TectonicsChapter 3

Evidence for plate motionsMeasurement of plate motions

Makeup of the crustal platesPlate boundaries

Environmental Geology jbartlett@national.edu

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Plate TectonicsTectonics is the study of large-scale movement and deformation of the earth’s

outer layersA small number of rigid pieces = plates form ocean basins & continentsThese move on top of flowing earth materials beneathPlate tectonics studies the interaction of crust plates over a weak or partly

molten layer in the earth’s upper mantle

Magnetic evidence

Magnetic stripes occur in ocean plates As new crust is formed, magnetic field occasionally reverses and changes cooling rocks

These stripes are great evidence for new crust formation pushing ocean crust out from the center – thus called“divergent” boundaries

Crustal plates

Different boundaries show up on the earth’s surface

Midocean ridges: Divergent plates

Coastal mountains:Convergent plates

In between:Transform boundaries

Plate Boundaries

Divergent Plate BoundaryLithospheric plates move apart; form oceanic ridgesRising magma forms oceanic ridges and new oceanic crustForces plates apart

Convergent Plate BoundaryLithospheric plates move togetherHeavy plate subductsMountains, quakes, volcanoes

Transform Boundaries – short segments of a ridgeTransform faults offset ridge, move ‘sideways’San Andreas Fault –

Plate Boundaries

Divergent Plate BoundaryLithospheric plates move apart; form oceanic ridgesRising magma forms oceanic ridges and new oceanic crustForces plates apart

Convergent Plate BoundaryLithospheric plates move togetherHeavy plate subductsMountains, quakes, volcanoes

Transform Boundaries – short segments of a ridgeTransform faults offset ridge, move ‘sideways’San Andreas Fault –

Relative plate motions

Direction and speed vary a lot over the worldLonger arrow = faster rate

5cm.yr = ~1 mile per 32000 years

EarthquakesChapter 4

Faults: earthquake sitesEarthquake processEarthquake hazards

Environmental Geology jbartlett@national.edu

Faults are planes where rocks can slip past each otherThree different types are based on three different motionsFriction between rocks against slipping generates elastic deformation and builds up energy before failureWhen the stress exceeds the friction (or rupture strength of the rock), a sudden movement occurs to release the stress

Faults

Normal fault

Earthquakes Energy releases from a dynamic earth occur

along faults Earth’s crust moves very slowly

over time enough stress builds up and a brittle release occurs – an earthquake

Stress is suddenly released and transferred

Actual site of the first movement along a fault is the focus (or hypocenter)Actual point on the earth’s surface directly above the focus is the epicenter

Figure 4.7 World seismicity (1979-1995)

Earthquake Energy Energy is released during an earthquake

As the waves of energy are transmitted through the rock, this energy with be felt by people at the surface

Magnitude – the amount of ground motion related to an earthquake

Intensity – effect on humans, and their structures, caused by the energy released by an earthquake

Earthquake Magnitude Measured by a seismograph Richter magnitude scale most common Richter scale is logarithmic

• An earthquake of magnitude 4 causes 10 times more ground movement as one of magnitude 3

• The energy released by an earthquake of magnitude 4 releases about 30 times more energy than an earthquake of magnitude 3

Direct and Indirect Effects Shaking is the most obvious and violent effect, but is highly variable and

depends on initial conditions at location Large magnitudes may have small or large human impact

Tsunamis are seismic sea waves. When an undersea or near-shore earthquake occurs, sudden movement of the sea floor may set up waves traveling away from that spot, hitting shore with devastating effect

Fire is caused by broken gas lines and infrastructure Power outages, water disruption are major impacts Famine and disease have been major historical impacts

Pakistan earthquake kills more than 200, creates

new island off southwest coast

Magnitude 7.7 , 15 km deepStrike slip fault~300,000 people involved

Predicting Earthquake Hazards

Like much of geology, prediction is difficult on human scalesModels generate probabilities but not predictionsCurrent focus looks at indicators like ‘seismic gaps’Active hazard areas can go sometime soon – like in the next 100, 1000 or 10,000 years!

Prevention and preparation

Predicting ground shaking and movement along the fault – the obvious hazards

Considering both structures and bedrocks Designing “earthquake-resistant” buildings Knowing the characteristics of the earthquakes in a particular

region The best building codes are typically applied only to new

construction Liquefaction and Landslides can be a serious secondary

earthquake hazard in hilly areas

VolcanoesChapter 5

Associated with boundariesVariety of compositions

Variety of stylesVolcanic hazards

Environmental Geology jbartlett@national.edu

Where the action is

Associated with divergent or convergent plate boundaries

Different locations mean different composition & type of volcanoes

Iron rich mantle stuff Subducted melting stuff

Magma Sources and Types Mafic magmas

produce dark, dense , liquid lavas• Relatively calm,

low drama events• Ocean crust

Felsic magmas produce light colored, sticky lavas• Stiffness causes

violent eruptions• Very dangerous• Continental crust

Composition

Depth

Felsic

low ironlight toned

Intermediate

Mafic

high irondark toned

Volcanic:Surface, fine grained

Rhyolite Andesite Basalt

Plutonic:Deep,coarse grained

Granite Diorite Gabbro

Make up ocean crust

Make up continental

crust

Composition and type

Magma composition mafic felsic affects the lava properties

Silica is very stiff, tends to stick and then explodeContinental environments tend to have water and volatiles melted along with magma

Explosive versus calm eruptions

Hazards Related to Volcanoes Lava, not the principal hazard! Actually not life-threatening generally Airborne/pyroclastic flows, way more dangerous than lava flows Ash falls from eruptions can be much more devastating than lava Gas, steam and poisons can also spew from volcanoes Lahars, a volcanic ash and water mudflow

Pulaweh, Indonesia

Climate effectsAsh cloud goes up and up into stratosphere

Can circle for years or decades, blocking sunlight

With enough blockage, global temperature can be altered with huge economic effects

Very large eruptions have happened geologically recentlyResults:Global ‘volcanic winter’ ~10 yearsEnhanced cooling ~1000 years

Effect on humans… dramatic

6 supervolcanoes of >1000 km3

In the recent past

Big, rare events

Historic eruption: Mt Tambora, 1815, ~160 km3

Result:‘year without summer’

{toba}

Minnesota volcanic hazard? Far from plate boundaries or magma

plumes We are “near” one major risk:

Yellowstone, site of major potential supervolcano

US Geologic Survey: “Thick ash deposits would bury vast areas of the United States… injection of huge volumes of volcanic gases into the atmosphere could drastically affect global climate. Fortunately, the Yellowstone volcanic system shows no signs that it is headed toward such an eruption in the near future”

BBC: 12/10/13Large Yellowstone magma chamber may be 3 times larger than estimated before, so these maps may underestimate effect

Yellowstone supervolcano

'even more colossal'