Back To CourseGED Science: Tutoring Solution
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Sheila has a master's degree in geology and has taught middle school through university-level science courses.
To begin, let's consider the Earth and its layers. The Earth is layered with the core at its center, then the mantle, and the crust on the outside. The crust and uppermost part of the mantle is the layer between the crust and the outer core. Together the crust and the upper mantle make up the solid layer called the lithosphere. The lithosphere is made up of large moving and broken pieces of rock called tectonic plates.
The Earth and its tectonic plates are sometimes described using a hard-boiled egg model with a yolk for a core, an egg-white mantle, and a cracked but still intact shell representing the Earth's lithosphere. The issue with this model begins when we start to discuss the movement of the tectonic plates on the Earth's surface. When you think of a hard-boiled egg, the shell does not do much sliding or moving. So, as we think about the plates on the Earth's surface moving, let's move on to a sandwich cookie model even though it doesn't have a sphere or Earth-like shape.
Imagine the bottom layer of a cookie sandwich as the Earth's core. Maybe you can even picture a large Whoopie pie, with its dark top and bottom layers and creamy center! The creamy filling is the Earth's mantle and the top layer is the lithosphere. We can break the cookie lithosphere into two tectonic plates and find ways to slide them across the mantle. The cookie tectonic plates can slide sideways past each other, likely getting stuck and leaving cookie crumbs as they move. They can move together, crashing into each other and being pushed upwards or one piece can sink beneath another. They can also pull apart as some tectonic plates do, exposing the sugary filling below.
These tectonic processes work similarly on the Earth's crust, but on a much bigger and slower scale. There are several large tectonic plates and many small ones on Earth's surface.
One of the large plates is the North American Plate. This plate includes most of the North American continent, Greenland, and part of Iceland and Siberia. Its motion can be measured on the order of a few centimeters per year.
The edges of the North American Plate exhibit all three of the major plate boundary type of movements. These types of movement include transform, where plates slide past each other; convergent, where plates come together; and divergent, where plates are pulled apart from each other. The movement of the hot mantle below the lithosphere drives the plate motion.
The North American Plate has a transform boundary with the Pacific Plate, dividing California at the San Andreas Fault. Here, the Pacific Plate moves northwest with respect to the North American Plate. Much like in our cookie model, the plates get stuck and, rather than moving smoothly and continuously, they push past each other in short bursts that we feel as earthquakes.
Near Alaska, the North American Plate meets the Pacific Plate in a convergent boundary, meaning the plates are coming together. Here, the Pacific Plate sinks under the North American Plate in a process called subduction. As the Pacific Plate subducts, some pieces of the crust scrape off onto the North American Plate. However, most of the dense sinking plate gets pulled into the mantle where it melts into the surrounding material. This helps to create volcanoes inland from the subduction zone that marks the border between the two plates.
In the middle of the Atlantic Ocean, the North American Plate borders the Eurasian Plate. The two plates move apart from each other at this divergent boundary. As they pull apart, the mantle material beneath rises to create new crust on the ocean floor. The continually rising mantle material created the Mid-Atlantic Ridge, an underwater chain of mountains.
So far, we've only discussed the North American Plate's boundaries. In addition to the plate boundaries, we should consider that there is a geologically active hot spot within the North American Plate. A hot spot is where magma rises from the mantle, disrupting the lithosphere within the tectonic plate. This is considered volcanic activity and can happen in the middle of a plate. Yellowstone, which is located hundreds of miles from the North American Plate's border, sits over a hot spot. This is thought to explain the cause of the park's hot springs and geysers and its potential for a massive eruption.
Rocks can be young or old, and formed today or billions of years ago. We have already mentioned plates moving and new crust being formed where mantle material rises to the surface. If you think back to the North American Plate's major divergent plate boundary at the Mid-Atlantic Ridge, we said that the North American Plate is continually pulling away from the Eurasian Plate. This means new rock is also being formed continually as the mantle material cools. Recently formed rocks can also be found anywhere lava erupts from a volcano, which we mentioned often happens near subduction zones.
On the North American Plate, we find volcanoes in Alaska that result from the subduction of the Pacific Plate. Volcanoes are also found in the Pacific Northwest. Mount St. Helens, which is located in the state of Washington, is an example of an active volcano. It results from the subduction of the relatively small Juan de Fuca Plate beneath the North American Plate.
The oldest rocks on the North American Plate tend to be found centered in Canada and the northern parts of the continental United States. This is a tectonically stable area known as a craton. Here, the rocks are up to 4 billion years old, having been formed when the Earth's tectonic plates looked very different from how they appear today.
The North American Plate is in motion, sliding over a major hot spot and bordering other plates with active boundaries. We feel earthquakes as plates slide past each other at transform boundaries, and we witness active volcanoes where other plates subduct beneath the North American Plate. Both convergent and divergent movement is also found on the North American Plate. As we consider the rocks that make up the North American Plate, we find that new rocks are continuously forming at the Mid-Atlantic Ridge. At the center of the North American craton, in the plate's least tectonically active areas, we find the oldest rocks.
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Back To CourseGED Science: Tutoring Solution
34 chapters | 724 lessons