Presentation on theme: "Plate Tectonics A Brief History of a Unifying Theory."— Presentation transcript:
Plate Tectonics A Brief History of a Unifying Theory
Accumulation of Observations - Evidence Patterns of continents Paleontology Geology Patterns of sea floor ages Patterns of seafloor depth Patterns of seafloor sediments Patterns of magnetism Patterns of volcanoes Patterns of earthquakes Plate Tectonics as the Unifying Concept of Earth Science
Earth’s Great Puzzle Pieces http://pubs.usgs.gov/gip/dynamic/historical.html 1620 – Sir Francis Bacon observed similarities of coasts of Africa and South America … “no mere accidental occurrence.” A few years later it was suggested that they were once one, but had been separated by the Flood. 1782 – Benjamin Franklin, based on observed oyster shells on mountain tops “The crust of the Earth must be a shell floating on a fluid interior.... Thus the surface of the globe would be capable of being broken and distorted by the violent movements of the fluids on which it rested.” 1799 – Alexander Von Humbolt, German explorer and naturalist, observed the similarities in the geology and features of the west coast of Africa and east coast of South America (separated by a valley filled by the flood) 1858 - Geographer Antonio Snider-Pellegrini made these two maps showing his version of how the American and African continents may once have fit together, then later separated
Current: Contracting Earth 1912: Continental Drift Observations Fit of Continents Geology Paleontology Climate belts Pangaea (“all lands”) 300 to 200 Ma Breakup 180 Ma Rigid bodies moving through yielding seafloor Alfred Wegener A Man and His Model
Arthur Holmes (Late 1920’s) Interior of Earth has sluggish convection (transport of heat from core); hot stuff rises, cool stuff sinks New ocean crust injected into ocean floor (where?) Mechanism for Plate Movement!
Maurice Ewing Mapping the seafloor 1947- 1959 Lockney Texas Rice University Trained UTMB - Division of Earth and Planetary Sciences of the Marine Biomedical Institute
Maurice Ewing Mapping the seafloor 1947-1959 Surprises: –Thin sediment –Basalt crust – glasses –Age less than 150 Ma (hadn’t identified a pattern yet) –Ridges – later shown to circle globe –Valley within ridge (Tharp) –Earthquakes along ridges –High heat flow (Bullard)
Harry Hess and Seafloor Spreading 1962 – startling new theory “History of the Oceans” New ocean crust at mid- ocean ridges Ocean crust dragged down at trenches; mountains form here Continental crust too light; remains at surface Earthquakes occur where crust descends “It explains everything….”
Rocks and Magnetism - Tools When magma cools, takes on signature of Earth’s prevailing magnetic field Three magnetic measurements can be taken from rocks –Inclination - ~ latitude ~distance to the pole –Declination - ~ direction to the pole –Positive (normal) or negative (reversed) - depending on what Earth’s field is doing Add age = powerful tool You are Here!
Earth’s present magnetic field is called normal –magnetic north near the north geographic pole –magnetic south near the south geographic pole At various times in the past, Earth’s magnetic field has completely reversed –magnetic south near the north geographic pole –magnetic north near the south geographic pole –171 times in last 76 million years … take 5,000 to 10,000 per reversal; last 10’s of thousands to millions of years … review your storage media … Magnetic Reversals
Symmetric patterns of magnetism on either side of mid-ocean ridge Vine and Matthews … The Final Push
Seafloor as a magnetic tape recorder Magnetic Stripes on Seafloor
When magma cools, takes on signature of Earth’s prevailing magnetic field magnetic iron-bearing minerals align with Earth’s magnetic field
Age of Continents Original copyrighted image removed; ;there is an image available at http:// www.lithosphere.info/TC1- 2006/TC1_Fig2-ages-062006.jpghttp:// www.lithosphere.info/TC1- 2006/TC1_Fig2-ages-062006.jpg that may be copyrighted.
Tuzo Wilson A Final Blow … Transform faults: opposite sense of movement than expected. Proven correct (Sykes) Sealed theory of sea-floor spreading and plate tectonics for most scientists
Theory of Plate Tectonics The upper mechanical layer of Earth (lithosphere) is divided into rigid plates that move away from, toward, and along each other Most deformation of Earth’s crust occurs at plate boundaries
How can you calculate the rate of plate movement?
Pick an object and watch it … Better on glaciers than on slow moving plates … Use magnetic reversals … long time periods Date rocks across a mid-ocean ridge really really carefully … tedious
Plate Movement Rates using Hot Spots http://www.classzone.com/books/earth_science/terc /content/investigations/es0810/es0810page01.cfm?c hapter_no=investigation http://www.classzone.com/books/earth_science/terc /content/investigations/es0810/es0810page01.cfm?c hapter_no=investigation