Plate Tectonics

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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 [phone]
• FAX [phone]
• Email tah_at_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

Time between arrivals determines how far away
earthquake was.
Surface waves
Fast P-wave
S-wave
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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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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
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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 Centerhttp//www.
  scec.org
• GPS times Serieshttp//sideshow.jpl.nasa.gov/mbh
  /series.html
• Earth Science course http//pubs.usgs.gov/publica
  tions/text/dynamic.html