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Lecture 14

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

EPSC201 - Lecture 14 Notes If an earthquake is along a transform fault, they don’t generate as much surface waves. Surface waves are responsible for damage. If its along a transform fault, the direction it should slide is already defined and set up to move. Here are some types of intrusive rock Laccolith – a huge underground body of mag- ma, that can cool, and pushes up the sedimen- tary layers above it Batholith – main magma chamber that feeds all these other terms. If the sedimentary layers wear down enough, it will become exposed. They look like rounded out- crops formed from igneous rocks. These can be very large, up to 50 km wide. So these are large bodies of magma. Dike – vertical wall of hard rock, it would have fed a fissure eruption of magma go- ing upwards, can be thin but continue onwards for kilometers. They feed volca- noes by flowing upwards. Sill – horizontal intrusions of magma that go in-between layers of sedimentary rock, fed by dikes, can be very thick, or very thin. Hundreds of meters to cm thick. Diabase – intrusive basaltic rock, does not make it to the sur- face – form of gabbro Gabbro – intrusive igneous rock chemically equivalent to basalt Basalt – extrusive, the magma makes it out to the surface Why do we get magma formation and volcanoes? Why do we get hot spots, such as Hawaii and Iceland? With spreading activity, you get basalt. With subduction, you get andosite and rhyolite. At spreading centers, we have decompression melting. As the plates spread, mantle moves upwards, and releases pressure. This causes the mantle to melt. Rocks are great insulators. As the mantle moves upwards, the surrounding rocks stay hot (the temperature they were when deep). So the mantle rises up, and is under less tem- perature, but is at the same hot temperature. This causes the mantle to melt into magma, and is called decompression melting. Shown here is a geotherm. From this, we see that as pres- sure increases going deeper into the earth, higher tempera- tures are required to make the rock liquid. At the liquids line, everything right of the line is liquid, which shows that at higher temperatures, the rock turns to liquid. Anything on the solidus line that is moved upwards due to mantle upwelling (decompression melting) will then turn to liquid. This graph explains why decompression melting works. However, the mantle must move enough to reach the liquidus line. If it only moves a little bit, and doesn’t reach the next line, it won’t melt completely. Of- ten, the mantle will be in a state of partial liquids and partial solidus. Subducting slabs come from the surface, which is at a much colder temperature then the mantle. Nonetheless, we still observe magma formation due to melting of the subducting slab. The friction of the subducting slab also causes a small amount of heating, but is not the source of melting. The source of melting for the subducting plate is water. If the subducting plate has water content, the melting point of the solidus is much lower. The above geotherm assumes the conditions are anhydrous. However, if there is water present then things melt much easier. Subducting plates often come from ocean, which causes them to have high water content. Here the subducting plate is under the ocean. This drags down seawater, which hydrates them. Note the different geotherms for hydrated and dehydrated plate melting. The hydrated form melts much easier. Amphibole can be considered a hydrated form of pyroxcine. When water is added, this transformation can occur. As amphibole is heated, it realeases its water, and dehydrates back into pyroxcine. This water is released into the surrounding mantle. Key point – amphibole releases its water into the mantle when heated, causing melting due to in- crease in water content Subduction melting does not cause that much rock to be melted. Decompression melting does cause a lot of rock to be melted. In the case of the spreading centre, it is easy to get the melting going. A lot of mantle rises up and melts. At the spreading center, the rock is mostly basalt. At the Subduction zone, there is rhyolites and andeosites. At the spreading centre, the rock is similar to most of the man- tle on the Earth. Basalt has high amounts of silica, and less olivine. So the magma at spreading cen- ters, has high amounts of silica. By contrast, Subduction melting melts only a small amount of rock, but generates magma high in silica. Decompression melting produces magmas are poor in silica. Pr
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