05/14/021 Overview Development of the Plate tectonic theory Geological Data –Sea-floor spreading –Fault types from earthquakes –Transform faults –Today's measurements of plate tectonics

05/14/022 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”

05/14/023 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?

05/14/024 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 tectonics5 Major Features of Earth

05/14/026 Global Topography High Mountains Highs in middle of oceans

05/14/027 Coastline matches By rotating and translating Africa and South America, coastlines can be matched (Wegener) Geology and fauna also match across coastlines

05/14/028 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.

05/14/029 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

05/14/0210 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

05/14/0211 Magnetic ``Stripes'' on Seafloor

Actual Data (South of Iceland)

Generation of sea floor magnetic anomalies

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

Earthquakes: Seismology Measurement of “sound” waves from earthquake Fast P-wave S-wave Surface waves Time between arrivals determines how far away earthquake was.

Transform Faults: Directions

05/14/0217 Transform Faults: Bathymetry High Resolution bathymetry showing linear transform faults

05/14/0218 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

05/14/0219 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

05/14/0220 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?

05/14/0221 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.

05/14/0222 Measurement of plate motions with GPS GPS Sites in California; hundreds of stations around the world

05/14/0223 Hawaii North Motion (Pacific Plate)

05/14/0224 GPS Measured Motions using the Global Positioning System

05/14/02Lexington HS Plate tectonics25 California Details Details of measured motions in the California Plate boundary These motions result in earthquakes along the whole California coast

05/14/0226 Measured motions in Turkey

05/14/0227 Izmit Earthquake (1999)

05/14/0228 Motions after Earthquake

05/14/0229 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?

05/14/0230 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.

05/14/0231 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.

05/14/0232 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.