Plate TectonicsII & The Origin of Magma

Moasic of Plates

Earthquake Epicenters

Wegeners View of Continental Drift

The problem with Wegeners view of Continental Drift

Continental crust plow through oceanic crust

Fail to provide a reasonable driving mechanism

Criticized by most geologists and geophysicists in his time

Continental DriftWegener noted the near jigsaw puzzle fit of the coastlines on opposite sides of the Atlantic.He proposed that a great single supercontinent (Pangea) had once existed.He collected a great deal of data in support of his ideas but failed to propose a plausible driving mechanism.

Evidences for a drifting continentFossils and modern organisms

Continental DriftIsland Stepping StonesRaftingIsthmian LinksCompeting Hypotheses for Explaining Similar Species on Different Continents

Evidences for a drifting continent (cont.)Paleoclimate evidencesRock age and structuresSimilarity of mountain rangesPaleomagnetism

Future spreading center

Spreading RatesFastest: ~12 cm/year at East Pacific RiseSlowest: ~1 cm/year at Mid-Atlantic RidgeAverage: ~6 cm/year

Calculating Spreading Rates

What is the driving mechanism for Plate Tectonics?HypothesesPush-PullPlates pushed by weight of ridges (Slab-push)Plates pulled by lithosphere sliding downslope (Slab-pull)

PushPull

HypothesesUpper mantle convecting

Entire mantle convecting

Hot spots from Earths deep interior drive irregular motion (lava lamp) Mantle Convection

Seismic TomographyProcess of making drawings of slices through an object.Detect velocity of seismic waves as they pass through earth.Waves travels faster in cold, denser rock and slower in hot, softer rock

Plate BoundariesDivergent Boundaries are where two plates move apart.Upwelling mantle material at divergent boundaries creates new ocean seafloor at divergent boundaries (ie. The Mid Atlantic Ridge).

Plate BoundariesSome convergent boundaries involve the collision of two continents (ie. Hymalaya Mountains)

TransformfaultsFigure 2.24

EarthsplatesFigure 2.19(right side)

EarthsplatesFigure 2.19 (left side)

Earthquakes

EarthquakesAn earthquake is the vibration of Earth caused by the rapid release of energy.The energy released radiates as waves in all directions from its source called the focus.The waves of energy are analogous to waves produced by a stone dropped into a calm pond.

San Francisco, 1906

Offset Along the San Andreas Fault

CausesElastic rebound theory is a widely accepted explanation of how earthquakes are generated.The theory name is a reference to rocks behaving elastically like a stretched rubber band.

Elastic rebound theory

CausesIn summary most quakes are produced by the rapid release of stored energy in rocks that have been subjected to stress.The energy is released as the rocks snap back into their original shape.Most large earthquakes are produced by this phenomena along preexisting faults.

CausesVolcanism can also produce earthquakes.In fact, small earthquakes often indicate an impending volcanic eruption.

Seismic WavesEarthquakes produce three main types of seismic waves that radiate at different velocities from the focus.P-waves are compressional and can travel through solids, liquids or gases.S-waves propagate with a shearing motion and travel only through solids.

Seismic WavesP-waves travel through granite crust at about 6km/sec. That is roughly 1.7 times faster than S-waves travel.S-waves have a higher amplitude.Both P-waves and S-waves are termed body waves because they travel through the Earth.

Seismic WavesA third type of wave that travels along the surface of the Earth are termed L-waves.L-waves are particularly destructive.

Seismic WavesThe unique attributes of the various seismic waves have allowed geologists to decipher a good deal about the Earths interior.Knowledge of the different wave velocities also enables one to locate an earthquake epicenter, the location on the surface of the Earth above the focus.

SeismographsSeismographs are earthquake detection and recording instruments.They are located all around the globe.As you will see, the seismograms created by seismographs can easily used to locate an earthquake epicenter.

QuestionsHow does the great New Madrid earthquake of 1811 fit with what you know about earthquakes and plate tectonics?

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