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: [email protected]• 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