Plate Tectonics oProf. Thomas Herring, Department of Earth, Atmospheric and Planetary Sciences, MIT oRoom oPhone oFAX o oWeb:

Overview oDevelopment of the Plate tectonic theory oGeological Data éSea-floor spreading éFault types from earthquakes éTransform faults éToday's measurements of plate tectonics

Solid-Earth Science oMajor paradigm: Theory of plate tectonics oTheory states: Earth's surface is made up of rigid plates that move relative to each other. oThis theory is the equivalent of General relativity and quantum theory to Physics oQuote: “There is nothing like data to ruin a perfectly good theory”

Plate Tectonics o As with all theories it is not perfect; but it explains much of the behavior of the solid Earth. oEarth scientists ask: How do we test this theory and if it is correct what can it tell us about the behavior of Earth?

History oWegener proposed “Continental Drift” in oTheory not accepted at the time -- Problem how to drive motions oBased on new geologic data theory re- emerges in the 1960's as “Plate Tectonics” oTestable hypotheses.

Major Features of Earth Major Features of Earth

Global Topography

Development of Plate Tectonics oObservation: Basic feature of the Earth éMountains, flat areas, oceans - Topography éMixture of old and new rocks - Fossils/Geology éDynamic processes - Earthquakes, volcanoes oQuestion: Is there a single explanation for all these phenomena? oNote the variety of data used.

Data used to address problem o Basic Data used in formulating plate tectonics: éMagnetic stripes on the sea-floor éDirections of slip vectors during Earthquakes éDirections of “transform faults” in sea floor

Magnetic reversals oObservations: éMagnetic field of Earth reverses on semi- regular basis éShort History: é1.0R, 1.8N, 1.9R, 2.5N, 2.9R?,3.1N,3.4R Myrs. éHow do we know dates? Isotope decay (Potassium/Argon) dating; fossils é“Hot” rocks record the direction of the magnetic field as they cool: Curie Temperature

Magnetic ``Stripes'' on Seafloor Magnetic ``Stripes'' on Seafloor

Actual Data (South of Iceland)

Generation of anomalies

Earthquakes: Types of faults oThree types: Normal, thrust and strike-slip

Earthquakes: Seismology oMeasurement of “sound” waves from earthquake

Transform Faults: Bathymetry

Transform Faults: Directions

Synthesis: oMagnetic anomalies tell us speed of motion oTransform faults the direction of motion over long periods of time oEarthquakes the instantaneous direction of motion oExplanation: Rigid plates moving over surface that interact at their boundaries

What does plate tectonic explain? oMuch of the topography of Earth: Where the high mountains are (Andes, Himalayas etc); Shape of seafloor (mid-ocean ridges, deep trenches) oLocations of most earthquakes and volcanoes

What does this tell us? oFuture motions of plates oWhere earthquakes are likely to occur oImportantly: Plate Tectonics gives a framework in which to formulate hypotheses that can be tested. éInteraction between plate tectonics and climate éCan we directly measure these motions?

Measurement of Plate Motions oOne prediction of plate tectonics is the rate and direction on motions of plates oCan we measure these motions today? oProposed by Wegener (1929) and again in 1969 (at the time intercontinental distances measured ±200m) oSucceeded in 1986.

GPS Measured Motions

Hawaii Motion

California Details

Motion in Turkey

Izmit Earthquake (1999)

Motion after Earthquake

Is the theory perfect? No! oProblems: éHow do we explain earthquakes that occur in the middle of plates? éWhat drives the whole system? éIs there another major process occurring? oProvides a starting point for addressing éHow variable are the plate motions? éWhy have motions changed in the past?

General Issue: oScience is not absolute. It is not a series of facts, but rather a way of looking at problems, oNone of the theories used in science are complete! oGeneral Relativity, Quantum Theory, Newtonian Mechanics all have problems at some level oDoes this mean they are useless? No.

Summary of Earth Science oPhysics and Chemistry provide the fundamental laws of nature oMathematics and statistics provide the methods for solving the laws oIn Earth Science, these are all put together in an attempt to fundamentally understand the workings of our planet.

Relevance: oThere are complex issues facing the future of Earth which are as much political as scientific oThere are typically no absolute answers to these questions -- but in many cases, Earth science provides the framework in which we can evaluate the problem.

Web resources oEAPS: oMine: oUnavco: oEarth Science course