GEOL 101 1st Edition Lecture 4 Outline of Last Lecture Age of Solar System Earth Unique Planet Geology History Geologic Time Earth s Sphere Earth s Layered Structure Rocks and Rock Cycle Outline of Current Lecture Continental Drif Great Debate Continental Drif and Paleomagnetism Polar Wandering Apparent Wandering Scientific Revolution Marine Magnetic Anomalies Magnetic Reversals Piece Observation and Hypothesis Together Current Lecture Continental Drif o Alfred Wegner first proposed Continental Drif Hypothesis in 1915 Super continent Pangea began breaking up 200 million years ago o Evidence Fit of continents Fossil evidence Rock type and structural similarities Paleoclimatic evidence Glacial scratches grooves that tell movement direction Recent glacial period Southern continents joined near South Pole Great Debate objections o Lack of mechanism for movement o Incorrectly suggested continents broke through oceanic crust Was correct in principle but contained in correct details Continental Drif and Paleomagnetism o Renewed interest in continental drif o Initially came from rock magnetism GEOL 101 1st Edition o o Earth s magnetic field grant bar movement Magnetized minerals in rocks Align in direction magnetic force lines Provides means of determining magnetic poles at time of formation o Polar wandering Curves for North America and Europe have similar paths but are separated by about the width of North Atlantic Ocean 3000 mi o Apparent Wandering Apparent movement of magnetic poles indicated that continents have moved Scientific Revolution o 1950 s and 1960 s technological strides Extensive mapping Mid Ocean ridges elevated Deep ocean trenches Thin sediment and young oceanic crust 180 o Sea floor spreading hypothesis proposed by Hess o Magma upwelling of mantle along MOR creates new seafloor o Mantle convection carries seafloor away from MOR o Seafloor descends into mantle at deep trenches Marine Magnetic Anomalies o Earth s magnetic field has 2 parts Main field bar magnet in core Magnetic anomalies caused near surface objects detect submarines find metal resources o Pattern Parallel symmetrical magnetic field across ridge Alternating bands of positive negative anomalies Magnetic Reversals o Paleomagnetic measurements of lava layers on land reveal magnetic North and South Poles flip periodically magnetic reversals Normal polarity same as current pole configuration Reverse polarity opposite to current pole configuration o Magnetic time scale radioactive dating for age of lava flow Chron 1 million years Subchron thousands to 200 000 years Piece Observation and Hypothesis Together o What seafloor spreading marine magnetic anomaly magnetic reversal o Who Vine and Matthews at Cambridge 1963 o How ocean floor as magnetic tape recorder Normal polarity Rocks enhance current magnetic field Negative anomalies o Seafloor spreading Magnetic reversals marine magnetic anomaly pattern o 1968 Continental Drif Seafloor Spreading unite Plate tectonics GEOL 101 1st Edition Plate tectonics o Earth s major plates Associated with Earth s strong rigid outer layer Lithosphere Consists of uppermost mantle and overlying crust Overlies a weaker region called asthenosphere 7 major lithosphere plates North American South American Pacific o largest African Eurasian Australian Indian Atlantic Several plates include entire continent and large areas of seafloor Major difference in Wegner s Continental Drif continents move through ocean floor Plates are in motion and continually changing in shape and size Plates move relative to each other at very slow but continuous rates o 5cm 2in per year Two locations on different plates gradually move away or toward while on the same plates remains relatively constant Plate boundaries o Manifested by earthquake belts o Type of plate boundary depends on its relative motion Divergent constructive move apart Convergent destructive move together Transform conservative grind past each other o Plate may be bounded by combination of three types of boundaries Juan de Fuca o Divergent and convergent out for 40 EACH o Transform 20 o Interactions among individual plates occur along boundaries