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Lecture 9

EPSC201 Lecture 9 Notes.doc

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

EPSC201 - Lecture 9 Notes Earthquakes pick out the boundaries of the tectonic plates, zones of spreading and subduction quite accurately. An upward push of the mantle causes the crust to crack. The oceanic crust is quite thin; about 6- 10 km thick. This causes earthquakes. The dots on the figure below indicate earthquakes. Pushing the brittle crust downwards causes it to fracture and break. Anywhere these fractures oc- cur, there is a potential for deep focus earthquakes. Subduction = deep and shallow focus earthquakes Here is a map of the tectonic plates. There is and equal amount of subduction and spreading, which keeps the size of the earth constant. The ocean floors are very young. The age of the rock can be deter- mined by the mag- netic striping. The rock nearest the mid ocean ridge is the youngest, and it gets older moving out- wards. Obviously, it would be very difficult to obtain rock samples from under the oceans; however, it is easy to drag a magnetometer across the ocean. Comparing the magnetic patterns to rock samples gathered on the surface with the same magnetic pattern, it is possi- ble to date the sea floor rocks. The surface rocks can be dated. Notice the rate of movement is extremly vari- able. In someplaces the expansion is rapid, and other areas, for the same color, it didn’t expand as quickly. Zig-zag pattern is called a transform fault. Only observed in areas of spreading. It is very easy to recognize our conti- nents from Pangea. However, looking at Rodinia, our continents are not obvious at all. Sometimes conti- nents try to separate. They are not always successful. Quebec tried to separate from North America and failed. There is a mid continent valley going through Africa, but current calcula- tions suggest it will also fail to separate. Passive margin – bit of continental crust stuck to the oceanic crust. If the crust had been subducting, it would have been an ac- tive margin. On the East coast of N.A. we have a passive margin, but on the West coast of N.A. we have an active margin. Note the oceanic crust is thinner, and more dense then the continental crust. The idea of the continental crust floating on the mantle is comparable to an ice- berg, which is mostly below water. At an ocean trench, the depth of water could be up to 6-7 km deep, which is much deeper then the rest of the ocean. The rest of ocean is about 1-3 km deep. The subducting plate generates a trench. The subducting slab will even- tually break up. As it moves deeper, the subducting plate becomes more ductile, and will assimilate with the mantle. Key Point – the mantle is heteroge- nous Transform Faults: (Professor
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