OSU OC 103 - e-OC103_Lesson07-08 (13 pages)

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e-OC103_Lesson07-08



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e-OC103_Lesson07-08

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13
School:
Oregon State University
Course:
Oc 103 - Exploring The Deep: Geography Of The World's Oceans
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OC103 Lessons 7 8 The Creation Aging and Recycling of Ocean Floor The Shifting Crust How it Works Last time we introduced the concept of Plate Tectonics and how new oceanic crust is created at seafloor spreading centers then slides across Earth s surface and subducts back into the mantle at subduction zones This double lesson will illustrate how that happens and why it is important for the oceans Components of the Plate Tectonic Conveyor Belt When Harry Hess proposed that the oceanic crust acts as a conveyor belt that carries continents with it he probably had in mind something like the figure below As the tectonic plates pull apart at seafloor spreading centers magma hot molten rock called lava when it reaches the surface rises into the crack from below and the magma solidifies into new oceanic crust The process repeats when the new crust later splits and more magma rises from below As this process continues over millions of years entire ocean basins floored by oceanic crust are formed Lithosphere and Asthenosphere You will notice in the figure on the previous slide the use of the terms lithosphere and asthenosphere It turns out that the plates that move around on Earth s surface are made of not just the crust but also the uppermost part of the mantle that is a little cooler and harder than the mantle just below it so it is strong enough to behave as part of the plates see figure below The cold and strong outer layer that includes the crust and uppermost part of the mantle is called the lithosphere from the Greek for stone which is what tectonic plates are made of The lithosphere can be brittle or elastic that is it can bend to a certain extent but will break if bent too far or too fast Below the lithospheric plates the mantle is solid but hot and weak and is called asthenosphere from the Greek for weak The asthenosphere is hard but plastic and deformable picture cold peanut butter or honey so plates can move slowly across it by deforming it Below the asthenosphere the mantle gets harder again because of the intense pressure at that depth The Tectonic Plates The tectonic plates cover Earth like the cracked shell on a hardboiled egg see figure below They are of various sizes from the huge Pacific Plate that underlies most of the Pacific Ocean to the small Juan de Fuca Plate just off of the coast of Oregon and Washington too small to be seen clearly on the map below but it will be obvious on a later figure Where they interact and jostle against each other they create earthquakes black dots on the figure below The Cookie Demonstration Does the description of the lithosphere asthenosphere and lower mantle remind you of anything Something with a hard upper plate a ductile deformable middle and a hard lower layer The figure below left provides a hint You can do your own demonstration to show how the plates move around and interact using an Oreo cookie I suppose you could use any similar sandwich cookie but I have not had a chance to test that scientifically yet When I do this as a demonstration in class I give everyone their own Oreo but you guys are on your own Some people claim the doublestuffed Oreo is the best for this demonstration but I personally prefer the Halloween Oreo because its orange creamy filling reminds me of hot magma from the mantle Start out by separating one of the chocolate cookies from the rest of the Oreo see figure below right The separate cookie will be your lithosphere while the creamy filling and the cookie still attached to it will be your asthenosphere and lower mantle respectively Break the lithosphere cookie in half and put the halves back together on top of the asthenosphere so that the Oreo looks whole again It will now be ready to demonstrate plate tectonics 3 Types of Plate Boundaries Divergent As tectonic plates move around and interact they create three types of boundaries between where they meet Divergent Plate Boundary where plates move apart At divergent plate boundaries plates separate and move away from each other Lithosphere is created in between the separating plates when magma rises to fill the gap and cools to form new lithosphere This is seafloor spreading see figure below left the plates spread apart rising magma fills the gap and new lithosphere is created on the trailing edge of the plates If you gently slide your two halves of the lithosphere cookie apart see figure below right you will see how hot magma could rise from the asthenosphere to fill the gap If you tried this on a hot enough day the creamy filling might even be gooey enough to actually do this for you Examples of seafloor spreading centers appear on the map of plates and earthquakes two slides ago as pairs of diverging arrows such as in the middle of the Atlantic Ocean in the eastern Pacific Ocean just west of South America and in the Indian Ocean south of Australia Credit USGS Credit Bob Lillie OSU 3 Types of Plate Boundaries Convergent Convergent Plate Boundary where plates collide Where plates collide one sinks or is shoved beneath the other Lithosphere is destroyed here as it sinks back into the hot mantle A convergent plate boundary is called a subduction zone if one plate sinks beneath the other see upper part of figure below left or a collision zone if neither plate wants to sink but they instead crumple together like accordions see lower part of figure below left Old oceanic lithosphere is cold and dense and will usually sink back into the mantle at a subduction zone but if the lithosphere is young and hot and thus still fairly buoyant it will be very reluctant to sink into the mantle Also thick continental crust is fairly light and will not sink so if two continents meet at a convergent plate boundary neither one will want to sink but instead they will crumple against each other creating large mountains such as the Alps or Himalayas see photo of Himalayas below center You can create your own subduction zone by shoving one half of the lithosphere cookie under the other as shown below right On the map of plates and earthquakes a few slides ago the convergent plate boundaries are shown by some of the many earthquakes black dots caused by one plate sinking underneath another Examples are the west coast of South America the south coast of Alaska and the western margin of the Pacific Plate where it subducts beneath Japan the Philippines etc The convergent boundaries at New Zealand and India are labeled with arrows pointing toward each other Credit Bob Lillie OSU 3


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