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Transcript
THE STRUCTURE OF THE EARTH CRUST MANTLE OUTER CORE INNER CORE 6000 OC 4000 OC The diameter of the Earth is approximately 12,800km. 3000 OC 2000 OC ATMOSPHERE Crust The crust, a thin layer of solid rock only 50km thick, has a mean density of about 2.5g/cm3. Continental (or granitic) crust, which is rich in silicon and aluminium, is pale in colour. Oceanic (or basaltic) crust, which is rich in iron and magnesium, is dark in colour. It is here that we obtain all of the resources that we depend on. Mantle The mantle, which is approximately 5700km thick, has a mean density of about 4.5g/cm3. The mantle consists of SOLID rock which is rich in magnesium, iron and silicon. Although the mantle is solid, its high temperature means that the rocks are able to move slowly (creep). Convection currents in the mantle are responsible for the movement of the continents. Outer Core The outer core, which is a very hot, dense liquid, consists mainly of iron and nickel. It extends to a diameter of about 6900km (just over half the Earth’s diameter) and has a mean density of about 11g/cm3. The Earth’s magnetic field arises here. Inner Core The inner core, like the outer core, consists mainly of iron and nickel, but because of the immense pressure, it is solid. It extends to a diameter of about 2500km and has a mean density of about 15g/cm3. TOPIC 10.1.7: OUR CHANGING PLANET 1 PLATE TECTONICS The Earth originated as a cloud of hot gas which condensed as it cooled to form a sphere of hot liquid. The densest substances sank to the middle of this sphere, forming the core, while the least dense materials stayed on the surface, where they cooled and solidified to form the crust. Until as late as the 1960’s, some scientists believed that the Earth continued cooling to the present day. The interior cooled and contracted as it did so, the solid crust buckled and puckered, forming mountain ranges. This effect is similar to the effect seen on the surface of hot toffee as it cools. Theory of Plate Tectonics Alfred Wegener Today it is believed that the Earth’s crust is cracked into a number of large pieces (tectonic plates). These plates are constantly moving at relative speeds of a few centimetres per year as a result of convection currents within the Earth’s mantle, driven by heat released by natural radioactive processes. This causes the continents, which sit on the plates, to change positions relative to each other. This theory was first put forward by Alfred Wegener in 1911; at that time people dismissed it, believing him to be a crank. Drifting Continents Material is subducted in an ocean trench and added to at a mid-ocean ridge, with the net result that material is transferred from one edge of a plate to the other. As plates move, driven by convection currents in the mantle, the continents ride on them. Although the process is very slow, in geological time (millions of years) the continents can move hundreds or even thousands of kilometres. The current arrangement of the continents is thought to have developed in the following way: Pangaea TOPIC 10.1.7: OUR CHANGING PLANET 2 The original land mass on the Earth was named Pangaea. Laurasia Gondwanaland 180 million years ago 180 million years ago, Pangaea began to split into two major parts, Laurasia and Gondwanaland, and Gondwanaland itself was beginning to break up. 135 million years ago Laurasia and Gondwanaland drifted northwards. The North Atlantic and Indian Oceans widened. The South Atlantic rift lengthened. 65 million years ago South America had separated from Africa. Australia and Antarctica were still joined. India was moving towards Asia. The Mediterranean Sea had appeared. Today South America has joined with North America. Australia has separated from Antarctica. India has collided with Asia. TOPIC 10.1.7: OUR CHANGING PLANET 3 EVIDENCE TO SUPPORT THE THEORY OF PLATE TECTONICS 1. Different Climates Although the Earth’s climatic regions are not thought to have changed significantly over time, it is possible to find in great Britain examples of rocks which were formed: under desert conditions under tropical swamp conditions from corals in warm seas under glacial conditions etc. This could be explained, however, if Britain itself had moved through the different climatic regions over time. About 300 million years ago, in the Carboniferous period, Britain was near the equator and tropical forests grew there. These later decayed to form coal deposits. By the Permian period, Britain had moved further north and had a desert climate. Most of the rocks deposited during this period are desert sandstones. 2. Continents Which Were Once Joined The edges of land masses (continents) which are separated by thousands of kilometres of ocean (e.g. the east coast of South America and the west coast of Africa) have shapes which fit quite closely. This suggests that they were once part of a single land mass which has split and moved apart. Also, if South America and Africa are rejoined, they can be seen to have similar patterns of rocks and fossils. For example, an old mountain belt and fossil remains of the Mesosaurus can be seen to run continuously between South America and Africa. 3. Earthquake Zones Most earthquakes and volcanoes occur along clearly defined lines in the Earth’s crust which correspond to the margins of the plates, where they rub against each other. If a large amount of stress builds up, the plates may deform. The built up tension can be released by a sudden movement: this is an earthquake. Scientists are unable to predict when earthquakes will happen. TOPIC 10.1.7: OUR CHANGING PLANET 4 TOPIC 10.1.7: OUR CHANGING PLANET 5 The Earth’s atmosphere The earth was formed about 4.6 billion years ago. During the first billion years of the Earth.s existence there was intense volcanic activity. This activity released the gases that formed the early atmosphere and water vapour that condensed to form the oceans. For the last 200 million years, the proportions of different gases in the atmosphere have been much the same as they are today: − about four-fifths (78%) nitrogen − about one-fifth (21%) oxygen − small proportions of various other gases, including carbon dioxide, water vapour and noble gases. Some theories suggest that during this period, the Earth’s atmosphere was mainly carbon dioxide and there would have been little or no oxygen gas (like the atmospheres of Mars and Venus today). There may also have been water vapour and small proportions of methane and ammonia. Bacteria and algae began to thrive and started to produce oxygen. As more plants spread across the earth’s surface, oxygen became richer and richer and eventually it was possible for animals to evolve Some of the earliest bacteria could not tolerate oxygen and eventually died out.. The seas absorb huge amounts of carbon dioxide forming both soluble and insoluble carbonates. Most of the carbon from the carbon dioxide in the air gradually became locked up in sedimentary rocks as carbonates and fossil fuels. The level of CO2 has remained constant as a result of a natural cycle. Nowadays the release of carbon dioxide by burning fossil fuels increases the level of carbon dioxide in the atmosphere. As CO2 levels in the atmosphere increase energy from the sun is trapped within our atmosphere leading to global warming. The Carbon cycle TOPIC 10.1.7: OUR CHANGING PLANET 6 Life on Earth Have you ever wondered where the molecules which make up the cells that all living things are made of? One theory (Miller-Urey) suggests that methane (CH4), ammonia (NH3), water (H2O) and hydrogen (H2), which were all present in the earth’s early atmosphere, may have reacted to form the amino acids which we are all made of. The energy required to make this happen could have been provided by lightning. In the laboratory they mixed these gases and produced sparks to simulate lightning over 7 days. When the mixture produced was analysed they found 22 amino acids. Other scientists suggested that the composition of the earth’s atmosphere was different to those used by Miller and Urey, however, amino acids are still produced when they replicated the original experiment. Other theories suggest that meteors crashing into our planet brought a variety of organic molecules with them. These are only theories and it has not been proven that life on earth did start in this way. TOPIC 10.1.7: OUR CHANGING PLANET 7 Gases in the atmosphere The air we breathe is a mixture of several very useful gases. Oxygen is used in welding as well as to help people with breathing difficulties in hospital. Nitrogen is unreactive and is used to stop food going off (eg a packet of crisps). Liquid nitrogen is used to freeze food rapidly (eg frozen fish fingers). Argon is used in light bulbs. In order to use these gases we need to separate the mixture. To do this we use fractional distillation. The liquefied air (-200oC) is passed into the bottom of a fractionating column. The column is warmer at the bottom than it is at the top. The liquid nitrogen boils (-196oC) at the bottom of the column. Gaseous nitrogen rises to the top, where it is piped off and stored. Liquid oxygen (-183oC) collects at the bottom of the column. The boiling point of argon - the noble gas that forms 0.9% of the air - is close to the boiling point of oxygen, so a second fractionating column is often used to separate the argon from the oxygen. TOPIC 10.1.7: OUR CHANGING PLANET 8