Ch 03 - 05
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Transcript of Ch 03 - 05
TectonicsChapter 3
Evidence for plate motionsMeasurement of plate motions
Makeup of the crustal platesPlate boundaries
Environmental Geology [email protected]
Your news items: ripped from the headlines
Giant Alien Planet Discovered in Most
Distant Orbit Ever SeenVERY large planetVERY strange configurationThanks
Shedavia!
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 [email protected]
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 [email protected]
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'