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Lecture 8

EPSC201 Lecture 8 Notes.doc

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McGill University
Earth & Planetary Sciences
EPSC 201
Anthony Williams- Jones

EPSC201 - Lecture 8 Notes Continental Drift (Wegner, 1920s) is explained by plate tectonics (1960s). Key line of evidence for plate tec- tonics was the alternating magnet- ic pattern observed on the Atlantic seafloor. It was anomalously posi- tive or negative, but in equal mag- nitude. This was unexpected because there is only one type of rock on the seafloor. Basalt – lava that flows out onto the ocean floor. Why are these patterns observed? Halfway between Canada and Eu- rope, there is the Mid-Atlantic Ridge. It is a topographical high point in the sea floor. The pattern of anomalies is the same moving outwards from the ride. It is symmetric about the mid ocean ridge. The rocks get older away from the ridge, and at the ridge, the rocks are young. There is lava flowing out of the ridge forming basalt. There are volcanoes along the ridge. Iceland is part of the Mid-Atlantic ridge that has come above the surface. Iceland is loaded with active volcanoes. They actually shut down air travel over the Atlantic after volcanic erup- tions. The country uses geothermal energy. Key Point – the earth is shifting its magnetic field, and the rocks are recording it. It is symmetrical about the ridge because new rock is being created at the ridge, pushing the old crust outwards. The new rock is formed by lava flowing out, forming basalt. This formation of new oceanic crust at the mid Atlantic ridge pushes the continents apart. This is called the sea floor spreading theory. At every new ocean ridge, the old crust is being pushed apart, moving the continents apart. The reversal of the magnetic field is a rapid transition. In the transition state, solar radiation is able to enter the at- mosphere. The transition state is on the magnitude of a few hundred to a couple thousand years. Exact amount of time is not known. It’s difficult to take samples from the bottom of the ocean. Finding surface rocks is much easier. Find an area with lots of lava flow from different time periods. An old cliff would be a good example Sampling different layers will tell you about the Earth’s magnetic field history. We are able to date these changes quite accurately. They can determine the age of a rock, and its distance from the ridge, which gives you the rate at which new crust is being created. The age is determined by the magnetic striping. You can measure the changing rate
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