67%(6)4 out of 6 people found this document helpful
This preview shows page 11 - 13 out of 36 pages.
Earth’s Magnetic Field-Flow in the liquid outer core creates a magnetic field-It is similar to the field produced by a bar magnet-The magnetic pole is tilted approximately 11.5 degrees from the axis ofrotation -A compass points to the magnetic North, not geographic north. -The difference between geographic North and magnetic North is called declination-Angle between magnetic field line and surface of the earth is called inclination Magnetic Poles-The magnetic poles move with the earth’s rotation axis-Geographic and magnetic poles are not parallel-Magnetic poles are located near the geographic poles-Magnetic poles move constantlyPaleomagnetism-Rock magnetism can be measured in the laboratory -The study of fossil magnetism is called paleomagnetism -Iron (Fe) minerals in rock preserve information about the magnetic fieldat the time the rocks formed. oDeclination and inclination preserved in rocks often vary from present latitude/longitudeoInstruments used in paleomagnetism record changes in positionoThese data are used to trace continental drift-Iron minerals archive the magnetic signal at formation-Hot magma has randomly oriented magnetic poles-Cooled magma: the magnetic dipoles become frozen in alignment with field because the thermal energy of the atoms is slow. Polar Wandering-Layered basalts record magnetic changes over time-Inclination and declination indicate change in positionApparent Polar Wandering-Polar wandering paths were initially misinterpreted -Not the signature of a wandering on a fixed continent-It is actually the continent that is moving not the poles.-The location of the magnetic pole is fixed. Sea-floor Bathymetry from Echo-Sounding
-Sonar waves reflect from the bottom -Sea-floor maps created by ships -There are deep sea trenches, oceanic islands, seamounts and guyots-Heat flow = evidence for mantle upwelling-Basalt is an igneous rock formed from mid-ocean ridges Earthquakes and the oceanic crust-Earthquakes in the oceanic crust is limited to:oParts of oceanic fracture zonesoMid ocean ridges axesoDeep ocean trenches-Earthquakes are happening along the same boundary that were found on the ocean floor-This all led to Hess’ concept of sea-floor spreading (explaining continental drift)-The ocean floor is formed at the ridges, drifts away from them on both sides, then plunges into the mantle at the trenches-The seafloor was viewed as a conveyor belt for marine material Evidence of seafloor spreading-Magnetism in sea-floor rocks varies father from mid ocean ridges-Stripes of positive and negative magnetism-The magnetism was recorded in the sea basalt-Magnetic anomaly = difference between the expected strength of the Earth’s magnetic field at a certain location and the actual measured strength-The magnetic poles have reversed many times-The magnetic data is symmetric on either side of the mid ocean ridges Sea floor spreading-There is new material coming out from mid ocean ridges and then that material moves away and then new material comes out.