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Lecture 10

EPSC201 Lecture 10 Notes.doc

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

EPSC201 - Lecture 10 Notes Last nights earthquake was around 4.5 on the Richter scale, and was rather deep focus. This earthquake was on a fault line. This fault had a little bit of move- ment on it, which caused the earthquake. The probable cause is elastic rebound from the last glaciation. At one point there was a few km of ice on top of the crust in the Montreal area, and now the crust is trying to bounce back upwards. In a geologi- cal time scales, the last ice age was extremely recent, and the crust is still rebounding. The aftershocks from this earthquake will be extremely small. They will be in the next day or two after the initial earthquake. Key point – glacier pushes plate downwards because its heavy, when it melts the plate bounces back upwards Perturba- tions in the mantle- core boundary cause mantle to be melted by the hot core, and rise up a mantle plume. This plume is a verticle column of hot rising material. As it rises, it is under less pressure, and the hot material melts into a liquid. This is called decompression melting. The plume seems to be stationary for millions of years. The evidence of this is the linear chain of islands formed over hot spots. The plates are moving over a stationary plume, which causes a chain of islands to form. Presumably, over a long period of time, the plume will collapse and quit producing islands. The temperature in plumes in constant for a long time, but will eventually cool down. The islands can be dated, by surface rock analysis, and the distance between them can be measured. Thus, you can measure the velocity of the plate, and see if the plate has changed speed over time. Not all of the volcanoes formed by the hot spot make it to the surface. The underwater area is very bumpy. The chain of islands produced by a hot spot can show the di- rection the plates have moved over time. There are rocks in India from a hot spot in the Indian sea. This shows the Indi- an sub-continent moved from Antarctica northwards. Continental Rifting: Continental rifting is occurring between Africa and Arabia. The two plates are moving apart, and an ocean is flowing in between. The spreading occurs from mantle uprising, which could be caused by mantle convection, or a hot spot. This causes the crust to be stretched out and thinned. As the crust is thin, it melts easier, and turns into lava and volcanoes. Even- tually the two plates will separate. Continental rifting is essentially the same as sea floor spreading. The crust is old because it doesn’t subduct. The ocean floor is very young. Remember, the asthenosphere is ductile. The mantle is solid. The outer core is liquid, the inner core is solid. Mineralogy – Minerals and Crystals: A crystal is a manifestation of a mineral. A mineral is a solid that has a specific chemical composition. Quartz has a chemical composition of SiO2. Halite is NaCl, also known as salt. Minerals are manifested as crystals. Wikipedia – minerals are a naturally occurring solid, represented by a chemical formula, and have ordered atomic structure. They differ from rocks, which are made of many different chemicals The crystal represents the geometric form that that mineral takes. Halite forms a cube. Quartz forms a hexagon shape with a pyramid on top. (prism) Crystals can be very big. In mines, they are found at sizes of up to 30 meters long. While the length and diameter can vary, the shape is exactly the same. Gypsum – the mineral CaSO4·2H2O. There was a big mine in Mexico that found a cave of huge gypsum crystals. To grow crystals, you evaporate the water very s
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