9.4 – Testing Plate Tectonics

Do Now What is the study of changes in Earth’s magnetic field called ?

Do Now What is the study of changes in Earth’s magnetic field called ? Paleomagnetism

Key Words Boundary Alignment

Vocab Words Hot Spot Paleomagnetism

Paleomagnetism Earthquake Patterns Ocean Drilling Ocean Hot Spots Evidence for Plate Tectonics Paleomagnetism Earthquake Patterns Ocean Drilling Ocean Hot Spots

Paleomagnetism The study of changes in Earth’s magnetic field, as shown by patterns of magnetism in rocks that have formed over time. Earth is much like a bar magnet, with invisible lines of force extending from one pole to another. A compass needle aligns with these lines of force and points toward Earth’s magnetic poles. Some rocks contain iron-rich minerals, such as magnetite. When heated above a certain temperature, these magnetic minerals lose their magnetism. However, when these iron-rich mineral grains cool down, they become magnetized in the direction parallel to the magnetic field and the magnetism stays frozen in this position.

Paleomagnetism Earth’s magnetic field periodically reverses polarity. The north magnetic pole becomes the south magnetic pole, and vice versa. When rocks have the same magnetism as the present magnetic field, they are described as having normal polarity. When rocks have the opposite magnetism as the present magnetic field, they are described as having reverse polarity. The discovery of strips of rocks of alternating polarity, which lie as mirror images across the ocean ridges, provided strong evidence of seafloor spreading.

Paleomagnetism

Earthquake Patterns Scientists found a close link between deep-focus earthquakes and ocean trenches. Also, the absence of deep-focus earthquakes along the oceanic ridge system was shown to be consistent with the new plate tectonic theory.

Ocean Drilling The data on the ages of seafloor sediment confirmed what the seafloor spreading hypothesis predicted. The youngest oceanic crust is at the ridge crest, and the oldest oceanic crust is at the continental margins.

Hot Spots A hot spot is a concentration of heat in the mantle capable of producing magma, which rises to Earth’s surface; The Pacific plate moves over a hot spot, producing the Hawaiian Islands. Hot spot evidence supports that the plates move over the Earth’s surface.

Group Challenge Question If the spreading rate at an ocean ridge increased, how would that affect the width of the paleomagnetic strips found on the ocean floor?