel typically half as fast as P waves (~4-5 km/s)

travel direction is perpendicular to direction of material movement

do not pass through liquids

Surface waves

travel at and near the earth-air interface, and are the slowest and last to arrive. Travel at speeds lower than shear waves

amplitude of motion decreases exponentially with depth

largest amplitude, longest period, most destructive

Locating Earthquakes

Seismographs record arrival times of P and S waves 

Time intervals between P and S waves at seismic stations with distances from each other are used to estimate the location of the epicenter

Determining Epicenter and Focal Depth by Triangulation

With multiple stations, the location of the epicenter can be estimated

If seismologist draws a circle with a radius calculated from the travel-time curves around each seismographic station, the point at which the circles intersect will locate the earthquake epicenter

Richter magnitude

a logarithmic measure of how much the ground moved at the seismograph as seismic waves pass by

Moment Magnitude (Mw)

a logarithmic measure proportional to total area of fault rupture and seismic energy released

Modified Mercalli scale

a “measure” of the perception of the earthquake –  what people felt, and how much damage there was. Scale is useful for studying historic earthquakes that occurred prior to modern seismographs

Richter Scale

measures the amplitude of largest seismic wave, and the time interval between the P-wave and S-wave arrivals to determine the distance from the epicenter to the seismograph. Plotting the two measurements on the graph and connecting the points will determine the Richter magnitude

Earthquake and Energy Release

Amount of energy released by fission bombs can range between the equivalent of less than a 1000 kg of TNT upwards to around 500,000,000 kgs

Tohoku Earthquake (2011)

occurred at the subduction zone, where the Pacific Plate subducts underneath the Eurasian plate. The megathrust earthquakes are among the world’s largest, with moment magnitudes (Mw) that can exceed 9.0

Deep Earthquakes at Subduction Zones

earthquakes tend to be shallow (less than 20 km due to brittle deformation) but at subduction zones, earthquakes can extend down to almost 700 km  

Different Faults and Forces

Normal Fault: tension forces

Thrust fault: compression forces

Strike-slip fault: shearing forces

Studying Earthquakes Using…..

seismometers, GPS measurements of “silent” earthquakes, creep events and continuous creeps

North Anatolian Fault: Right Lateral

An active right lateral strike-slip fault in northern Anatolia which runs along the boundary between the Eurasian plate and Anatolian plate. It is thought that an earthquake will soon strike near the city of Istanbul.

Divergent/Transform Boundary Fault Mechanisms

shallow earthquakes coincide with normal faulting at divergent boundaries and with strike-slip faulting at transform-fault boundaries

Convergent Boundary Fault Mechanisms

Large shallow earthquakes occur mainly on thrust faults at the plate boundary. Intermediate focus and deep focus earthquakes occur in the descending slab

Transform Boundary Fault Mechanisms

occurs especially on continental crust 

Intraplate Fault Mechanisms

Can occur distant from plate boundaries, typically with shallow-foci. Some of the most destructive earthquakes (e.g. New Madrid, Missouri in 1812) occurred on old faults that were once part of ancient plate boundaries. These faults remain parts of the crustal weakness that releases stress

Earthquake Hazards

Cause loss of life and property damage through: faulting and shaking, landslides and ground failures, tsunamis, and fires 

Collapse of buildings and structures are the leading cause of casualties

PGA (peak ground acceleration) in gravity unit

expected with 5% probability in next 50 years

Tsunamis

Generated by earthquake motions or slumps on the seafloor

Can travel at speeds of 400-800 km/hr and form waves over 20m high as they break on shore 

Deadliest and most destructive hazards associated with the world’s largest earthquakes – megathrust events in subduction zones, e.g. Tohoku earthquake

Earthquake Prediction

Large earthquakes do tend to follow a cycle of rupture, followed by declining aftershocks and a period of quiescence. During the quiet period strain is building towards another rupture

Recurrence intervals vary from tens of years to thousands

Real time forecast and warning 

Reducing Earthquake risks

Hazard characterization, land-use policies, earthquake engineering, emergency preparedness 

Diagrams of Forces causing Deformation and at Plate Boundaries

Seismic Wave Velocities

When waves move from one type of material to another, they changed speed and direction 

Seismic velocity depends on the composition of the type of material and pressure-temperature

• More dense = faster velocities 

• Cold = stiff = fast velocity 

• Hot = soft = slow velocity

Velocity Profiles of Earth’s Center

Velocity of P waves and S waves increases with increasing depth, because the rocks become denser as they are squeezed by the weight above

Pressure (compression) is a major factor while temperature usually is not

Jumps in velocity at ~400 and ~660 km reflect structure changes in minerals – the transition zone. Used as anchor points for geotherm

Seismic Wave Reflection and Refraction

Changes in rock properties at boundaries in Earth’s interior can cause seismic wave reflection (bounce off at boundary) or refraction (bent within a layer)

P-wave Paths through Earth’s Interior

Refraction of P-waves at core/mantle boundary results in a shadow zone between 105° and 142° from epicenter 

• Refraction helps constrain core’s density

S-wave Paths through Earth’s Interior

Shear waves cannot pass through the outer core

This produces a “shadow” region at greater than 105° from an earthquake epicenter where no S-waves are observed (liquid outer core)

Why can S waves be found in Earth’s inner core if they can’t pass through liquid?

S waves can be found in Earth’s inner core because P waves can split into the two types after reaching the inner core 

Seismic Inversion Method: Seismic Wave Velocities 

Recorded arrival times of P and S waves and their locations (distances from each other) are used to model P-wave and S-wave velocities in Earth’s interior through seismic inversion method

How do we know Earth has an iron core?

Earth is too massive to be made of just silicate rocks

Primitive meteorites (building blocks for planets) are much richer in iron and nickel than crustal or mantle rocks

Seismic wave velocity profiles of core match that of Fe-Ni alloy well

Earth’s angular momentum (moment of inertia) suggests much of its mass is near the center. Most likely candidate element is iron. 

Earth’s Internal Temperature

Heat flow through Earth’s Interior: conduction (lithosphere), convection (mantle/core), radiative (smaller contribution)

Geothermal gradients: normally 20 to 30 K/m while 0.3 K/m in mantle

Lithosphere: Conductive Cooling

Relationship between the age and depth of the seafloor is fundamentally a function of temperature

• Conduction is main heat transport mechanism

Mantle Convection Heat Transport

Transport heat efficiently as compared with heat conduction 

Temp gradient in mantle is ~0.3 K/m

Earth’s Magnetic Fields

produced through vigorous convections (velocity at ~mm/s) of liquid iron in outer core 

Geotherm of Earth’s Interior 

Temperature increases with depth 

Anchor points at transition zone and inner core boundary 

Heat flux across mantle powers geodynamo

Magnetic Polar Wonder

Magnetic north pole is tilted about 10° from North Pole

Location of North magnetic pole changes over time indicating dynamics of outer-core convection patterns

Paleomagnetism in Rocks

Magnetic mineral grains transported to ocean with other sediments become aligned with Earth’s magnetic field while settling through water

Orientation is preserved in lithified sediments, which thus “remember” the field that existed at the time of deposition (e.g. Magnetite)

Paleomagnetic Time Scale

Oldest magnetised rocks formed about 3.5 Ga (early magnetic field)

• Good record of geomagnetic reversals back to about 60 Ma

Magnetic Reversals

Polarity of Earth’s magnetic field has changed thousands of times in Phanerozoic

Most recent reversal was about 30,000 years ago; end of last significant reversal was approximately 700,000 years ago 

We are in magnetic reversal period (lasts about 1000 years)