Plate Tectonics

Prof. Thomas Herring

MIT

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Contact Information

• Prof. Thomas Herring, Department of Earth, Atmospheric and Planetary Sciences, MIT

• Room 54-618 • Phone 253-5941• FAX 253-1699• Email: tah@mit.edu• Web: http://www-gpsg.mit.edu/~tah

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Overview

• Development of the Plate tectonic theory• Geological Data

– Sea-floor spreading– Fault types from earthquakes– Transform faults– Today's measurements of plate tectonics

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Solid-Earth Science

• Major paradigm: Theory of plate tectonics• Theory states: Earth's surface is made up of

rigid plates that move relative to each other.• This theory is the equivalent of General

relativity and quantum theory to Physics• Quote: “There is nothing like data to ruin a

perfectly good theory”

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Plate Tectonics

• As with all theories it is not perfect; but it explains much of the behavior of the solid Earth.

• Earth scientists ask: How do we test this theory and if it is correct what can it tell us about the behavior of Earth?

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History

• Wegener proposed “Continental Drift” in 1915.• Theory not accepted at the time — Problem

how to drive motions• Based on new geologic data theory re-

emerges in the 1960's as “Plate Tectonics”• The theory provided hypotheses that could be

tested: Not at the time but some twenty years later. Can we see plates move today?

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Major Features of Earth

•

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Global Topography

High Mountains

Highs in middle of oceans

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Coastline matches

By rotating and translating Africa and South America, coastlines can be matched (Wegener)

Geology and fauna also match across coastlines

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Development of Plate Tectonics

• Observation: Basic feature of the Earth– Mountains, flat areas, oceans - Topography– Mixture of old and new rocks - Fossils/Geology– Dynamic processes - Earthquakes, volcanoes

• Question: Is there a single explanation for all these phenomena?

• Note the variety of different data types that need to be explained.

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Data used to address problem

• Basic Data used in formulating plate tectonics:– Magnetic stripes on the sea-floor– Directions of slip vectors during Earthquakes– Directions of “transform faults” in sea floor

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Magnetic reversals

• Observations:– Magnetic field of Earth reverses on semi-regular

basis– Short History: – 1.0R, 1.8N, 1.9R, 2.5N, 2.9R?,3.1N,3.4R Myrs.– How do we know dates? Isotope decay

(Potassium/Argon) dating; fossils– “Hot” rocks record the direction of the magnetic field

as they cool: Curie Temperature

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Magnetic ``Stripes'' on Seafloor

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Actual Data (South of Iceland)

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Generation of sea floor magnetic anomalies

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Earthquakes: Types of faults

• Three types: Normal, thrust and strike-slip

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Earthquakes: Seismology

• Measurement of “sound” waves from earthquake

Fast P-wave

S-wave

Surface wavesTime between arrivals determines how far away earthquake was.

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Transform Faults: Directions

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Transform Faults: Bathymetry

High Resolution bathymetry showing linear transform faults

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Synthesis:

• Magnetic anomalies tell us speed of motion• Transform faults the direction of motion over

long periods of time • Earthquakes indicate the instantaneous

direction of motion• Explanation: Rigid plates moving over surface

that interact at their boundaries

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What does plate tectonic explain?

• Much of the topography of Earth: Where the high mountains are (Andes, Himalayas etc); Shape of seafloor (mid-ocean ridges, deep trenches)

• Locations of most earthquakes and volcanoes

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What does this tell us?

• Future motions of plates• Where earthquakes are likely to occur• Importantly: Plate Tectonics gives a

framework in which to formulate hypotheses that can be tested.– Interaction between plate tectonics and climate– Can we directly measure these motions?

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Measurement of Plate Motions

• One prediction of plate tectonics is the rate and direction on motions of plates

• Can we measure these motions today?• Proposed by Wegener (1929) and again in 1969 (at

the time intercontinental distances measured ±200m)• First measurement of “real-time” global plate motions

was in 1986 when the distance between radio telescopes in Westford MA and Onsala Sweden (5600km) was seen to change linearly over a period of 6-years.

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Measurement of plate motions with GPS

GPS Sites in California; hundreds of stations around the world

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-300

-250

-200

-150

-100

-50

0

50

100

1980 1985 1990 1995 2000 2005

VLBI KOKEE N VLBI Kauai NGPS KOKE N

North (mm)

Year

North position of KOKEE, Pacific(Mean removed from each system)

Hawaii North Motion (Pacific Plate)

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GPS Measured Motions using the Global Positioning System

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California Details

• Details of measured motions in the California Plate boundary

• These motions result in earthquakes along the whole California coast

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Measured motions in Turkey

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Izmit Earthquake (1999)

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Motions after Earthquake

0

10

20

30

40

170

180

190

200

210

220

1999 1999 2000 2001 2001 2002

LDES Pre-Hector

LDES Post Hector

Pre-Hector North (mm) Post Hector North (mm)

Year

Time of Hector Mine Earthquake

Motion of GPS site near Hector Mine Earthquake

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Is the theory perfect? No!

• Problems:– How do we explain earthquakes that occur in the

middle of plates?– What drives the whole system?– Is there another major process occurring?

• Provides a starting point for addressing– How variable are the plate motions?– Why have motions changed in the past?

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General Issue:

• Science is not absolute. It is not a series of facts, but rather a way of looking at problems,

• None of the theories used in science are complete!

• General Relativity, Quantum Theory, Newtonian Mechanics all have problems at some level

• Does this mean they are useless? No.

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Summary of Earth Science

• Physics and Chemistry provide the fundamental laws of nature

• Mathematics and statistics provide the methods for solving the laws

• In Earth Science, these are all put together in an attempt to fundamentally understand the workings of our planet.

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Relevance:

• There are complex issues facing the future of Earth which are as much political as scientific

• There are typically no absolute answers to these questions -- but in many cases, Earth science provides the framework in which we can evaluate the problem.

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Web resources

• EAPS: http://www-eaps.mit.edu • Mine: http://www-gpsg.mit.edu/~tah• UNAVCO (University GPS Consortiurm):

http://www.unavco.ucar.edu• Southern California Earthquake Center:

http://www.scec.org• GPS times Series:

http://sideshow.jpl.nasa.gov/mbh/series.html• Earth Science course

http://pubs.usgs.gov/publications/text/dynamic.html