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ATER GEOGRAPHY a Level ATER GEOGRAPHY A Level AS Unit 1 Section B Tectonic Hazards A. Prickett Barry Comprehensive School 0 Section B: Tectonic Hazards Focus Geographical content Revision 1.3.1 Tectonic Characteristics of the Earth's structure including core, mantle and processes and crust and the boundaries between them. Mechanisms of plate movement including internal heating within hazards the Earth, convection currents, ridge push and slab pull. Plate distribution and the processes operating at different margins including diverging, converging and conservative margins; and tectonic activity at hot spots. Global distribution of tectonic hazards and their link to tectonic processes. Characteristics of the physical hazard profile that influence its impact including magnitude (as measured on Mercalli and Richter scales and Volcanic Explosivity Index (VEI), predictability, frequency, duration, speed of onset and areal extent. 1.3.2 Volcanoes, Types of volcano including shield, composite and cinder and processes, types of volcanic eruption including explosive and effusive. Volcanic processes and the production of associated hazards hazards and their including pyroclastic flows, lava flows, ash falls, lahars, jökulhlaups, impacts volcanic landslides and toxic gases. Demographic, economic and social impacts of volcanic hazards on people and the built environment including primary and secondary effects. Local scale, regional scale and global scale impacts of volcanic activity. Use example of one eruption to demonstrate the varied degree of risk and impacts of volcanic activity. 1.3.3 Earthquake characteristics to include P and S waves, focus, Earthquakes, depth and epicentre. Earthquake processes and the production of associated hazards processes, including ground shaking, liquefaction and landslides. hazards and their Demographic, economic and social impacts of earthquake activity impacts on people and the built environment including primary and secondary effects. Local scale, regional scale and global scale impacts of earthquake activity. Use example of one place specific event to demonstrate the varied degree of risk and impacts of earthquake activity. 1.3.4 Human Economic factors including level of development and level of factors affecting technology. Social factors including the population density, population profile risk and (age, gender) and levels of education. vulnerability Political factors including the quality of governance. Geographical factors including rural / urban location, time of day and degree of isolation. 1.3.5 Responses Monitoring, predicting and warnings of volcanic eruptions and to tectonic earthquakes. Short-term and long-term responses to the effects of earthquake hazards and volcanic hazards (the hazard management cycle). 1 INTRODUCTION What are Natural Hazards? What are Tectonic Hazards? 2 1.3.1 Tectonic processes and hazards Patterns of Plates and Plate Boundaries The World map on page 4 shows the distribution of volcanoes and earthquakes around the globe. They are found in similar locations in narrow belts along plate boundaries. A map of the World’s major plates and plate boundaries (margins can be seen on page 5) To understand why they occur here we must firstly understand the internal structure of the Earth, which is shown in the annotated diagram below (not shown to scale). Internal Structure of the Earth Km Lower mantle Lower mantle – hot, dense, solid Asthenosphere – hot, solid with partial melt Lithosphere – solid outer shell 3 The Layers of the Earth The uppermost layer of the Earth is known as the lithosphere. This is formed of the crust and top layer of the mantle. These two layers move together, floating on top of the next layer. The crust is split into two types: Continental (C) and Oceanic (O).The continental crust is thicker but less dense and so ‘floats’ a little higher than the oceanic crust which is denser but much thinner. In the asthenosphere there is tremendous heat and pressure which causes the otherwise solid material to become partially melted. This material can then flow in a series of convection currents much as the wax in a lava lamp does. It is the motion in this layer of the Earth that causes the plates to move. The rest of the lower mantle is hot, dense and mostly solid. The outer core is a layer of rapidly moving liquid metal (mainly iron and nickel). It is thought that the Earth’s magnetic field is generated by strong convection currents in this layer. The inner core is an extremely hot and dense ball of solid metal at the centre of the Earth. It is so dense that it generates the Earth’s gravity field. Decay of radioactive elements in the core generates the heat that drives all of these internal processes. The study of the movement of the Earth’s plates, and the landforms formed as a result of the movement, is called Plate Tectonics. Plate tectonics explains why volcanoes and earthquakes are found in narrow belts or active zones along the boundaries or margins of the plates. 4 Distribution of Earthquakes and Volcanoes Major earthquake centres Active volcanoes 5 Plate Boundaries and Plate Movement 6 Plate Tectonic Theory Plate tectonics has its origins in a theory put forward by German scientist Alfred Wegener in 1915. He suggested that all of the continents of the world were once joined together to form one super-continent called Pangaea. Pangaea then became split into two sections – Laurasia and Gondwanaland, and over the last 200 million years, the continents have drifted slowly to their present positions. Wegener called his theory Continental Drift. This theory was later refined into plate tectonic theory in the 1960s as observations were taken of the sea floor. What evidence supports plate tectonic theory? 1) Jigsaw fit of some continents 2) Rock formations – The same age, type and formations of rocks have been found to match up in places such as Brazil and Namibia (SW Africa), Norway and USA, India and Australia. 3) Palaeontological evidence – Fossils of the Mesosaurus (a small reptile) have only ever been found on the East coast of South America and the South West coast of Africa. 4) Palaeomagnetic evidence – When lava cools and crystallises, the magnetic minerals (e.g. iron) that it contains will align with the Earth’s magnetic field. This evidence will be stored in the rocks allowing us to work out where the rock was formed. 5) Palaeoclimatic evidence – The existence of coral fossils in the rocks in the UK suggest that we once had a much warmer climate indicative of locations nearer to the Equator. 6) Sea floor spreading – Oceanography suggests that the ocean crust is much younger than the continental. Studies also show that in some places the ocean plates are moving apart and new crust being formed in between. This process can be measured (on land in Iceland) and changes traced back in time to produce maps of where the continents were in the past. 7 Plate movement over the last 200 million years 8 Processes Operating Inside the Earth There are a number of processes that govern the plate movements occurring on the surface of the earth, though not all of these are fully understood. This is because most of these processes are happening deep down inside the planet. Where is all the heat from? Tectonic processes are driven by radioactive decay in the core. This decay generates heat inside the earth, which drives convection currents or cells. This convection is largely responsible for driving the plate movement in the crust above. There are two main theories as to how these convection currents actually move the plates. These are illustrated below: 1 2 9 1) Ridge push – this is the theory suggesting that the main mechanism driving the plates occurs at divergent (constructive) plate margins. As magma rises it forces the crust to rise at the ridge. As the crust rises gravity then forces it away from the margin, thus driving the crust. 2) Slab pull – this however suggest that the main driving force is the subduction of the colder, denser oceanic crust into the mantle. This effectively pulls the oceanic crust along behind it. Many scientists today believe that this is the main driving force behind plate movement, far more so than ridge push. 10 Types of Plate Boundary/Margin There are three main types of plate boundary: 1) Divergent (Constructive) – Ocean plate is made as plates move apart 2) Convergent (Destructive) – Plates collide (ocean plate can be destroyed) 3) Conservative – Plates move along side each other (none destroyed or made) Also: 4) Volcanic Hot Spots – Intra-plate volcanoes Divergent Plate Boundaries (Constructive, Spreading zones) Plate Movement Plates move apart, spread Processes Large rift valley occurs as plates move apart Magma pours into split between plates Lava solidifies to form new ocean crust Volcanoes form to produce ocean ridge on either side of central rift Speed of plate movement differs along boundary so large parallel faults are found Volcanic Activity Lots of volcanic activity near plate boundary Shield (gentle slopes) or basaltic volcanoes Eruptions not very violent Lava is fluid and mobile Earthquake Activity Less activity than other plate boundaries Small earthquakes are common, larger ones are rare Few of these events affect people Examples Mid – Atlantic ridge (formed as a result of N&S American plates moving away from Africa and Europe) Antarctic plate moving away from the Indo Australian plate African plate splitting along the rift valley 11 Mid ocean ridges and sea floor spreading Iceland Atlantic
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