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EESB15H3 (1)
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Chapter 2

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Environmental Science

Chapter 2 Plate Tectonics  the plate tectonics theory suggests that the surface of the earth is divided into several large plates that change in position and size  continental drift  moving continents  moving sea floor  sea floor spreading  tectonics is the study of the origin and arrangement of the broad structural features of the earth’s surface  intense geologic activity occurs at plate boundaries where plates move away from one another, past one another, or toward one another.  The concept of plate tectonics was born in the late 1960s by combining two pre-existing ideas – continental drift ad sea floor spreading. o Continental drift – the idea that continents move freely over the earth’s surface, changing their positions relative to one another. o Sea floor spreading – a hypothesis that the sea floor forms at the crest of mid-oceanic ridges, then moves horizontally away from the ridge crest toward an oceanic trench  Pangea initially separated into two parts: o Laurasia  The northern supercontinent, containing what is now North America and Eurasia (excluding India) o Gondwanaland  The southern supercontinent, composed of all the present-day southern-hemisphere continents and India (which has drifted north)  These continents all have deposits of Late Palaeozoic age  The study of ancient climates is called paleoclimatology  Polar wandering – Wegener used this term to describe the fact that ancient poles were in different positions than present poles Study of the sea floor  oceans cover more than 70 percent of the earth’s surface  sample of rocks and sediments can be taken from the sea floor in many ways: o rocks can be broken from the sea floor by a rock dredge, which is an open steel container dragged over the ocean bottom at the end of a cable o sediments can be sampled with a corer, which is a weighted steel pipe dropped vertically into the mud and sand of the ocean floor o both rocks and sediments can be sampled by means of sea floor drilling.  Offshore oil platforms drill holes in the relatively shallow sea floor near shore  A ship with a drilling derrick on its deck can drill a hole in the deep sea floor far from land  The drill cuts long, rod-like rock cores from the ocean floor  A basic tool for indirectly studying the sea floor is the echo sounder o Draws profiles of submarine topography o A sound sent downward from a ship bounces off the sea floor and returns to ship o The water depth is determined from the time it takes the sound to make a round trip  A seismic profiler works on the same principle as the echo sounder but uses a louder noise at lower frequency o Gives more information  The magnetism of old rocks can be measured to determine the direction and strength of the magnetic fields in the past o The study of ancient magnetic fields is called paleomagnetism o Old pole positions can be determined from the magnetism of old rocks  Pangea was formed by the collision of many small continents long before it split up o Continents have been in motion for at least the past 2 billion years What is Sea Floor Spreading  Harry Hess proposed that the sea floor might be moving too o This proposal was named sea floor spreading o Suggests that the sea floor moves away from the mid-oceanic ridge as a result of mantle convection o According to the initial concept of sea floor spreading, the sea floor is moving like a conveyor belt away from the crest of the mid-oceanic ridge, and across the deep-ocean basin, finally to disappear by plunging beneath a continent or island arc  Subduction  the sliding of the sea floor beneath a continent or island arc  The sea floor moves at rate of 1-10 cm per year  Hess hypothesized that the sea floor moves because the sea floor spreading is driven by deep mantle convection o Convection is a circulation pattern driven by the rising of hot material and/or the sinking of cold material o Hot material has a lower density, so it rises o Cold material has a higher density, and sinks  If convection drives sea-floor spreading, then hot mantle rock must be rising under mid-oceanic ridges o Hess showed how the existence of ridges and their high heat flow are caused by the rise of this hot mantle rock. o As hot rock continues to rise beneath ridge crests, the circulation pattern splits and diverges near the surface. Mantle rock moves horizontally away from ridge crests on each side of the ridge. This movement created tension at the ridge crest, cracking open the oceanic crust to form rift valleys and associated shallow-focus earthquakes  sea floor spreading implies that the youngest sea floor should be at the ridge crest, with the age of the sea floor becoming progressively older toward a trench What are Plates and how do they Move?  a plate is a large, mobile slab of rock that is part of the earth’s surface  the plates are part of a relatively rigid outer shell of Earth called lithosphere o includes
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