Disasters Lithosphere/crust: the earth’s crust which includes landforms, rocks and soils. Mostly solid rock, rigid and contains high strength. 5-7km Mantle: Partially molten rock. 500 to 2000 degrees. It is 2900km thick. The asthenosphere is the hot, soft, semi-fluid upper part of the mantle. Outer core: molten iron and nickel. 4000 to 6000 degrees. It is 2300km thick. Inner core: Mostly iron. Solid because of pressure. 7000 degrees max. It is 1200km thick. Hydrosphere: The water on or at the surface of earth. Eg- oceans, lakes, rivers Atmosphere: Blanket of gas surrounding Earth. It is 145km above sea level, and nitrogen and oxygen make up most of it. Theory of Plate Tectonics: The theory says that the Earth’s crust is divided into a number of plates, which move across the layer of partially molten rock in the asthenosphere. Convection currents and gravitational forces cause the plates to move. Heat from the core causes mantle rock to rise and spread out. It then cools and falls due to gravity. The plate’s movement vary from 2-10 cm per year. There are 13 major plates. Movement of the crust can cause earthquakes and most volcanoes. Evidence: 1. Geometric fit of the continents: jigsaw can make super continent called Pangaea 2. Age of rocks and fossils: rocks and fossils are similar and of the same age on sides of continental margins. 3. Earthquakes and volcanic distribution: these occur on location of plate boundaries 4. Mid-oceanic ridges: they show where new plates are formed, pushing plates apart. 5. Magnetic reversals Sea-floor spreading: The idea of sea-floor spreading is that new crust is being made in the mid-oceanic ridges. Half of the new crust goes to each side of the ridge. The growing ridge pushes the continents apart. The rock to make crust is supplied in the mantle. Mid-Oceanic Ridges: They are an underwater mountain range, and are usually made by tectonic plates. Magma constantly emerges onto the ocean floor through a rift. Usually, the magma builds up and forms new crust. The rocks that make up the sea floor are usually younger near the centre of the ridge and the older ones are further away. It is evidence of the emergence of new magma. As the crustal plates move about, the magma emerges. Page 1 Divergent boundaries/Spreading zones eg-Mid Atlantic Ridge At divergent boundaries, plates are moving apart from each other and the space that this creates is filled with new crust due to the process of sea-floor spreading. Convergent boundaries eg- Himalayas At convergent boundaries, plates collide with each other. However, if one plate is pushed under the other plate, it is called a subduction zone. They are usually indicated by chains of volcanoes and ocean trenches. If a thin oceanic plate collides with the thick continental plate, the oceanic plate is usually pushed down. The plate that is pushed downward into the mantle melts and molten rock rises in a line of volcanoes. A deep-sea trench forms where 1 plate is subducted under the other. When two continental plates collide, they either crumple and compress, or one plate burrows under or overrides the other. This creates mountain ranges. When two oceanic plates converge, they form an island arc as oceanic plate is subducted. Transformational boundaries: They occur when plates slide and grind along transform faults and usually causes earthquakes. An example is the San Andreas Fault. Volcanoes: It is a vent or chimney, and its surrounding is built up of volcanic material, which connects the magma in the magma chamber with the surface. The study of volcanoes is called vulcanology. A person who studies volcanoes is called a vulcanologist. Volcanoes can be classified by their recent history of activity: 1. active: any volcano with some form of current activity eg- eruption release of lava or seismic activity. 2. dormant: any volcano that hasn’t erupted for a long time but could do so again 3. extinct: any volcano that will not erupt again The viscosity of the lava extruded from a volcano is the main force behind the eruption type and that is the main factor controlling the shape of a volcano. 1. Composite (stratovolcanoes): formed from viscous lava, made up of pyroclastic (erupted rock fragments such as pumice) and lava, they have steep slopes eg-Mt. Fuji. Formed from convergent boundaries. 2. Shield volcanoes: formed from less viscous lava which flows away from the vent and over time forms low sloping volcanoes. Page 2 3. Caldera: erupt so explosively that little material builds up near the vent. Eruptions empty part or the entire underlying magma chamber, causing the region to sink under its weight. This results in a basin shaped depression. It is roughly circular, and can be several kms in diameter. What comes out of a volcano? Gases eg- carbon dioxide, nitrogen, carbon monoxide, hydrogen, sulphur dioxide Aerosols: sulphur dioxide reacts with water to form liquid aerosol particles of sulphuric acid Liquids: lava Solids: lava, fine dust A volcano erupts when the pressure of magma within the volcano gets so high that it moves upwards through a weakness in the crust. Earthquakes: A sudden violent shaking of the ground caused by movements within the crust. Most earthquakes occur on or near plate boundaries. They usually occur when transform fault plates grind pass each other. Rocks in these plates contain a lot of elastic potential energy. When the rock reaches its breaking point, it breaks the rocks and releases that energy, causing earthquakes. The energy travels as a wave motion. Waves that travel through the earth are called seismic waves. The epicentre is the place on earth above where the earthquake has occurred. The focus is the exact point where an earthquake occurred. The waves that travel out, causes the earth to shake. The shaking is detected using a seismograph, which measures and records details of an earthquake. The chart it records to is called a seismogram. The most commonly used scale for measuring earthquakes is called the Richter scale, which measures the power of an earthquake. Earthquakes can cause a lot of damage to buildings and other structures. They can cause landslides. Page 3