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Lecture 3

3. Lecture Three - September 15.docx

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Jessica Hawthorn

PLATE TECTONICS Plate tectonics is the movement of rigid tectonic plates that compose the outer crust of the Earth. The crust is composed of continental and oceanic crust. Oceanic crust is heavier, denser, and younger than continental crust. As we discussed last class, oceanic crust subducts under continental crust. With plate tectonics comes plate rotation and plate movement. As these plates are rotating around, they are creating new connections. Plates are moving. The continent that we‟re on has not always been the North American continent. There is a cycle of continental construction and deconstruction. Approximately 200 million years ago, a large continent called Pangaea deconstructed into two smaller continents (Gondwana and Laurasia). About 100 million years ago Gondwana fragmented into Africa, South America, India, Australia, and Antarctica. At the same time, Laurasia fragmented into North America and Eurasia. India and Australia are on one large plate called the Indo-Australian plate. About 100 million years ago, India and Australia start to fracture off and begin to rotate. India moves up as an isolated continent and docks with Southern Asia about 40 million years ago. The oceanic crust on the outskirts of the Indian plate comes into contact with the continental crust of the Eurasian plate and subducts under it (i.e., oceanic crust subducts under continental crust). All the oceanic crust is eventually subducted and eventually, we have continental crust (from the Indian plate) contacting continental crust. Subduction cannot take place. The two pieces come into contact and both are pushing against each other and lifting and lifting and lifting. As a result, a mountain range forms on the Tibetan plateau called the Himalayas. This mountain range is the result of the crumpling of India with Asia. Right now, India is still moving north. The Himalayas are still moving up. Eventually, as India keeps pushing up, it will knock off China. China will become an island. When mountain ranges emerge, it changes global temperatures. These things affect the animals, plants, and fungi. So, you have tectonics creating new continents, destroying old continents, creating and destroying basins, and you have connections between continents being made and destroyed. Some of the most dramatic changes in life on Earth are the result of plate tectonics. The Earth is a dynamic planet. There is a cycle. Within this cycle, we can describe what happens to the elements that compose the crust. What is a mineral? A mineral is a physical arrangement of solid elements. What is a rock? A rock is a collection of minerals. The rock cycle describes the pattern of rocks and minerals through this dynamic, evolving planet. There are three main types of rock. Igneous Rock Igneous rock forms from the cooling/crystallization of magmas or lavas. Magmas or lavas are mantle elements that are in a fluid or near-fluid state. They have not crystallized completely. Their constituent elements have not cooled down into regular minerals. When this material is underneath the surface, it is called magma. When it is exposed to the surface, it is called lava. As magma or lava cools, all the elements in that material will begin to slowly aggregate and because of their electrochemical properties, will crystallize and form regular minerals. Granite is igneous rock. It is the primary continental rock. Basalt is also igneous rock. It is the primary oceanic igneous rocks. Metamorphic Rock Metamorphic rocks form through metamorphosis, or the changing of pre-existing rocks by exposure to pressure or temperature. This is important. These rocks get exposed to pressure or temperature. With respect to pressure, you can exert pressure on a rock by placing more rocks on top of it. As for heat, you can bury a rock and thereby increase the temperature. You can also expose it to magmas or lavas. Metamorphic rocks do not simply melt. If you melt a pre-existing rock, and it re-crystallizes, it forms an igneous rock. If, however, you take it and force it to re-crystallize through a change in pressure or temperature, it becomes a metamorphic rock. Sedimentary Rock Sedimentary rocks are formed from the physical and chemical weathering of pre-existing rock. If you take a pre-existing rock and you erode it, weather it, and break it down, and alter its chemical composition by exposure to water, and turn it into sediment, and the sediments get glued back together, you have a sedimentary rock. Sandstone is essentially chemically cemented pieces of sand. A mudstone is chemically cemented pieces of clay. Any one type of rock can transform into any other type of rock. Any rock in the Earth is in a cycle of transformation from one type to another type of rock. This cycle has no beginning. It does not go from to the other in a fixed pattern. Any rock can undergo a physical or chemical transformation into another type of rock. Why do we care about rocks in a course about the history of life? Well we‟re dealing with fossils. We find fossils in rocks. Understanding the rock cycle allows us to understand how to read the geology of the planet and reconstruct the past. Chemical Rock Types: If you take mud (which is actually the weathered product of granite) and cement it together, you‟ll end up with a mudrock. Now, if you expose it to temperature and pressure, what you get is slate (i.e., chalkboards). Then, you take chalk (which is calcium carbonate) and make a conveniently sized chalk out of it. The rock cycle is important. It preserves the history of life. Fossils are preserved almost exclusively in the sedimentary rock record. We have igneous rock which forms from the cooling of lavas/magmas (
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