Compare the isochron map to the topographic map of the seafloor in the previous investigation. Which seafloor features are associated with young crust? What can you infer from this?
Answers
Plate tectonics is the grand unifying theory in geology. It gets that title because many topics in geology can be explained, in some way, by the movement of tectonic plates. Tectonic plates are composed of Earth’s crust and the uppermost, rigid portion of the mantle. Together they are called the lithosphere. Earth’s crust comes in 2 “flavors”: oceanic and continental
Composition Fe, Mg, and Ca silicates K, Na, and Al silicates
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Lithospheric plates move around the globe in different directions and come in many different shapes and sizes. Their movement rate is millimeters to a few centimeters per year, similar to the rate that your fingernails grow. Motion between tectonic plates can be divergent, convergent, or transform. In divergent boundaries, plates are moving away from each other; in convergent boundaries, plates are moving toward each other; and in transform boundaries, plates are sliding past each other. The type of crust on each plate determines the geologic behavior of the boundary (
The six types of plate tectonic boundaries and their motions.
Figure 1.1 – These models show 6 main types of plate tectonic boundaries. Blue indicates ocean, green indicates land, brown indicates the lithosphere, and orange is the asthenosphere. The bold arrows on the plates indicate their relative motion. Also shown are gray volcanoes. Ocean-ocean transform boundaries (not shown) exist on a small scale associated with spreading at mid-ocean ridges, and continent-ocean transform and divergent boundaries are rare (former) or don’t exist (latter). Image credit: Adapted from Wikimedia Commons user Domdomegg, CC BY.
The foundations of plate tectonics began with a German scientist named Alfred Wegener, who proposed the idea of continental drift in 1915. Think about it, 1915. What kind of evidence could someone possibly have to propose such a big idea? It turns out that Wegener had 4 pieces of evidence that he claimed supported his idea: 1) The continents looked like they fit together like pieces of a puzzle; 2) There were matching fossils on continents that were separated by oceans; 3) There were matching mountain ranges on continents that were separated by oceans; 4) There was paleoclimate evidence that suggested that in the past some continents were closer to the polar regions and some were close to the equator. Wegener took his idea one step further and proposed that all of the continents were together in one giant supercontinent 200 million years ago called Pangea. Like many great ideas in science, Wegener’s idea of continental drift was not accepted by his peers, in part because he did not have a well-developed hypothesis to explain what was causing the continents to drift. It wasn’t until the 1960’s that his idea was expanded upon by scientists like Harry Hess.
Exercise 1.1 – Reconstructing Positions of Continents Using Wegener’s Evidence
When Alfred Wegener came up with his continental drift hypothesis in the early 1900s, he used several lines of evidence to support his idea. He also proposed that 200 million years ago, all continents were together in a single supercontinent called Pangea. In this exercise, you will use the fit of the continents and matching fossil evidence to piece together Pangea. This exercise is adapted from “This Dynamic Planet” by the USGS.
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