GLG101:PhysicalGeology&...

G. Calderone- GLG101: Physical Geology Lecture Reviews, Exam#3, October 19, 2015 of 20

GLG101: Physical Geology Lecture Review Series Instructor: Gary Calderone 623.845.3654; PS105

GLG101: Physical Geology Lecture Outlines for Exam #3:

Geologic Time (p. 2-‐6) Geologic Structures & Mountain Building (p. 7-‐9)

Earthquakes (p. 20-‐16) Earth Interior (p. 16-‐20)

References to Tarbuck, Lutgens & Tasa (or T & L) refer to the 11th or custom GCC edition.

20 pages including this cover


GLG101: Geologic Time Lectures-‐ Reviews Time and Terrestrial Change

Our concept of time Time and Geologic Processes

Dramatic Changes = Catastrophic change •Volcanic Eruptions •Earthquakes & sudden uplifts of land •Tsunamis •Floods •Point: some processes occur episodically as dramatic events through time. In short, over large

amounts of time, these "catastrophes" are quite usual, although we tend to think of them as somewhat rare.

Subtle Changes = Slower almost imperceptible uniform changes •Sea level rise (Seaport measurements, 1.2 mm/year) •Uplifts of land (California example-‐ 1.5 meters/century) •Cutting of the Grand Canyon (.7 mm/year) •Point: some processes occur slowly and uniformly through time. Although seemingly inconsequential

even over the last century, uniform processes can create impressive geological features. Relative versus Absolute (Numerical) Time

Relative Time: a sequence of events-‐ what came first, second, third…last etc. Absolute Time: assigning some numerical estimate of age to an event.

Principles of relative dating-‐ no jokes please

•Superposition •Original Horizontality •Lateral Continuity •Cross-‐cutting Relationships •Inclusions/Baked Contacts

Unconformities (or erosional surfaces):

Angular unconformity = erosional surface between two originally horizontal rocks now at different angles.

Nonconformity = erosional surface between a surface rock and a rock that must have formed deep beneath the surface.

Disconformity = erosional surface between two parallel surface rocks


Development of the Modern Geologic Time Scale •Faunal Succession (another principle of relative dating) -‐Life has varied through time -‐Fossil assemblages can be distinguished from one another -‐Relative ages of fossil assemblages can be determined using the other principles of relative time.

The succession of fossils or fossil assemblages from oldest to youngest is the same everywhere. This allows us to make a relative time scale based on the life forms found in the rocks.

Relative Geologic Time Scale (see attached)

•Period-‐ the basic unit of Geologic Time: Name of a place where a rock unit contains a distinct faunal assemblage. Cambrian system named for old Roman names for Wales = Cambria. Silurian named for the Silures, an ancient Welsh tribe that occupied that region-‐ and so forth. • Periods of the Relative Geologic Time Scale developed by Sedgewick and Murchison-‐ 1835. Cambrian Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, Permian, Triassic,

Jurassic, Cretaceous, Tertiary, Quaternary. Memory device for periods from oldest to youngest: •Can Old Senators Demand More Political Power Than Junior Congressmen? Tough Question.

•Other Time units: Periods are the basic time units but these are grouped and subdivided for

convenience. •Periods grouped into Eras. Eras named for the degree to which life is similar to present.

Paleozoic-‐ "ancient life"; life very different from today (Cambrian – Permian) Mesozoic-‐ "middle life"; life between ancient and recent (Triassic-‐ Cretaceous) Cenozoic-‐ "recent life; life resembles today's fauna and flora. (Tertiary-‐Quaternary)

•Eras grouped into Eons. Eons named for visibility of life Phanerozoic-‐ "visible life"; forms are visible to naked eye. Proterozoic-‐ "early or proto life"; microscopic forms-‐ mostly primitive algae Archean-‐ this is actually named for a distinct assemblage of rocks assumed to be older than

life-‐ (recent evidence suggests life also existed in Archean time)


Rock Units and Correlations Just as the fundamental unit of Geologic Time is the Period, the fundamental unit of rock is called the

Formation. A Formation is a single mapable rock type or lithology (e.g. the Coconino Sandstone); or multiple lithologies having some related or common characteristics (e.g. the Kayenta Formation).

Rock Formations can be divided into Members (e.g. the Shinerump Member of the Chinle Formation). Rock Formations can be parts of larger Groups (e.g. the Unkar Group of the Grand Canyon area). Groups can be further gathered into Supergroups (Grand Canyon Supergroup) Lithologic correlations: Rock Formations may be traced laterally for hundreds of miles. This is called

lithologic correlation. Such correlations do not necessarily indicate same time because of environment transgressions and regressions

Time correlations •Key bed correlations (ash layers are best as they indicate a single time event) •Biostratigraphic correlations-‐ rocks with the same fossil assemblages are the same age. Guide or Index

Fossils are easily recognized, abundant & geographically widespread, and have a narrow range of existence. These are useful when a full assemblage is not present.

Absolute or Numerical Time

Principles of Absolute Dating: Any regularly occurring event can be used to numerically date at geological or archaeological feature. •Tree ring dating: Can count ring sequences in trees to obtain age of wood and times of drought and

abundant rainfall. •Glacial Varve dating: Can count couplets of dark and light layers of sediment in glacial lakes to

determine years that it has been depositing sediment. •Radiometric or isotopic dating

Radioactive isotopes: An unstable isotope of an element. Emits radiation to decay to a stable daughter isotope of another element.

Half-‐life: the time it takes for half of the radioactive parent isotope to decay to a stable daughter. Calculation of ages: Determine amount of parent/daughter in a mineral or glass to determine age.

For most circumstances, these quantities are then fed into the radioactive decay equation for the particular isotope system: N = Noe-‐λt, where N is the amount of parent remaining, No is the original amount of the parent (= remaining parent + daughter), λ is the radioactive decay constant for that parent-‐daughter pair, and t is time. The equation is then solved for time.


For even multiples of ½, however, the age can be calculated by multiplying the half-‐life value by the number of multiples of ½ that it takes to equal the amount of radioactive parent left in the sample. For example, ½ = 1 half life; ¼ = 2 half lives; 1/8 = 3 half lives; and so on.

Commonly used isotopic systems Long-‐lived (half lives are in billions or millions of years)

Uranium-‐Lead (238U decays to 206Pb with a half life = 4.5 billion years) Rubidium-‐Strontium (87Rb decays to 87Sr with a half life = 48 billion years) Potassium-‐Argon (40K decays to 40Ar with a half life = 1.3 billion years)

Short lived (half lives are in thousands of years) Carbon-‐Nitrogen (14C decays to 14N with a half life = 5700 years)

•What are we dating? Minerals, glasses at the time of their isotope closure Usually minerals in igneous and metamorphic rocks Cannot easily date sedimentary rocks-‐ so how do we attach numbers?

Attaching numbers to the Geologic Time Scale

Combination of radiometric dating and superposition and crosscutting relationships of igneous and metamorphic rocks relative to the sedimentary sequences. Example: Sequence intruded by dikes and interbedded with lava flows Do this all over the globe and the system boundaries can be defined and refined in absolute time. This process is ongoing and continues to this day. In other words, the absolute age assignments to the Geologic Time Scale are constantly being revised

•Summary diagrams-‐ attached •Summary films -‐ Geologic Time-‐ Annenberg (Earth Revealed Series); Geologic Time-‐ Britannica Both available in media center library.


GLG101: Structural Geology and Mountain Building Lectures-‐ Reviews Basic concepts-‐ Stress and strain-‐ [Free physics lesson-‐ no charge!]

Stress = force applied over a given area. Forces caused largely by interaction of plates as a result of the Earth's internal heat engine trying to lose heat.

Strain = response of rocks to stress-‐ also called deformation. Stress-‐ types of:

Compressional = convergent plate boundaries Tensional = divergent plate boundaries Shear = transform plate boundaries

Strain Elastic strain = return to unstrained state with removal of stress Inelastic strain = not returned to unstrained state

Brittle = fracture-‐ material physically breaks along discrete and separate planes Ductile strain = Plastic-‐ material changes shape

Factors governing how a substance will deform/strain under stress Strain rate-‐ how fast material is deforming Material type Temperature (& confining pressure) conditions -‐ When cold materials will exhibit brittle behavior

under most stresses. When hot, materials behave plastically. Folding-‐ Inelastic, ductile deformation under compressive stress

Basic parts of a fold and description of its geometry: Hinge & Limb-‐ the bent and straight parts of a fold, respectively Axial plane-‐ imaginary plane that divides the fold in half Fold axis-‐ imaginary line running through the hinge of the fold Plunge of fold-‐ angle that the fold axis makes relative to horizontal Strike and Dip = describing tilt of bedding or other planes Strike and dip symbols

Horizontal beds & vertical beds, overturned beds


Anticlines and Synclines: Map or outcrop pattern is mirror symmetry of rock units and map pattern after erosion is stripes of different rock units Anticlines-‐ Up-‐turned fold-‐ memory device-‐ resembles the letter "A" or "Anthill". Beds always dip away

from axial plane or fold axis and oldest rock is in center after erosion Syncline-‐ Down-‐turned fold -‐ memory device -‐ resembles a smile that begins with the letter "S"; or "Sink".

Beds always dip towards axial plane or fold axis and youngest rocks are in center after erosion Plunging folds-‐ map pattern changes to "S" or “zigzag” or “horseshoe” shaped geometry. BUT all the rules we

just established are still valid. Plunging anticlines & Plunging synclines. Structural Domes & Basins-‐ map pattern forms "bulls-‐eye" or target pattern

Domes-‐ oldest rx in center, all beds dip away from center Basins-‐ youngest rx in center, all beds dip toward center

Monoclines-‐ folds formed over fault blocks-‐ common in AZ & UT

Intensity of folding-‐ Limit of how much folding a rock can endure under compression. Eventually rocks will break

Faults

Dfn: Fracture with movement. Joint = fracture without movement Basic components of fault

Fault plane Strike & Dip Displacement Relative motion Hanging wall & footwall

Dip slip faults Normal Fault-‐ hanging wall down relative to footwall

Detachment fault= normal fault dipping less than 45° Horsts and Grabens

Reverse Fault -‐ hanging wall up relative to footwall Thrust Fault= reverse fault dipping less than 45°

Strike-‐slip faults Right lateral = dextral Left lateral = sinistral Strike-‐slip faults and transform faults

Oblique-‐slip faults Note that although most faults have some oblique component, they tend to be dominantly dip-‐ or

strike-‐ slip. So we simply classify most of them as if they were completely one way or the other.


Mountains Types of mountains

•Fold and Thrust Belts-‐ Compressional stress e.g. Himalayas •Block Faulting & Rift Valleys-‐ Tensional stress e.g. Basin & Range province of SW N. America.

Horsts & Grabens •Single volcanoes or volcanic chains (island or continental arcs)-‐ found around subduction zones •Erosional remnants-‐ present day Appalachians formed originally as a fold and thrust belt when Africa

collided with North America at the end of the Paleozoic. Eroded to flat. Exhumed by later uplift and differential erosion of weaker rock units.

Anatomy of an orogenic belt-‐ The Plate Tectonic or Wilson Cycle Passive and active continental margins Rifting and Sea-‐Floor spreading to produce Passive margin

Deposition of sediments Change in tectonic environment to produce subduction Beginning of andesitic volcanism Compressional deformation

Folding Faulting Metamorphism

Continental Collision and return to passive margin tectonics Microplate tectonics-‐ Suspect Terranes Supplemental Materials: Films: Earth Revealed Series-‐ Mountain Building & Earth Structures


GLG101: Earthquakes Lectures-‐ Reviews Elastic Rebound Theory: Stress can be stored as elastic strain, then suddenly released in part as seismic

waves. The waves travel some distance-‐ vibrating the ground or quaking it as they pass. This action is what we call an earthquake.

Basic definitions:

Earthquake (EQ) = shaking of ground by passage of seismic waves resulting from the release of stress through breakage of rock

Focus the point of rupture-‐= usually occurs below the surface of the Earth. Epicenter = the surface location directly above the point of rupture Aftershocks = smaller Earthquakes representing adjustments of remaining rock to new motion.

Causes of earthquakes

Faulting = two masses of rock sliding past one another along fault plane-‐ Elastic rebound theory. Buildup of strain, release in an earthquake, repeat

Magma motion = as magma forces its way to surface, rocks may break causing earthquakes Explosions = Volcanic eruptions or nuclear bombs Possible mineral phase changes for really deep earthquakes

Seismic Waves-‐

Body Waves-‐ Waves travel through the "body of rock or body of Earth" p waves-‐ Primary Waves-‐ compressional waves-‐ particle motion is back and forth-‐ very fast waves-‐ 4-‐7

km/sec can travel through almost any material s waves -‐ Secondary or Shear waves-‐ particle motion is up/down and sideways (perpendicular do

direction of wave travel)-‐ slower than p-‐waves for the same material-‐ 2-‐5 km/sec. Because waves propagate by shearing particles, s-‐waves cannot travel through fluids such as gasses and liquids

Surface Waves-‐ waves travel along Surface of the Earth-‐ slower than body waves

Love Waves -‐ particle motion is a horizontal shear wave-‐ cannot travel through fluid. Rayleigh Waves -‐ particle motion is retrograde elliptical-‐ similar to ocean waves. Rayleigh waves

produce very large ground motion and are responsible for much of the destructive power of EQs.


Effects of Earthquakes (see movie "When the Bay Area Quakes") Ground motion-‐ from the passage of seismic waves

The Mercali Scale: a measure of Earthquake Intensity-‐ Ground shaking “scale” Factors influencing earthquake intensity. Distance from epicenter Rock type Construction Type

Landslides-‐ triggered by passage of seismic waves Liquefaction-‐ creation of "quicksand" in wet muddy sediments Surface rupture-‐ actual ground breakage Fires-‐ from rupture and ignition of gas lines etc. Seismic Sea Waves (Tsunamis)-‐ large waves (hurricane sized or larger) caused by land shifts in earthquakes Seiches (pronounced "Say-‐shays")-‐ rocking motion of water in closed bays Aftershocks-‐ "smaller earthquakes" following the main shock. These may further damage structures

weakened by the main shock. Although there will be hundreds or thousands of aftershocks after a large earthquake, there will usually be 2 or three aftershocks of one magnitude less than the main shock. If the main shock was a magnitude 8, some of the aftershocks may be magnitude 7-‐ very large aftershocks!

Power outages/ Water shortages/ communications out etc. Coping with the Threat of Earthquakes (see attached FEMA what to do list] Earthquake prediction.

Long term Seismic frequency-‐ some areas seem to have earthquakes at "regular" intervals of time (Parkfield

Experiment) Space-‐time patterns-‐ patterns in where and when earthquakes occur. Seismic gaps-‐ areas along faults that have not ruptured in a long time. Stress is building.

Short term-‐ Parkfield Experiment Foreshocks-‐ smaller earthquakes that may signal onset of larger one Water level changes-‐ possible changes in groundwater well levels due to shift prior to rupture Radon emissions-‐ possibly due to shifts in Earth prior to rupture Geodetic changes-‐ shifts in location of areas prior to rupture Low Frequency Radio Waves-‐ for whatever reason, these may indicate immanent rupture Animal behavior


Measuring Seismic Waves Seismometers-‐ device that can detect the passage of waves through the earth. Many types of

seismometers used to measure different periods of waves. World-‐wide seismic network (WWSN)-‐ set up after W.W.II to monitor nuclear testing.

Seismographs -‐ seismometer with a recording mechanism to show passage of waves Seismogram -‐ graph from a seismograph-‐ and the fundamental source of seismic data. Record of arrival of

various waves and their times. Seismograms from all the stations of the WWSN are stored and archived by the National Earthquake Information Center in Golden Colorado and several other sites as well. Nowadays the seismograms are recorded digitally for instant data dissemination all over the world.

Locating Earthquakes from seismograms

Difference in p and S velocities allow construction of Travel time curves. Difference in wave type allow for easy recognition of p and S waves on seismograms. The two combined allow us to Figure out from any given seismic station, how far away an Earthquake occurred. Triangulation is needed to give real location of epicenter. Can also get depth to focus.

Shallow focus 0-‐70 km Intermediate Focus 70-‐350 km Deep focus 350-‐700 km.

Can also determine type of faulting first motion studies (don't worry about how we do this-‐ just that we can do it.)

Distribution of aftershocks indicates fault plane. Distribution of Earthquakes Plate boundary quakes-‐ most earthquakes occur on and help define plate boundaries Shallow focus

Divergent boundary Convergent boundary Transform boundary

Deep focus-‐ Convergent boundary only Benioff zones-‐ can determine which plate is subducting—know this. Subduction angles


Earthquake Magnitude Earthquake magnitude-‐ relative amplitudes of waves and energy released

The Richter Scale-‐ logarithmic scale based on amplitudes of waves. An increase of one on the scale corresponds to a factor of 10 increase in wave amplitude. Consequently, the waves produced by a Richter Magnitude 5 earthquake are 10 times greater than those produced in a Richter magnitude 4 earthquake; and 100 times greater than in a magnitude 3 earthquake.

However, it takes about 30 times the stored strain energy to create a ten-‐fold increase in wave

amplitude. Thus, a magnitude 7 earthquake releases 30 times the strain energy of a magnitude 6 earthquake and roughly 900 times the energy of a magnitude 5 earthquake. In short, it would take 900 magnitude 5 earthquakes to release the same amount of energy as one magnitude 7 earthquake.

Other magnitude scales-‐ such as total moment magnitude and surface wave magnitude are actually

used more often by geologists. These are converted to hypothetical Richter magnitudes for Dan Rather (the press).

•Summary diagrams-‐ attached FEMA what-‐to-‐do list •Summary films -‐ When the Bay Area quakes; available in media center library; •Supplemental film-‐ Earthquakes (Earth Revealed); Nova-‐ Earthquake; The San Andreas Fault


What to do Before, During, and After an Earthquake

Source: FEMA (http://www.fema.gov/library/quakef.htm). Earthquakes strike suddenly, violently and without warning. Identifying

potential hazard ahead of time and advance planning can reduce the dangers of serious injury or loss of life from an earthquake.

BEFORE Check for hazards in the home.

Fasten shelves securely to walls.

Place large or heavy objects on lower shelves.

Store breakable items such as bottled foods, glass, and china in low, closed cabinets with latches.

Hang heavy items such as pictures and mirrors away from beds, couches, and anywhere people sit.

Brace overhead light fixtures.

Repair defective electrical wiring and leaky gas connections. These are potential fire risks.

Secure a water heater by strapping it to the wall studs and bolting it to the floor.

Repair any deep cracks in ceilings or foundations. Get expert advice if there are signs of structural defects.

Store weed killers, pesticides, and flammable products securely in closed cabinets with latches and on bottom shelves.

Identify safe places in each room. Under sturdy furniture such as a heavy desk or table or against an inside wall.

Away from where glass could shatter around windows, mirrors, pictures, or where heavy bookcases or other heavy furniture

could fall over.

Locate safe places outdoors. In the open, away from buildings, trees, telephone and electrical lines, overpasses, or elevated

expressways.

Make sure all family members know how to respond after an earthquake.

Teach all family members how and when to turn off gas, electricity, and water.

Teach children how and when to call 9-‐1-‐1, police, or fire department and which radio station to tune to for emergency

information.

Contact your local emergency management office or American Red Cross chapter for more information on earthquakes.

Have disaster supplies on hand.

Flashlight and extra batteries Nonelectric can opener

Portable battery-‐operated radio and extra batteries Essential medicines

First aid kit and manual Cash and credit cards

Emergency food and water Sturdy shoes

Develop an emergency communication plan.

In case family members are separated from one another during an earthquake (a real possibility during the day when adults

are at work and children are at school), develop a plan for reuniting after the disaster.

Ask an out-‐of-‐state relative or friend to serve as the "family contact." After a disaster, it's often easier to call long distance.

Make sure everyone in the family knows the name, address, and phone number of the contact person.

DURING If indoors:

Take cover under a piece of heavy furniture or against an inside wall and hold on. Stay inside. The most dangerous thing to do

during the shaking of an earthquake is to try to leave the building because objects can fall on you.


If outdoors:

Move into the open, away from buildings, streetlights, and utility wires.

Once in the open, stay there until the shaking stops.

If in a moving vehicle:

Stop quickly and stay in the vehicle.

Move to a clear area away from buildings, trees, overpasses, or utility wires.

Once the shaking has stopped, proceed with caution. Avoid bridges or ramps that might have been damaged by the quake.

AFTER Be prepared for aftershocks.

Although smaller than the main shock, aftershocks cause additional damage and may bring weakened structures down.

Aftershocks can occur in the first hours, days, weeks, or even months after the quake.

Help injured or trapped persons. Give first aid where appropriate. Do not move seriously injured persons unless they are in

immediate danger of further injury. Call for help.

Listen to a battery-‐operated radio or television for the latest emergency information. Remember to help your neighbors who

may require special assistance-‐-‐infants, the elderly, and people with disabilities.

Stay out of damaged buildings. Return home only when authorities say it is safe.

Use the telephone only for emergency calls.

Clean up spilled medicines, bleaches or gasoline or other flammable liquids immediately. Leave the area if you smell gas or

fumes from other chemicals.

Open closet and cupboard doors cautiously.

Inspect the entire length of chimneys carefully for damage. Unnoticed damage could lead to a fire.

INSPECTING UTILITIES IN A DAMAGED HOME

Check for gas leaks-‐-‐If you smell gas or hear blowing or hissing noise, open a window and quickly leave the building. Turn off the

gas at the outside main valve if you can and call the gas company from a neighbor's home. If you turn off the gas for any

reason, a professional must turn it back on.

Look for electrical system damage-‐-‐If you see sparks or broken or frayed wires, or if you smell hot insulation, turn off the

electricity at the main fuse box or circuit breaker. If you have to step in water to get to the fuse box circuit breaker, call an

electrician first for advice.

Check for sewage and water lines damage-‐-‐If you suspect sewage lines are damaged, avoid using the toilets and call a plumber.

If water pipes are damaged, contact the water company and avoid using water from the tap. You can obtain safe water by

melting ice cubes.

Pets after an Earthquake The behavior of pets may change dramatically after an earthquake. Normally quiet and friendly cats

and dogs may become aggressive or defensive. Watch animals closely. Leash dogs and place them in a fenced yard. Pets

may not be allowed into shelters for health and space reasons. Prepare an emergency pen for pets in the home that

includes a 3-‐day supply of dry food and a large container of water.


GLG101: Earth Interior Lecture Reviews Methods used to determine properties and characteristics of the Earth Interior

•Seismology-‐ the study of seismic waves produced by Earthquakes and explosions •Gravity-‐ the study of the Earth's gravitational field •Geomagnetism-‐ the study of the Earth's magnetic field •Meteorites-‐ study of the extra-‐terrestrial bits of rock that occasionally hit the Earth •Heat Flow-‐ study of the Earth's internal heat •Physical and Chemical constraints-‐ laboratory study of material behavior

Seismology-‐ Evidence from seismic waves

Seismic waves travel at different speeds depending on the material. In general, denser, more rigid material (crystalline rocks) transmits waves faster than lighter, less cohesive rocks. In addition, experimental and theoretical considerations indicate that some waves (S-‐waves) cannot pass through liquids.

•Wave fronts versus ray paths-‐ seismic waves traveling through the Earth can be represented either as

wave fronts or as rays perpendicular to wave fronts. •Arrivals at some seismic stations at medium distances from EQ epicenters come too fast for material to

be completely homogeneous. Example: Distance = rate x time. Given epicenter and station A (a short distance away), we can measure distance and time and thus calculate the velocity for each type of wave. Given these velocities we can predict the arrival times at other seismic stations, say at distance B (a longer distance from the epicenter). In reality, however, the arrival times at farther stations are almost always too soon-‐ indicating that the waves must have been traveling faster. Stations farther away receive waves that travel deeper within the Earth. Therefore, rocks deeper within the Earth are more rigid or denser than those at the surface. Point: layered Earth.

•Seismic refraction-‐ Waves are bent by passage into a layer of different velocity. If the new layer is more dense (hence faster velocity), waves are bent more shallowly than entrance angle. If new layer is less dense then waves are bent more steeply.

•Seismic reflection-‐ experiments with explosions indicated that some waves are not refracted but bounce right back-‐ they are reflected. This is also difficult to explain in a homogeneous Earth.


•Seismic velocity versus depth curves-‐ Upper Earth Seismic refraction and reflection of the various waves allow us to construct a picture of the velocities

versus depth within the Earth Basically, velocities increase with depth with a few exceptions and jumps called discontinuities

-‐Moho Discontinuity-‐ crust-‐mantle boundary. Jump is consistent with change of rock type from felsic/intermediate/mafic in crusts to ultramafic in mantle. confirmation from mantle xenoliths of peridotite

-‐LVZ (Low Velocity Zone) = Asthenosphere-‐ both S and P waves transmitted so material must behave like solid in short time frames. Large drop in wave velocities, however, suggests that material is not all that rigid.

-‐400km Discontinuity-‐ sharp velocity increase. Possibly due to mineral phase change or chemical change at this level. Not well known-‐ either would work.

-‐670km Discontinuity-‐sharp velocity increase. Possibly due to mineral phase change or chemical change at this level. Not well known-‐ either would work.

•Seismic velocity versus depth curves-‐ Whole Earth Shadow zones and velocities of the various waves allow us to construct a picture of the velocities

versus depth within the whole Earth •S-‐Wave shadow zone-‐ Since S-‐Waves cannot transmit through liquids and no S-‐Waves are observed

to arrive beyond 103° from epicenter, then there must be a liquid layer of the Earth. Depth can be calculated by geometry. Evidence for existence of liquid core.

•P-‐Wave Shadow zone-‐ No P-‐wave arrivals are know between 103 and 143° from epicenter. P-‐Waves can transmit through liquids although their velocities will be slowed down. Therefore, refraction will make them enter core more steeply. This will create a shadow zone in the way portrayed by the illustration in book. Depth can be calculated by geometry. Consistent with S-‐Wave Shadow zone for existence of liquid core.

-‐Gutenberg discontinuity -‐ major drop in P-‐wave velocity. S waves not transmitted. Therefore, material is most likely fluid. Core/mantle boundary

-‐Slight discontinuity in p-‐wave arrivals traveling through inner most part of core -‐ Chemical considerations dictate that extra pressure may allow liquid core to go to a solid at depth. Appears that there may be a solid inner core. Other evidence from free oscillations suggests solid inner core.


Evidence from gravity •Gravity is the name given to the attraction between masses. Gravity is a force and the strength of this

force is proportional to the masses involved and the square of the distance between them. •Observations of gravitational attraction of the Earth indicate that the Earth must have more mass than is

visible from crustal rocks and those of the upper mantle. •Density within the Earth

Average Earth Density is about 5.5 grams/cc. Average Crust & Upper mantle densities, however, are only about 2.7-‐3.5 g/cc Calculations indicate that core must have density of about 10 g/cc-‐ consistent with iron/nickel

•Gravimeters and Gravity anomalies Bodies of heavier rock perturb the usual value of the Earth's gravity field. Heavier rock beneath the surface produces positive gravity anomalies (field strength is higher than

normal) Lighter rock beneath the surface produces negative gravity anomalies (field strength is less than

normal) Gravitational anomalies can be used to locate metal ore bodies or depth to crystalline rock in deep

sedimentary basins •Principles of Isostacy

In mountain belts, we would expect a positive gravity anomaly due to extra mass of mountains In most mountain belts, however, we see no gravity anomaly Lithosphere blocks of extra mass can actually sink into the asthenosphere (like icebergs sink into

water). The displacement of heavy mantle material away from the roots of mountains removes positive gravity anomaly.

Compensation of weight of lithosphere by displacement of asthenosphere is isostacy. As erosion removes mountains, the lithosphere rises.

Evidence from magnetism

•Shape of magnetic field-‐ geometry of Earth's magnetic field is the same AS IF a bar magnet were centered in the Earth's core and aligned with the magnetic poles.

•Generation of magnetic field-‐ temperatures are too hot in the core, however, for a bar magnet to exist. An electrical current moving around the Earth’s spin axis in the core can generate the field. Therefore, the Earth's core must be an electrically conducting fluid. Again, iron-‐nickel would work

•The Earth's magnetic field occasionally reverses (NOT every 500,000 years though, I don't care what your text says) irregular through time. Cause: unknown.

•Magnetic field anomalies Bodies of magnetic rock perturb the usual value of the Earth's magnetic field.


More magnetic rock beneath the surface magnetized in the direction of the Earth's present magnetic field produces positive magnetic anomalies (field strength is higher than normal)

Less magnetic rock beneath the surface OR rock magnetized opposite of the direction of the Earth's present magnetic field produces negative gravity anomalies (field strength is less than normal)

Magnetic anomalies can be used to locate metal ore bodies or depth to crystalline rock in deep sedimentary basins AND play an important role in the determinations of plate motions

Evidence from meteorites

•Origin of meteorites-‐ trajectories place most of them in asteroid belt between Jupiter & Mars. They are thought to represent either a broken up planet or original material of the solar system that never coalesced into a planet because of Jupiter's enormous gravity field. •Composition of meteorites-‐ Nickel-‐iron. Stony-‐ much like Peridotite in composition Point: Earth interior is probably made up of similar stuff

•Age of meteorites-‐ gives ultimate age of Earth at 4.6 billion years (supported by moon rocks)

Summary of Earth's interior •Seismology gives layers of Earth

Crust-‐mantle-‐outer core-‐inner core Lithosphere-‐asthenosphere-‐lower mantle (= mesosphere)-‐outer core-‐inner core

•Xenoliths (inclusions), seismology, constrain properties of mantle, meteorites constrain composition. Mantle is most probably solid (more-‐or-‐less) peridotite. Seismology, however, suggests that some composition changes may occur below asthenosphere (400 & 670 km discontinuities.

•Gravity, seismology, and magnetism constrain properties of core, meteorites constrain composition. Outer core is most probably liquid iron-‐nickel. Seismology, however, suggests that innermost core is solid.

•Age of meteorites-‐ gives ultimate age of Earth at 4.6 billion years •Summary films: Earth Interior (Earth Revealed)

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