Plate NotesThe earth is made up of 3 main layers
Core Mantle Crust The crust is where we live, the Earth’s crust is made of Continentals and Oceanic crust. Continental is thick (10-70km) it’s buoyant (less dense than oceanic crust) and mostly older Oceanic crust is thin (7km) and its dense (sinks under continental crust) and is young Continental drift Alfred Wegener in the early 1900’s proposed the hypothesis that continents were once joined together in a single large land mass he called Pangaea (Greek for “all land”) He proposed that Pangaea had split apart and the continents had moved gradually to their present positions gradually to their present positions- a Process that became known as continental drift According to his hypothesis of continental drift, continents have moved slowly to their current location Pangaea about 200 million years ago, before it began breaking up. Wegener named the southern portion of Pangaea Gondwana and the northern portion Laurasia The positions of the continents today. The continents are still slowly moving, at about the speed your fingernails grow. Satellite measurements have confirmed that every year the Atlantic Ocean gets a few inches wider. Continents fit together like a puzzle. Fossils of plants and animals of the same species found on different continents. Wegener’s evidence for continental drift is the rock sequence (meaning her looked at the order of rock layers) in South America, Africa India, Antarctica, and Australia show remarkable similarities Wegener showed that the same three layers occur at each of these places. The same three layers are in the same order in areas now separated by oceans. Wegener proposed that the rock layers were made when all the continents were part of Pangaea. He proposed that they formed in a smaller small joined land mass that was later broken and drifted apart. Seafloor Spreading Everyone agreed that Wegener’s evidence was compelling. But wouldn’t we feel the movement? Also, wouldn’t there be evidence to show that continents were still moving today? Wegener was a meteorologist and his theory was not accepted. (he died on an expedition in Greenland collecting ice samples) One reason scientist had a hard time with Wegener’s theory is that there was no mechanism for the continents motion. In the 1960’s a scientist names Henry Hess made a discovery that would vindicate Wegener Using new technology, radar, he discovered that the seafloor has both trenches and mid-ocean ridges Henry Hess proposed the sea-floor spreading theory. Hess proposed that hot, less dense material below Earth’s crust rises toward the surface at the mid-ocean ridges. Then, it flows sideways, carrying the seafloor away from the ridge in both directions As the seafloor spreads apart at mid-ocean ridge, new seafloor is created The older seafloor moves away from the ridge in opposite directions. This helped explain how the crust could move – something that the continental drift hypothesis could not do. In 1968, scientist aboard the research ship Glomor Challenger began gathering info about rocks on the seafloor. Scientists found that the youngest rocks are located at the mid-ocean ridges Seafloor spreading provided insight to the mechanism for how the continents moved. The magma which pushes up at the mid-ocean ridge provides the new land pushing the plates, and the subduction zones gobble up the land on the other side of the plates The mechanism was convection currents. Plate Tectonic Theory Both Hess’s discovery and Wegener’s continental drift theory combined into what scientist now call the Plate Tectonic Theory Theory of plate tectonics: The Earth’s crust and part of the upper mantle are broke into sections, called plates which move on a plastic-like layers of the mantle. The Earth’s crust is divided into 12 major plates which are moved in various directions. This plate motion causes them to collide, pull apart, or scrape against each other. Each type of interaction causes a characteristic set of Earth structures or “tectonic” features. The word, tectonic, refers to the deformation of the crust as a consequence of plate interaction. Tectonic plates are made of rigid lithosphere, the lithosphere is made of the crust and upper mantle. Below the lithosphere, is the asthenosphere which makes up the tectonic plates. “Plates” of the lithosphere are moved around by the underlying hot mantle convection cells. Divergent- moves away from each other Convergent- move towards Transform- one goes up one goes down Spreading ridges, as plates move apart new material is erupted to fill the gap Iceland is an example of continental rifting, it has divergent plate boundary running through the middle of it. Convergent boundaries have 3 different styles, continental- continental collision, continental- ocean crust collision, and ocean- ocean collision. Continental- continental collisions forms mountains (European Alps, Himalayas) Continental- ocean crust collision is called subduction, the oceanic lithosphere subducts underneath the continental lithosphere, oceanic lithosphere hears and dehydrates as it subsides, and it then melts forming volcanism (the Andes) Ocean- ocean plate collision- when 2 oceanic plates collide, one runs over and the other sinks into the mantle forming a subduction zone. The subducting plates is bent downward to form a very deep depression in the ocean floor called a trench. (The world deepest parts of the ocean are found along trenches, the Mariana trench is 11km deep) Transform boundaries- where plates slide past each other (San Andreas) |
|