Transcript PowerPoint Presentation - 12.540 Principles of the Global

Plate Tectonics
Prof. Thomas Herring
MIT
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 reemerges 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
Surface waves
Time between
arrivals determines
how far away
earthquake was.
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)
100
North position of KOKEE, Pacific
(Mean removed from each system)
50
0
North (mm)
-50
-100
-150
-200
VLBI KOKEE N
VLBI Kauai N
GPS KOKE N
-250
-300
1980
1985
1990
1995
2000
2005
Year
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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
Motion of GPS site near Hector Mine Earthquake
40
LDES Pre-Hector
210
Time of Hector Mine Earthquake
30
200
20
190
10
LDES Post Hector
Post Hector North (mm)
Pre-Hector North (mm)
220
180
0
1999
1999
2000
2001
2001
170
2002
Year
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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
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