Tectonic Activity Earthquakes Introduction An earthquake is the shaking and vibration of the crust due to plate tectonics (movement of plates) Earthquakes can happen along any type of plate boundary. They also occur along faults which are large cracks in the earth s crust. Most faults are associated with large plate boundaries where violent earthquakes usually occur. Causes of Earthquakes Earthquakes are caused when the tension is released from inside the crust. This happens because plates do not move smoothly - sometimes they get stuck. When this happens a great deal of pressure builds up.
Eventually this pressure is released and an earthquake tends to occur. An earthquake starts deep below the earth s surface at the focus. The focus is the point inside the Earth's crust where the pressure is released. The epicentre of an earthquake is the position on the earth s surface directly above its focus. The jerking movement caused by plates sticking then moving releases built-up pressure inside the Earth's crust, in the form of seismic waves. The waves spread out from the focus. The strongest waves are found near the centre of the earthquake. This means that the most severe damage caused by an earthquake will happen close to the epicentre. It is almost impossible to predict when they will occur. The effect of an earthquake depends on the depth of an earthquake as well as its magnitude. If the focus is very deep or the shockwaves have to travel through dense rock, the effect will be less The following video shows how a Tsunamis works, which are often caused by Earthquakes.
Measuring Earthquakes Richter Scale The power (magnitude) of an earthquake is measured on the Richter scale, using an instrument called a seismometer. The Richter scale is numbered 0-10 with 10 being the most powerful. The Richter scale is logarithmic an earthquake measuring 7 is 10 times more powerful than one measuring 6 and 100 times more powerful than one measuring 5. Mercalli Scale The Mercalli scale measures the damage caused by an earthquake. The Mercalli scale goes from I to XII Effects of Earthquakes Primary effects Occur immediately All due to the shaking of the ground e.g. buildings collapsing, destruction of roads and bridges. Secondary effects
Happen afterwards, but can be even more devastating e.g. fire, tidal waves, disease and landslides Example: Northwest Turkey, Izmit 1999 Primary effects: - Over 20,000 deaths -More than 100,000 buildings were destroyed - Over 100,000 families homeless. - Transport links destroyed - Landslides Secondary effects: -Disease e.g. Cholera and typhoid (caused from polluted water) -Services damaged e.g. water, electricity and sewage -Intense heat and dust (people who were made homeless had no shelter, thus were exposed) Formation of Fold Mountains Fold mountains form along both destructive and collision plate boundaries. They form when two plates with landmasses on them move towards each other. The plates push layers of accumulated sediment in the sea into folds between them. This becomes a fold mountain range. Most fold mountains continue to grow as the plates constantly move. Examples: the Himalayas (Asia), Rockies (USA), Andes (South America), Alps (Europe) This short animation explains how tectonic activity forms moutain ranges
Human effect of Fold Mountain Ranges There are both positive and negative effects for the people living in these mountain ranges. Tourism is a positive effect with hill walking, attractive scenery, river rafting, and climbing attracting people. However, building tourist facilities such as hotels and restaurants is difficult due to the lack of flat land. Skiing is a popular tourist activity in winter e.g. in the Alps and Rockies. Tourism can have negative impacts on the local environment and people. There can be a constant threat of avalanches in winter which have to be monitored with huge amounts of money being spent to combat the avalanche threat, especially where many tourists use the mountains. Tourists also bring congestion, litter and pollution problems. Farming is the main primary activity but often only cattle and sheep rearing is possible due to the very cold, wet climate, the altitude and steepness of the slopes. Agricultural machinery is difficult to use and there is a very short growing season. In mountains such as the Andes and Himalayas, terraces (steps) have been cut into the hillsides to allow crops such as vines and fruit to be grown. Forestry is a key economic activity with the planting and harvesting of trees e.g. in the Alps. There are times, however, when forestry can be damaging e.g. in the Himalayas local people and logging companies have cut down large numbers of trees causing large-scale deforestation which leads to problems of soil erosion and flooding. Tectonic Plates Introduction
Tectonic plates are sections of the Earth's solid crust that float on top of the mantle. The mantle contains hot molten rock (magma) heated by energy from the earth's core The earth s crust is made up of seven main tectonic plates and numerous smaller plates. There are two main types of tectonic plate. Oceanic plates are relatively thin (about 6-10 km) and contain very heavy (dense) rocks such as basalt. Continental plates are considerably thicker (35-40 km) but contain lighter (less dense) rocks such as granite. The plates fit together like pieces of a large jigsaw. Tectonic plates meet along four main types of plate boundaries. The tectonic plates are in constant motion travelling at a few centimetres per year. The ocean floors are continually moving, spreading from the centre and sinking at the edges. Where plates meet, huge forces build up causing earthquakes and volcanoes and the formation of fold mountains and deep-sea trenches. This short video provide an introduction into Plate Tectonics and charts some of the historic movement of plates.
Convection Currents Hot currents of molten rock called convection currents move slowly within the earth's mantle which moves the tectonic plates above them in different directions. The source of heat driving the convection currents is radioactive decay which is happening deep in the Earth. Convection occurs because the density of a fluid is related to its temperature. Hot rocks lower in the mantle are less dense than their cooler counterparts above. The hot rock rises and the cooler rock sinks due to gravity.
Plate Boundaries Conservative Plate Boundaries Conservative Plate Boundaries (aka Passive) When two tectonic plates slide past each other without creating or destroying any land they form a conservative plate boundary. The plates often get stuck as they try to move past each other due to friction. Over time this builds up great pressure until finally they jolt past each other. The sudden movement causes an earthquake which may be violent and cause great damage. Example: the San Andreas Fault where the N. American Plate meets the Pacific Plate Constructive Plate Boundaries Constructive Plate Boundaries (aka Divergent or Tensional) When two plates move away from each other creating a gap between them Molten rock (magma) rises from the mantle to fill the gap forming a mid-ocean ridge
Constructive plate boundaries cause mid-ocean ridges, volcanoes and earthquakes, though often less violent than those on destructive plate boundaries. Volcanoes can form along the edges of the plate boundary due to the rising magma. These volcanoes are called shield volcanoes. Example: the North American plate moving away from the Eurasian plate formed the Mid-Atlantic Ridge and created Iceland through volcanic activity. Destructive Plate Boundaries Continental Plate < > Continental Plate When continental plates collide They crumple and create fold mountains Where two continental crusts collide neither can sink Instead they push into each other forcing material to be folded up into huge mountain ranges Often this movement and pressure can cause earthquakes, but no volcanoes occur on these boundaries
Example: Himalayas created where the Indian Plate meets the Eurasian Plate. Oceanic plate < > Continental plate Destructive Plate Boundaries (aka Convergent or Compressional) Oceanic plate colliding with a Continental plate
When an oceanic plate and a continental plate move towards each other the denser oceanic plate dives under the lighter continental one creating a deep ocean trench. As the oceanic plate goes deeper into the mantle, increased temperature and friction cause it to melt creating a subduction zone. Newly molten rock is lighter than the surrounding rock so it rises through the crust towards the surface. If molten rock (magma) reaches the earth s surface it may spread out along a fault line or it may erupt at a single point as a volcano. Example of an oceanic plate colliding with a continental plate is where the Pacific plate moving towards the South American plate at a rate of 9 centimetres per year. Oceanic Plate < > Oceanic Plate Oceanic plate colliding with oceanic plate When two oceanic plate meet each other (oceanic-oceanic) this often results in the formation of an island arc system. As the subducting oceanic crust melts as it goes deeper into the Earth, the newly-created magma rises to the surface and forms volcanoes. If the activity continues, the volcano may grow tall enough to breech the surface of the ocean creating an island.
Example of an island arc is West Indies and Japan The Early Earth & Plate Tectonics The Earth is formed by accretion of spatial particulates and large masses and eventually forms an outer crust. This short video follows speculation of early plates and land masses and their movement through time. Summary
Plates are sections of the Earth s crust that float on top of the mantle There are two types of plate - Oceanic & Continental Convection currents cause the plates to move Oceanic colliding with a Continental (destructive) = deep open trench, possible volcanic eruptions Continental colliding with a Continental (destructive) = fold mountains formed Oceanic colliding with an Oceanic (destructive) = Island Arc system formed Plates moving apart (constructive plate boundaries) = mid-ocean ridges, volcanoes and earthquakes can result Plates sliding past each other (conservative plate boundaries) = no landforms but they can cause damaging earthquakes Volcanoes Introduction Volcanoes are formed along two types of plate boundary: destructive and constructive. Volcanoes occur where molten rock (magma) comes to the surface of the earth. The magma rises to the surface through cracks in the crust called vents. Cone Volcanoes
When lava is thick and acidic it doesn t flow far before cooling and solidifying, this causes cone volcanoes (aka acid cone volcanoes) to have steep sides. Example: Mount Pelee on Martinique (an island in the Caribbean) Shield Volcanoes When lava is runny and thin it can flow a long way before cooling and solidifying, this causes shield volcanoes to have gentle slopes and wide bases built almost entirely of low viscosity basaltic lava flows. Shield volcanoes are the largest of the three types. The eruptions are generally non explosive due to the low silica content and may last for years Example: Mount Kilauea in Hawaii (USA), Galapagos Islands, Snake River Plain in Idaho, USA. Composite Volcanoes Composite volcanoes are the most deadly of volcano types. They are made of alternate layers of ash and lava and have steep sides built up by eruptions of intermediate viscosity andesitic lava and explosive tephra. Often the lava cools creating a plug which blocks the vent resulting in a huge explosion blowing out the plug
Example: Mount St Helens in Alaska (USA), Mount Shasta in California, Mount Rainier in Washington state, and Mount Fuji in Japan. Human effects of Volcanoes Primary and Secondary effects The effects of volcanoes can be divided into primary and secondary effects. Primary effects Produced directly by the volcanic activity. Examples include lava flows, ash-flows, lateral blasts, ash-falls, and gases. Secondary effects The result of primary effects e.g. mud flows Lava Flows Lava flows are streams of molten rock. Lava flows can erupt relatively non-explosively and move very slowly (a few meters to a few hundred meters per hour) or they can move rapidly (typically down steep slopes). Most lava flows are slow enough that they are seldom a threat to human life. Such flows generally follow a predictable course. However, lava flows can cause extensive damage or total destruction by burning, crushing, or burying everything in their paths.
Whole villages have been known to completely disappear beneath lava flows. To avoid such destruction, controlling a lava flow has become important and successful deflection has occurred in the past. photos of lava flows. Pyroclastic Hazards Volcanic explosions produce volumes of tephra. Tephra is the material blown out of the volcanic vent when an explosion occurs. Ash-flows, lateral blasts, and ash-falls are the types of pyroclastic activity that produce tephra, with composite volcanoes and large calderas the vent sources. Mud Flows Mud flows (Lahars / Debris flows) are mixtures of water, rock, ash, sand, and mud that originate from the slopes of a volcano. They can travel over 80 kilometres and commonly reach speeds of 35 to 65 kilometres per hour. They contain a high percentage of rock debris look like fast-moving rivers of concrete. Close to a volcano, they have the strength to rip huge boulders, trees, and structures from the ground and carry them for great distances. Farther downstream the coarser debris settles to the bottom of the flow, leaving mud to continue on to cover everything it passes. Mud Flows are formed when masses of unconsolidated, wet debris become mobilized, and are commonly start by: - Large landslides of water-saturated debris - Heavy rainfall eroding volcanic deposits - Radiant heat emitted from a volcanic vent suddenly melting snow and ice - Pyroclastic flows on the flanks of a volcano - Breakout of water from glaciers, crater lakes, or from lakes dammed by volcanic eruptions Historically, mud flows have been one of the most deadly of the volcanic hazards Pyroclastic Flows Pyroclastic flows (also called ash-flows) are high speed avalanches of hot ash, rock fragments, and gas
which move down the sides of a volcano during explosive eruptions. These flows occur when the vent area or ash column collapses. Because pyroclastic flows can reach 1500 degrees F and travel at high speeds (160-250 kilometres per hour and up), they are extremely destructive and deadly. Pyroclastic flows are typical of composite volcano eruptions, but are also associated with large caldera systems. Volcanic Gases All magmas contain dissolved gases that are released during and between eruptive episodes. These gases are predominately steam, followed in abundance by carbon dioxide, compounds of sulphur and chlorine, and lesser amounts of other gases. While they rarely reach populated areas in lethal concentrations, gases can be injected to great heights in the atmosphere by volcanic eruptions, in some cases spreading throughout the globe. QUESTIONS Quiz 1 Question 1 What is an earthquake? Answer An earthquake is the shaking and vibration of the crust due to plate tectonics (movement of plates) Question 2 What is the name of the point at which the earthquake occurs deep underground? Answer The focus Question 3
What is the Richter Scale used for? Answer The Richter Scale which goes from 1 to 10 is used to record the strength of an earthquake. Question 4 Give an example of the secondary effects of an earthquake. Answer Fire, tidal waves, disease and landslides Question 5 Which type of plate boundaries are fold mountains formed along? Answer Destructive and Collision Boundaries Quiz 2 Question 1 What economic activities are likely in an area of fold mountains? Answer Tourism (skiing and walking), forestry, pastoral farming or arable farming where terracing has been carried out. Question 2 What tectonic plate do we live on in the UK?
Answer The Eurasian plate Question 3 Which are thinnest the oceanic plates or the continental plates? Answer The oceanic plates are much thinner. Question 4 What is the link between the convection currents in the mantle and the tectonic plates? Answer The convection currents make the plates move. The solid crust rides on the moving mantle. Question 5 What type of plate boundary is the San Andreas Fault (N.America) an example of? Answer The San Andreas Fault is an example of a conservative plate boundary.