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THE STRUCTURE OF THE EARTH • The earth is made up of 4 distinct layers: • The inner core is in the centre of the earth and is the hottest part of the earth. The inner core is solid. It is made up of iron and nickel with temperatures of up to 5500°C. With its immense heat energy, the inner core is like the engine room of the Earth. The outer core is the layer surrounding the inner core. It is a liquid layer, also made up of iron and nickel. It is still extremely hot here, with temperatures similar to the inner core. The mantle is the widest section of the earth. It has a diameter of approximately 2900km. The mantle is made up of semi-molten rock called magma. In the upper parts of the mantle the rock is hard, but lower down, nearer the inner core, the rock is soft and beginning to melt. The crust is the outer layer of the earth. It is a thin layer between 0-60km thick. The crust is the solid rock layer upon which we live. There are two different types of crust: continental crust, which carries land, and oceanic crust, which carries water. The diagram below shows the structure of the earth. In geography, taking a slice through a structure to see inside is called a cross section. CROSS SECTION OF THE EARTH TECTONIC PLATES • Plates and plate boundaries • The earth's crust is broken up into pieces. These pieces are called plates. Heat rising and falling inside the mantle creates convection currents. The convection currents move the plates. The movement of the plates, and the activity inside the earth, is called plate tectonics. • Plate tectonics cause earthquakes and volcanoes. The point where two plates meet is called a plate boundary. Earthquakes and volcanoes are most likely to occur either on or near plate boundaries. Subduction zone • The San Andreas fault in Western California • Convection currents Conservative plate margins • Conservative margins occur where two plates slide horizontally past each other. There is no subduction zone with one plate being destroyed beneath another nor is there a constructive zone as at the Mid-Atlantic ridge. As the two plates slide past each other, the high levels of friction create large strains along the slippage zone faults. Eventually the strain energy is released in an earthquake. When this happens there is likely to be large scale movement along the transform fault with substantial damage to nearby buildings. • The best known example of a conservative plate margin can be found along the west coast of the USA where the Nazca plate is moving in a North Westerly direction and sliding past the North American Plate. The North American plate is also moving in a North-Westerly direction but not so fast as the Nazca plate so the two appear to be moving in opposite directions. The fracture zone between the two plates is the San Andreas Fault - a huge fault running for hundreds of km along the Californian coastal region from San Francisco to Los Angeles. The region has a number of other substantial faults running parallel to the major San Andreas fault - the Hayward Fault runs almost parallel and to the east of the San Andreas fault with the modern city built across the two active fault planes. Earth tremors and earthquakes regularly occur along these fault planes - in 1906 the city of San Francisco was destroyed in a magnitude 8.2 quake on the San Andreas fault; in 1989 substantial damage and some 69 fatalities resulted from the magnitude 7.1 Loma Prieta earthquake. The 1989 Loma Prieta earthquake The 1906 San Francisco earthquake Volcanism • Processes which lead to the eruption of lava, gases, and pyroclastic materials onto the surface and into the atmosphere • Active volcanoes An active volcano has erupted recently and is likely to erupt again. • Dormant volcanoes A dormant volcano has not erupted in 2000 years. • Extinct volcanoes An extinct volcano will never erupt again. What are Volcanoes? • Conical mountains formed around a vent where lava, gases, and pyroclastic materials are erupted – Variations in the type of lava and other factors distinguish three types – Most have a central crater, while calderas and fissures are also common Cone volcanoes: •These are usually found at destructive boundaries. •Cone volcanoes are tall and steep-sided. •Cone volcanoes are formed by eruptions of thick, viscous (sticky) lava. •The thick lava moves relatively slowly and hardens quickly to form new rock - this explains the formation of a cone shape. •Eruptions tend to be violent. Shield volcanoes: •These are usually found at constructive boundaries. •They are low, with gently sloping sides. •Shield volcanoes are formed by eruptions of thin, runny lava. •Eruptions here tend to be frequent but relatively gentle. Composite volcanoes: •These volcanoes are composed (made up) of alternating layers of lava and ash (other volcanoes just consist of lava). •The eruptions from these volcanoes may be a pyroclastic flow rather than a lava flow. A pyroclastic flow is a mixture of hot steam, ash, rock and dust. •A pyroclastic flow can roll down the sides of a volcano at very high speeds and with temperatures of over 400° C. What is this guy doing? Why is he dressed like this? Crater Lake Cinder Cone Lava Types Debris Avalanche, Mt. Adams Pyroclastic Flow, Mt. St. Helens Mt St Helens Eruption Volcanic eruptions can have a devastating effect on people and the environment. However, unlike earthquakes, volcanoes can also have a positive impact on an area. These positive impacts can help to explain why people choose to live near volcanoes. MONTSERRAT • Montserrat is a small island in the Caribbean. There is a volcanic area located in the south of the island, called Soufriere Hills. • The volcanic peak in this area is called Chances Peak, which had been dormant for over 300 years. Then in 1995, the volcano began to give off warning signs of an eruption (small earthquakes and eruptions of dust and ash). Once Chances Peak had woken up it then remained active for a period of 5 years. The most intense eruptions occurred in 1997. • During this time, Montserrat was devastated by pyroclastic flows. The small population of the island (11,000 people) was evacuated in 1995 to neighbouring islands. The evacuees became refugees. • Despite the evacuations, 19 people were killed by the eruptions. This is because a small group of people chose to stay behind on the island and watch over their crops. • Volcanic eruptions and lahars have destroyed large areas of Montserrat. The capital, Plymouth, has been covered in layers of ash and mud. Homes and buildings have been destroyed. (Lahars are similar to pyroclastic flows but contain more water ) The graphic shows the progress of the eruption and its impact on the island. Volcanic activity has calmed down in recent years and people have begun to return to the island. Eruption Ash Disaster Evacuation The island of Montserrat in the Caribbean. How deep is the ash in this photo? What effect might it have had on the people living here? Few people died in this eruption but it is still a disaster. Why? Why do people live close to volcanoes? People live close to volcanoes because Geothermal energy can be harnessed by using the steam from underground which has been heated by the Earth's magma. This steam is used to drive turbines in geothermal power stations to produce electricity for domestic and industrial use. Countries such as Iceland and New Zealand use this method of generating electricity. Volcanoes attract millions of visitors around the world every year. Apart from the volcano itself, hot springs and geysers can also bring in the tourists. This creates many jobs for people in the tourism industry. This includes work in hotels, restaurants and gift shops. Often locals are also employed as tour guides. Lava from deep within the earth contains minerals which can be mined once the lava has cooled. These include gold, silver, diamonds, copper and zinc, depending on their mineral composition. Often, mining develop around volcanoes. Volcanic areas often contain some of the most mineral rich soils in the world. This is ideal for farming. Lava and material from pyroclastic flows are weathered to form nutrient rich soil which can be cultivated to produce healthy crops and rich harvests.
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