EarthQuakes & VolcanoesEarthQuakes & Volcanoes(e.g. Shake and Bake)(e.g. Shake and Bake)

• The most visible The most visible manifestations of the manifestations of the earth’s internal heat earth’s internal heat are earthquakes and are earthquakes and volcanoes.volcanoes.

• There is an interesting There is an interesting relationship between relationship between the two that led the two that led researchers in the researchers in the 1960’s to overturn our 1960’s to overturn our view of the Earthview of the Earth

Igneous Rocks

• Formed when minerals and other rocks melt

• Are classified according to their chemistry and texture

• Bowen’s Reaction Series Explains all there is to know

Texture of Igneous Rocks

• Texture describes how the minerals fit together in a rock and their size

• Fine-grained are usually volcanic rocks

• Coarse grained are commonly plutonic rocks.

Definitions

• Plutonic = Intrusive (magma stays underground)

• Volcanic = Extrusive (magma flows to the surface)

• Lava = Magma that has reached the surface.

Geothermal Gradient

• Why do rocks melt?

• Because the earth is hot inside!

• The change in temperature with increasing depth is called the Geothermal gradient.

0 100 200 300 400 500 600 700 800 900 1000 1100 1200

Temperature (C)

-60

-50

-40

-30

-20

-10

De

pth

(km

)

Geothermal Gradient

What Happens When Rocks Melt?

• They expand which causes….

• The density to decrease (because you have the same amount of rock in a larger volume) which causes…..

• The melted rock to rise toward the surface!

Volcanic eruptionsVolcanic eruptions

• We have discussed why We have discussed why volcanic material volcanic material (magma) rises to the (magma) rises to the surface.surface.

• There are several There are several different types of volcanic different types of volcanic eruptions that are related eruptions that are related to the composition of the to the composition of the magma.magma.

• Volcanoes are found all Volcanoes are found all over the world, but tend over the world, but tend to occur along linear to occur along linear belts.belts.

Locations of Quakes/VolcanoesLocations of Quakes/Volcanoes

Earthquakes/Volcanoes Occur along Linear Belts. The most famous Earthquakes/Volcanoes Occur along Linear Belts. The most famous of these regions is called the “Pacific Ring of Fire”of these regions is called the “Pacific Ring of Fire”

Locations of Volcanoes

Types of Volcanoes/LocationsTypes of Volcanoes/Locations

• Rhyolitic or Andesitic VolcanoesRhyolitic or Andesitic Volcanoes: Concentrated near : Concentrated near the boundaries between oceans and continents.the boundaries between oceans and continents.

• Basaltic Volcanoes: Concentrated in the middle of the Basaltic Volcanoes: Concentrated in the middle of the Oceans (Mid-Atlantic Ridge).Oceans (Mid-Atlantic Ridge).

Types of VolcanoesTypes of VolcanoesStrato or Composite ConesStrato or Composite Cones

• StratoStrato Volcanoes: Made up of Volcanoes: Made up of alternating layers of rhyolitic alternating layers of rhyolitic to andesitic flows and ashfall to andesitic flows and ashfall deposits.deposits.

• Top picture is Mt. St. Helens Top picture is Mt. St. Helens in Washington and the bottom in Washington and the bottom is Mt. Fuji in Japan.is Mt. Fuji in Japan.

Types of VolcanoesTypes of VolcanoesShield VolcanoesShield Volcanoes

Shield Volcanoes tend to be Shield Volcanoes tend to be very large, but not as steep very large, but not as steep or tall (with respect to the or tall (with respect to the land surface) as Composite land surface) as Composite volcanoes.volcanoes.

Shield volcanoes tend to be Shield volcanoes tend to be dominated by basaltic dominated by basaltic volcanism.volcanism.

Eruption of Kiluaea, Eruption of Kiluaea, HawaiiHawaii

Types of VolcanoesTypes of VolcanoesCinder ConesCinder Cones

• Cinder conesCinder cones are the are the simplest type of simplest type of volcano. They are built volcano. They are built from particles and from particles and blobs of congealed blobs of congealed lava ejected from a lava ejected from a single vent. single vent.

• They tend to be much They tend to be much smaller than the other smaller than the other types of volcanoes.types of volcanoes.

Mt. Capulin, New MexicoMt. Capulin, New Mexico

Frequency of EruptionsFrequency of Eruptions

• Volcanic activity on Earth is nearly Volcanic activity on Earth is nearly constant. constant.

• This activity, more than anything else, This activity, more than anything else, indicates the dynamic hot nature of the indicates the dynamic hot nature of the planet.planet.

Predicting Volcanic EruptionsPredicting Volcanic Eruptions

• Predicting volcanic eruptions has become increasingly more accurate.

• Two Good Books– Surviving Galeras

by Stanley Williams– No Apparent

Danger by Victoria Bruce

Bowen’s Reaction Series (Chemistry of Igneous Rocks

Additional Notes on Bowen’s

• As SiO2 increases density decreases.

• As SiO2 increases melting temperature decreases.

• As SiO2 increases crystallization temperature decreases.

• As SiO2 increases viscosity increases.

• As SiO2 increases explosive potential increases.

More Interesting TriviaMore Interesting Trivia

• Why are continents, continents? Why are continents, continents?

• Why are oceans, oceans?Why are oceans, oceans?

• Answer:Answer:

• Continental Crust: Thick and Light Continental Crust: Thick and Light (2.7)(2.7)

• Oceanic Crust: Thin and Heavy Oceanic Crust: Thin and Heavy (2.9)(2.9)

EarthEarthQuakesQuakes

• Earthquakes, like Earthquakes, like volcanoes, reflect the volcanoes, reflect the dynamic nature of the dynamic nature of the planet earth.planet earth.

• Earthquakes, like Earthquakes, like volcanoes, tend to volcanoes, tend to occur along linear occur along linear belts.belts.

• There are different There are different types of earthquakes types of earthquakes in different regions.in different regions.

Details of EarthquakesDetails of Earthquakes

• What causes an earthquake?What causes an earthquake?• Where did the earthquake occur?Where did the earthquake occur?• How big was the earthquake?How big was the earthquake?• What type of fault generated the What type of fault generated the

earthquake?earthquake?• How frequently do earthquakes occur?How frequently do earthquakes occur?• Can Earthquakes be predicted?Can Earthquakes be predicted?

What Causes an Earthquake?What Causes an Earthquake?

• The simple answer is the buildup of stress The simple answer is the buildup of stress in in solidsolid rock. rock.

• Elastic Rebound TheoryElastic Rebound Theory states that stress states that stress will build to a critical level and then the will build to a critical level and then the rock will break suddenly causing an rock will break suddenly causing an earthquake.earthquake.

• Earthquakes occur on Earthquakes occur on faultsfaults which are which are breaks in the earth’s crust along which breaks in the earth’s crust along which movement occurs.movement occurs.

T im e

Stre

ssC ritical Stress

Ear

thqu

ake

Ear

thqu

ake

Ear

thqu

ake

Where did the Earthquake Occur?Where did the Earthquake Occur?

• We need the following information to accurately We need the following information to accurately locate an earthquake.locate an earthquake.

• Time that P and S waves arrive at 3 earthquake Time that P and S waves arrive at 3 earthquake monitoring stations.monitoring stations.

• P-waves are also known as ‘primary’ or ‘push’ P-waves are also known as ‘primary’ or ‘push’ waves because they are the first to arrive at a waves because they are the first to arrive at a seismic station. P-waves can travel through gas, seismic station. P-waves can travel through gas, solid or liquid.solid or liquid.

• S-waves are known as ‘shear’ waves or S-waves are known as ‘shear’ waves or ‘secondary’ waves because they are normally the ‘secondary’ waves because they are normally the second type of wave to arrive at a seismic station.second type of wave to arrive at a seismic station.

0

T im e-------->

P -w av e S-w av e

Seism ogram

D istance

Tim

e (m

inu

tes)

P-W ave

S-W ave

2 m inu tes

2 m inu tes

500 k m

Locating an EarthquakeLocating an Earthquake

Difference in P-S gives an estimate of Difference in P-S gives an estimate of distance, but not direction. In distance, but not direction. In essence, we end up with a ‘circle of essence, we end up with a ‘circle of possible locations’ for each station.possible locations’ for each station.

A

A

B

H ere?

H ere?500

500

750

750

A

B

C

500

500

750

300

750

A

B

C

500

500

750

300

750

FocusFocus-point of rupture on fault

Epicenter: Epicenter: Point on the earth’s Point on the earth’s

Surface directly above the focusSurface directly above the focus

How Big?How Big?

• The size of an earthquake is called its The size of an earthquake is called its magnitudemagnitude..

• Originally developed by Charles Richter, there Originally developed by Charles Richter, there are now many different magnitude scales. are now many different magnitude scales.

• Most of these depend on Most of these depend on DistanceDistance to the quake, to the quake, amplitudeamplitude of a particular wave and of a particular wave and periodperiod of that of that wave.wave.

• Station corrections are added to account for the Station corrections are added to account for the crust in the region of the seismometer.crust in the region of the seismometer.

MagnitudesMagnitudes

Period

Am

plitu

de

Sample Formula would look Sample Formula would look like:like:

Mag= log (A/T) + log D + 3.2Mag= log (A/T) + log D + 3.2

Where A=amplitude (mm)Where A=amplitude (mm)

T= period (seconds)T= period (seconds)

D=DistanceD=Distance

And the last number is a And the last number is a station correction.station correction.

• Because the scale is logarithmic, each Because the scale is logarithmic, each single digit increase in magnitude single digit increase in magnitude represents a represents a 10x10x increase in the amplitude increase in the amplitude of the wave.of the wave.

• Perhaps more importantly, each single Perhaps more importantly, each single digit increase in magnitude represents a digit increase in magnitude represents a 32x32x increase in the amount of energy increase in the amount of energy released by a quake.released by a quake.

What type of fault?What type of fault?

• The type of fault depends on the type of stress The type of fault depends on the type of stress generating the earthquake.generating the earthquake.

• Normal fault= Normal fault= Extensional vertical stress (pull-Extensional vertical stress (pull-apart)apart)

• Reverse fault= Reverse fault= Compressional vertical stress Compressional vertical stress (push-together)(push-together)

• Strike-slip faultStrike-slip fault=sliding horizontal stress=sliding horizontal stress

Hanging Wall: Miners hang their lamps on it. Foot wall is the side of the fault that miners could walk on.

Left-lateral motion: Observers standing on opposite sides of the fault would see each other moving to their left.

Right-lateral motion: Observers standing on opposite sides of the fault would see each other moving to their right.

Focal Mechanism MapsFocal Mechanism Maps

• Often called ‘beach balls’ they give geologists a Often called ‘beach balls’ they give geologists a way to determine fault type remotely.way to determine fault type remotely.

• Based on the first motion of the P-wave (either Based on the first motion of the P-wave (either up or down.up or down.

• If the first motion of the P-wave is ‘up’, then this If the first motion of the P-wave is ‘up’, then this signifies upward movement of the ground signifies upward movement of the ground surface beneath the seismograph.surface beneath the seismograph.

• If the first motion of the P-wave is ‘down’, then If the first motion of the P-wave is ‘down’, then this signifies downward movement of the ground this signifies downward movement of the ground surface beneath the seismographsurface beneath the seismograph

In this case, which direction is the movement on the In this case, which direction is the movement on the fault?fault?

Fault

A

B

C

D

AC

BD

Up

Up Down

Down

AC

BD

Up

Up Down

Down

Strike-Slip Fault Plane Solution---In this case a right lateral fault.

Pulls Pushes PullsPullsPushe s Pushe s

Reverse Fault Focal MechanismReverse Fault Focal MechanismNormal Fault Focal MechanismNormal Fault Focal Mechanism

Relationship between Faults/VolcanoesRelationship between Faults/Volcanoes

• NormalNormal faulting is associated with faulting is associated with basalticbasaltic volcanism.volcanism.

• ReverseReverse faulting is associated with faulting is associated with andesiticandesitic and and rhyoliticrhyolitic volcanism. volcanism.

• Strike-slipStrike-slip faulting is faulting is amagmaticamagmatic..

How Often?

• Earthquakes occur on a daily basis.

• In your homeworks, you will learn more about the frequency-energy relationships of earthquakes.

• Can Earthquakes be predicted?

Earth Structure From EarthquakesEarth Structure From Earthquakes

• Recall:– P-Waves are fastest and can travel through P-Waves are fastest and can travel through

all mediaall media– S-Waves cannot travel through liquid or gas.S-Waves cannot travel through liquid or gas.

Snell’s LawSnell’s Law

• As a wave (represented by its ray path) As a wave (represented by its ray path) moves through layers of different density it moves through layers of different density it will bendwill bend.

V > V2 1

Case 1: Layer 2 is ‘faster’ than Layer 1Case 1: Layer 2 is ‘faster’ than Layer 1

V < V2 1

Case 2: Layer 2 is ‘slower’ than Layer 1Case 2: Layer 2 is ‘slower’ than Layer 1

The Real EarthThe Real Earth

S-Wave Shadow ZoneS-Wave Shadow Zone

Can Earthquakes Be Predicted?Can Earthquakes Be Predicted?

NO!NO!

• I should say, not yet.• Anomalous Features associated with

quakes includes:• Cockroach scattering.• Lights• Gravity Changes• Animal Behavior• Velocity changes of P & S waves

EarthQuakes and Earthquake DamageEarthQuakes and Earthquake Damage

Earthquake damage depends on many variables. For example:Earthquake damage depends on many variables. For example:

How Deep was the quake? The deeper the quake, the less How Deep was the quake? The deeper the quake, the less likely likely to see damage at the surface.to see damage at the surface.

How Big was the quake? Larger magnitudes cause more How Big was the quake? Larger magnitudes cause more damage.damage.

What time did the earthquake occur? Crowded buildings, What time did the earthquake occur? Crowded buildings, streets etc may result in more death.streets etc may result in more death.

What are building codes? Places such as California and What are building codes? Places such as California and Japan Japan have rigid earthquake building codes so a mag 6 have rigid earthquake building codes so a mag 6 quake is less quake is less likely to cause severe damage there than likely to cause severe damage there than

in a in a highly populated highly populated region with poor/no building codes.region with poor/no building codes.

Number & Size of aftershocks- Large aftershocks can Number & Size of aftershocks- Large aftershocks can destroy damaged buildings.destroy damaged buildings.

Tsunami’s: What are they?Tsunami’s: What are they?

Tsunami is an wave generated most commonly by earthquakes; however, Tsunami’s can also form when large chunks of ice calve off into the oceanOr massive landslides fall into the sea. They are most commonly the result Of earthquakes.

Tsunami’s are sometimes called ‘tidal waves’ or seismic sea waves.

Why are they different?

Sumatra Sumatra Earthquake Dec Earthquake Dec 26, 200426, 2004