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How good is your memory? • Structure of the Earth • Plate tectonic theory Today: • What are the different types of plate boundary/margin? • What processes and features occur at these margins? There are three main types of plate boundary. •CONSTRUCTIVE /Divergent boundaries -- where new crust is generated as the plates pull away from each other. •DESTRUCTIVE /Convergent boundaries -- where crust is destroyed as one plate moves under another. •CONSERVATIVE /Transform boundaries -- where crust is neither produced nor destroyed as the plates slide horizontally past each other. Artist's cross section illustrating the main types of plate boundaries (see text); East African Rift Zone is a good example of a continental rift zone. (Cross section by José F. Vigil from This Dynamic Planet -- a wall map produced jointly by the U.S. Geological Survey, the Smithsonian Institution, and the U.S. Naval Research Laboratory.) Convection Currents • Hot magma in the Earth moves toward the surface, cools, then sinks again. • Creates convection currents beneath the plates that cause the plates to move. Constructive margins • Where plates move apart in oceanic areas they produce mid-ocean ridges. • Where they move apart in continental crust they produce rift valleys. • The space between the diverging plates is filled with basaltic lava upwelling from below. • Constructive margins are some of the youngest parts of the Earth’s surface, where new crust is continuously being created. Oceanic ridges • The longest continuous uplifted features on the surface of the planet. • Have a total length of 60,000km. • In some parts they rise 3,000m above the ocean floor. • Where two plates pull apart there is a weaker zone in the crust and an increase in heat near the surface. The hotter, expanded crust forms a ridge. • Volcanic activity occurs along the ridge. • If these eruptions persist, volcanoes may develop until they reach the surface; islands can be formed in this way. • Iceland is the largest example, on the MidAtlantic Ridge. Surtsey: created by eruptions in 1963 • These volcanoes have fairly gentle sides because of the low viscosity of basaltic lava. (It can flow a long way before it cools) • Eruptions are frequent but relatively gentle. • As new crust forms and spreads, transform faults occur at right angle to the plate margin. • The parts of the spreading plates on either side of these transform faults may be moving at different rates, leading to friction and ultimately to earthquakes. • These tend to be shallow-focus earthquakes, originating near the surface. Constructive plate boundary Rift valleys • Occur at constructive margins in continental areas. • The heating and updoming of the crust leads to fracturing and rifting. • As the sides of the rift move apart, central sections drop down to form rift valleys. • The largest of these is the Great East African Rift Valley. • 4,000m long (from Mozambique to the Red Sea) • From the Red Sea it extends north into Jordan, a total distance of 5,500km. • 50km wide • In some areas, the inward-facing scarps are 600m above the valley floor and often marked by a series of parallel step faults. Step faults Inward facing scarps Fracture Volcanic activity. • Active volcanoes are surface evidence of volcanic activity beneath the rift valley. Mount Kilimanjaro Mount Kenya The rift valley might widen still further, allowing the sea to inundate it, possibly cutting off part of East Africa. Task.... • Add a copy of Fig 1.5 to the handout. • As neat as possible, include all labels. Destructive margins. • There are two types of plate, so there are three types of destructive/convergent margin: 1. Oceanic plate moves towards continental plate. 2. Oceanic plate moves towards oceanic plate. 3. Continental plate moves towards continental plate. Oceanic-continental convergence. • The denser oceanic plate is forced under the lighter continental one, into the upper mantle. • This process is known as subduction. • As the oceanic crust descends, friction with the overlying continental crust builds up and causes major earthquakes. Rocks scraped off the descending plate and folding of the continental crust create young fold mountain chains on edge of the continental mass. E.g the Andes. Deep ocean trenches are found along the seaward edge of destructive margins. They mark where one plate begins to descend beneath another and can reach great depths. E.g. Peru-Chile trench, 8km deep As it is less dense than the surrounding asthenosphere, the molten material begins to rise up through fissures and by burning their way through overlying rock. Eventually these reach the surface to form volcanoes. The lava has a viscous nature (flows less easily). This creates complex , composite, explosive volcanoes. If the eruptions take place off shore, a line of volcanic islands known as an island arc can appear Benioff zone – the further the rock descends, the hotter the surroundings become. This, together with the heat generated from friction, begins to melt oceanic plate into the magma. What do you know? Destructive plate boundary Example: • The Peru-Chile Trench. • Also known as the Atacama Trench. • Eastern edge of Nazca Plate being subducted under the South American Plate. • Max depth: 8,065m • Length: 5,900km • Width: 64km • If the eruption of magma takes place offshore, a line of volcanic islands known as an island arc can appear. • Such island chains may develop over millions of years to become major land masses. • E.g. Japan – Pacific plate subducted beneath Eurasian plate. Oceanic - oceanic convergence. • Two oceanic plates moving towards each other. • One is forced under the other and the processes involved with subduction begin. • The crust that is subducted may be marginally the denser of the two plates or is the one which is moving more quickly. • Ocean trenches and island arcs are the main features, as the interaction takes place well offshore. • E.g. Marianas trench – western side of the Pacific Ocean. • Pacific plate is being subducted beneath the smaller Philippine plate. Mariana Islands • Made up by the summits of 15 volcanoes. • To the west of the Mariana Trench. Guam • U.S territory, population 178,000. • “Where Americas day begins”. Continental-continental convergence. • As continents have similar density and thus buoyancy, they will not be subducted. • They collide with each other. • Their edges and the sediments between them are forced up into fold mountains. • Deep roots in the lithosphere. • No volcanic activity. • Movement of the plates can trigger shallowfocus earthquakes. Himalayas Causing Mt Everest to rise by up to 3cm a year. Indo-Australian Plate Moving north and east Approx. 5.8cm/year Eurasian Plate (rigid) In parts the Indo-Australian plate is being pushed under to form the mountain roots up to 70km deep. This movement causes great stresses which are released by earthquakes. Often extremely violent and destructive. E.g. Sichuan, 2008 – China, 80,000 deaths. 350km wide Extend for 3,000km Collision plate boundary Conservative margins. • Occur when two plates move parallel or nearly parallel to each other. • There is no creation or destruction of crust. • No subduction, therefore no volcanic activity. • Movement of the plates creates stresses between the plate edges. As sections of the plates rub past each other, the release of friction triggers shallow-focus earthquakes. The San Andreas Fault. • California • Although both plates are moving north-west, the Pacific plate moves faster, giving the illusion that they are moving in opposite directions. • Los Angeles could eventually be on an island off the Canadian coast. San Francisco 1906 1989 Another big one predicted before 2032!! Conservative plate boundary Task… • Complete the summary table of the different types of plate margin. Homework: • Finish any notes/diagrams from today. • What are ‘hot spots’? (Tip: find out how the Hawaiian Islands were/are formed). • Are there any other examples of ‘hot spots’? • Find and annotate diagrams to support written notes.