GEOL 240Lg 1st Edition Lecture 5Outline of Last Lecture I. Where’s all that Lava?II. Plate movement over hot spotsIII. Alfred Wegener IV. Continental DrifV. PANGEA VI. Distribution of Ancient Glaciers Outline Current Lecture I. Gallium Magnetic DataII. Earth’s Interior IllustrationIII. Earth’s Magnetic FieldIV. DeclinationV. Rock MagnetismVI. Currie PointVII. Thermo- remnant magnetizationVIII. Harry HessIX. Age of the Sea floorX. “Movie” of past plate motionsCurrent 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, howeverWhat temperature the balt (how hot to melt rock) 1000 degreeRock 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 rockSimilar 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 timeGeo 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 placeThe plate tectonics of 1960’sI. Harry Hess “ seafloor spreadingII. He was in the navy during world war two… key was his backgroundKnown 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 fieldsTowed magnetometers behind navy ships in WWII (Das Boot)Magnetic stripes on the seafloorPaleomagnetic 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 observationScience is made of previous observations - Age of the sea floorThe 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 cCarbonaceous Chondrites Allende meteoriteThe Wilson