Hazards associated with earthquakes
- Ground displacement. Impacts structures such as buildings and bridges is very dangerous. Displacement of gas and electricity supply systems can lead to fires.
- Landslides. Movements of masses of rock, earth, or debris down a slope, triggered by a number of events, including earthquake tremors.
- Liquefaction. Occurs when the shaking of silts, sands, and gravel causes them to lose their load-bearing capacity. As a result, buildings and other structures may sink to the ground.
- Tsunamis. Ocean waves with extremely long wavelengths, generated by earthquake tremors.
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Hazards associated with volcanoes
- Lava flows. Sheets and tongues of liquid rock expelled from the crown or flank of an erupting volcano. The speed of lava flow depends on the viscosity of the lava, which itself is influenced by the temperature, the silica content and the incline of the volcano's slope.
- Explosive blasts. Outsbursts of fragments of rock and lava driven by gases expanding at great depths. These blasts may throw great blocks of rock many kilometres. However, the superheated blast cloud expelled by the volcano is more destructive.
- Ash flows/pyroclastic flows. Dense masses of gas and fragments of lava that flow down the sides of volcanoes at great speeds.
- Ash falls. Less devastating than ash flows but can be very disruptive, collapsing roofs, breaking branches and coating plants.
- Mudflows/lahars. As meltwater flows down the volcano's flank, it mixes with loose soil and ash to form a muddy liquid the consistency of wet cement.
- Glacial outbursts. Masses of water and ice suddenly released from a glacier by the heat from lava inside a volcano.
- Poisonous gases. Released in and around volcanoes before, during, and for many years after a volcanic eruption. The most abundant gases, water vapour and carbon dioxide, are not poisonous, but smaller quantities of more toxic gases are released, including sulphur dioxide and sulphur trioxide.
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The Richter scale
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The Mercalli scale
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Causes of tectonic hazards
- Crust types. The plates are made up of one or both of two different crust types: continental and oceanic. It is now known that the Earth's outer layers cannot simply be divided into crust and mantle. Rather, the outer part of the mantle consists of rigid material that is attached to the crust. Together, these two elements form the lithosphere. The upper, semi-molten part of the mantle is known as the athenosphere.
- Heat. The most likely cause of plate movement is convection cell currents in the mantle, caused by heat from the core. This heat comes from the combination of radioactive decay and residual primary heat and its currents cause the lithosphere to move. There is uncertainty about the forces involved but the movement is thought to be due to the pusing apart of the plates where two rising limbs of convection cells diverge below the surface, and the pulling downwards (known as drag) of the edges of plates at places where descending limbs exist. The downward drag seems to have the greater strength.
Plates are at their hottest near the mid-ocean ridges and cool as they move away. This increases their density and so they sink lower into the partially melted rock beneath and are dragged downwards into the subduction zone.
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Divergent plate margins
- Occur when plates are moving away from each other.
- May be a divergence of two oceanic plates (e.g. along the mid-Atlantic ridge) or of continental crust (e.g. east African rift valley).
- The divergence of convection cells beneath b ring magma from the mantle towards the surface.
- Pressure from the rising magma leads to a doming-up effect of the surface and the formation of a ridge, in which tensional faults are produced, into which the rising magma can enter. This cools and solidifies, producing new crust either within the existing crust or on the surface.
- Earthquake activity occurs at all plate margins, as it is the mechanism which releases the stress built up by the friction between moving plates.
- The exact nature of earthquake activity varies within the type of plate margin. At divergent margins most earthquake activity is shallow focus. low magnitude and high frequency.
- This is because the movements are at or near the surface, and the pressure is easily released as the plates diverge.
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Convergent plate margins
- Occur when two plates move towards each other, due to the descending limbs of convection cell currents in the mantle.
- This can happen when oceanic crust meets continental crust (e.g. Nazca plate and South American plate), or when two oceanic plates converge (e.g. Pacific plate and Philippine plate).
- Oceanic crust is denser than continental crust, and when the two converge the denser is subducted beneath the less dense. This forms an oceanic trench on the sea floor at the point of subduction. The continental crust, being more buoyant, is not subducted but uplifted, buckled and folded, forming a range of fold mountains.
- The subducted plate is heated and partially melts under pressure about 100km below the surface. This melt is less dense than the surrounding rock and so rises through any lines of weakness towards the surface. It may cool and solidify beneath the surface, forming intrusive igneous rock such as granite, or eventually reach the surface under great pressure, forming violent, infrequent volcanic eruptions.
- Earthquakes are also a common feature, occuring at a range og depths along the Benioff zone, the boundary between the subducting plate and the overlying crustal rocks, from shallow focus events at the ocean trench down to 700km.
- Where oceanic crust converges with oceanic crust, subduction still occurs and leads to the formation of a chain of volcanic islands above the subduction zone.
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Collision plate margins
- The convergence of two plates of continental crust is known as a collision margin,
- No subduction occurs, as both plates are buoyant and low-density.
- However, intervening oceanic sediments trapped between the two converging plates will be heaved upwards, resulting in the uplift of major fault mountain ranges.
- No volcanic activity is found at this type of margin, as no crust is being destroyed.
- Earthquakes do occur, although they are often deep-focus and have limited surface impact
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Conservative plate margins
- A conservative margin occurs when two plates move laterally past each other.
- No volcanic activity is found here.
- Shallow-focus earthquakes of varying frequency and magnitude are found.
- Low-magnitude, high-frequency events occur when pressure along the margin is easily released.
- Occasional major events take place after a significant build-up of pressure, typically when high levels of friction restrict movement of the crust along fault lines.
- Most plates have a constructive margin at one edge and a destructive margin at the other, with conservative margins making up the other two sides.
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- Hot spots are places where plumes of magma are rising from the asthenosphere, even though they are not necessarily near a plate margin.
- If the crust is particularly thin or weak, the magma may escape onto the surface as a volcanic eruption.
- Lava may build up over time until it is above present-day sea level, giving rise to a volcanic island.
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