GEOL 240Lg 1st Edition Lecture 5 Outline of Last Lecture I Where s all that Lava II Plate movement over hot spots III Alfred Wegener IV Continental Drif V PANGEA VI Distribution of Ancient Glaciers Outline Current Lecture I Gallium Magnetic Data II Earth s Interior Illustration III Earth s Magnetic Field IV Declination V Rock Magnetism VI Currie Point VII Thermo remnant magnetization VIII Harry Hess IX Age of the Sea floor X Movie of past plate motions Current Lecture Earth s interior picture Earth has a magnetic field we wouldn t be here if there want one or we will all be dead Iron filled Earth s magnetic field The field looks like that produced by a dipole located at the center of earth The field is inclined with respect to the axis of rotation Freely suspended compass needles line up along the magnetic lines of force Lines of force are more vertical near the poles and more horizontal near the equator Declination angle between magnetic flux lines true geographic North pole So with inclination you can tell the latitude how far N or S for ancient row Cannot use declination to determine Paleo longitude however What temperature the balt how hot to melt rock 1000 degree Rock Magnetism a Magnetic mineral grain at a temperature above the Curie point 600 C Magnetic domains are randomly oriented with no net magnetization b Magnetic mineral grain has cooled to a temperature below the Curie point in the presence of the Earth s magnetic field blue arrow Magnetic domains have lined up in the direction of the earth s field producing a net permanent magnetization in that direction Curie point 600 Magnetic particles in magma are randomly orientated When magma cools through Curie point the magnetic particles align in 3D with lines of magnetic flux The orientation of earth magnetic field is located in as magma ends to form rock Similar alignment of magnetic minerals occurs as sedimentary layers sand stone and shale are buried and gradually harden to form rock An example of the application of thermo remnant magnetization to the study of magnetic field reversals Earth s magnetic field changes molarity it seems to switch we don t know time Geo magnetic time scale or the geo magnetic reversal time scale 700 000 years the earth s magnetic field has been stabled but before that it wasn t Based on determining the magnetization of rocks of known age from both the oceans and the continents We have a good record of geomagnetic reversals back to about 60 MA Ready All the background work is now in place The plate tectonics of 1960 s I II Harry Hess seafloor spreading He was in the navy during world war two key was his background Known as the father of seafloor spreading His career was exhaustible went back and for because they were looking for submarines Detect them with sonar the magnetic fields Towed magnetometers behind navy ships in WWII Das Boot Magnetic stripes on the seafloor Paleomagnetic maps that were parallel to spreading ridges These ocean ridges solidifies the magma to form seafloor spreading Add picture of magnetic stripes on the seafloor Data that led to the biggest science ever until Harry Hess made the observation Science is made of previous observations Age of the sea floor The age of the sea floor increases away from the mid oceanic ridges 200 000 years old in west pacific Movie of past plate motions Sea floor lithosphere age mainly from magnetic stripes Slip directions in recent very large subduction mega thrust earthquake Orientation of transform faults Important because they constrain the direction of sea floor spreading How old is planet earth 4 6 billion years old 200 million years old oceanic lithosphere Oldest piece of rock on earth 4 2 carbonaceous chondrites have never been hotter than 200 c Carbonaceous Chondrites Allende meteorite The Wilson Cycle