Plate Tectonics Prof. Thomas Herring MIT

05/14/02Lexington HS Plate tectonics2 Contact Information Prof. Thomas Herring, Department of Earth, Atmospheric and Planetary Sciences, MIT Room Phone FAX Web:

05/14/02Lexington HS Plate tectonics3 Overview Development of the Plate tectonic theory Geological Data –Sea-floor spreading –Fault types from earthquakes –Transform faults –Today's measurements of plate tectonics

05/14/02Lexington HS Plate tectonics4 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”

05/14/02Lexington HS Plate tectonics5 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?

05/14/02Lexington HS Plate tectonics6 History Wegener proposed “Continental Drift” in 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?

05/14/02Lexington HS Plate tectonics7 Major Features of Earth

05/14/02Lexington HS Plate tectonics8 Global Topography High Mountains Highs in middle of oceans

05/14/02Lexington HS Plate tectonics9 Coastline matches By rotating and translating Africa and South America, coastlines can be matched (Wegener) Geology and fauna also match across coastlines

05/14/02Lexington HS Plate tectonics10 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.

05/14/02Lexington HS Plate tectonics11 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

05/14/02Lexington HS Plate tectonics12 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

05/14/02Lexington HS Plate tectonics13 Magnetic ``Stripes'' on Seafloor

05/14/02Lexington HS Plate tectonics14 Actual Data (South of Iceland)

05/14/02Lexington HS Plate tectonics15 Generation of sea floor magnetic anomalies

05/14/02Lexington HS Plate tectonics16 Earthquakes: Types of faults Three types: Normal, thrust and strike-slip

05/14/02Lexington HS Plate tectonics17 Earthquakes: Seismology Measurement of “sound” waves from earthquake Fast P-wave S-wave Surface waves Time between arrivals determines how far away earthquake was.

05/14/02Lexington HS Plate tectonics18 Transform Faults: Directions

05/14/02Lexington HS Plate tectonics19 Transform Faults: Bathymetry High Resolution bathymetry showing linear transform faults

05/14/02Lexington HS Plate tectonics20 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

05/14/02Lexington HS Plate tectonics21 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

05/14/02Lexington HS Plate tectonics22 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?

05/14/02Lexington HS Plate tectonics23 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.

05/14/02Lexington HS Plate tectonics24 Measurement of plate motions with GPS GPS Sites in California; hundreds of stations around the world

05/14/02Lexington HS Plate tectonics25 Hawaii North Motion (Pacific Plate)

05/14/02Lexington HS Plate tectonics26 GPS Measured Motions using the Global Positioning System

05/14/02Lexington HS Plate tectonics27 California Details Details of measured motions in the California Plate boundary These motions result in earthquakes along the whole California coast

05/14/02Lexington HS Plate tectonics28 Measured motions in Turkey

05/14/02Lexington HS Plate tectonics29 Izmit Earthquake (1999)

05/14/02Lexington HS Plate tectonics30 Motions after Earthquake

05/14/02Lexington HS Plate tectonics31 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?

05/14/02Lexington HS Plate tectonics32 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.

05/14/02Lexington HS Plate tectonics33 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.

05/14/02Lexington HS Plate tectonics34 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.

05/14/02Lexington HS Plate tectonics35 Web resources EAPS: Mine: UNAVCO (University GPS Consortiurm): Southern California Earthquake Center: GPS times Series: Earth Science course