4.1 Tectonics Flashcards
What are hazards? What are the types?
Threats that could injure people and damage environments
Natural hazards occur when humans interact with the physical world.
Disasters are hazards which cause so much damage that recovery without help is impossible
Risk is the exposure to a hazard
Vulnerability is the degree to which conditions make a population more likely to experience a hazard
What is the perception of risk?
Responses to an event depend on experience, material well being and personality
3 choices in an event - do nothing, adjust to the situation or leave the area.
Adjustment may occur by identifying hazards, estimating the risk and evaluating the cost caused by the hazard
What are the ways of reducing risk?
- Hazard mapping uses a body of theory where the build up to a disaster and its aftermath are assessed.
- Rehabilitation refers to people being able to make homes safe to live in again
- Government use rehabilitation, reconstruction and aid focused on reducing the impact of future events to improve safety of the region.
What is the global distribution of seismic hazards?
- 84% of seismic events occur as plate boundaries, in clear linear patterns determined by them.
- Collision and destructive plate boundaries result in linear chains and conservative and constructive result in lines of narrow earthquakes.
- The remaining 15% are intraplate earthquakes caused by human activity
- Broad belts associated with subduction zones
- Narrow belts associated with constructive plate boundaries as new material is formed and plates are moving apart.
- Collision boundaries are broad and narrow
How can humans trigger earthquakes?
- Groundwater extraction changes stresses and can trigger earthquakes
- Disposing liquid waste by injecting into underlying rock lubricates and reactivates faults
- Underground nuclear testing
- Fracking of rock
- Increased crustal loading - weight of water behind dams
What is the global distribution of volcanos?
- Most found at plate boundaries, some in hotspots such as Hawaii
- 3/4 of 550 volcanos in the ring of fire in the Pacific.
- Found along major plate boundaries - Pacific Rim is often related to subduction either below oceanic or continental crust.
- Chains of volcanic islands such as the Aleutian Islands are found here.
- Where oceanic crust subducts continental crust, young fold mountains and composite volcanos are formed, such as the Andes
- Plumes or hotspots are jets of hot material rising from deep within the mantle, such as Hawaii.
What are the different types of volcano and where they are found?
- At subduction zones, more viscous andesitic lava is found which is more explosive.
- Mid ocean ridges have more basaltic fluid lava deep from the mantle rising from convection currents, melting and forming new oceanic crust or shield volcanos/island arcs.
- At subduction zones cold ocean floor slides down and warms up slowly due to hydrogen in the slab. When water meets the hot rock there are explosions due to pressure. The eruption is generally influenced by processes producing magma at different compositions - mainly andesitic
What is found at Constructive O/O boundaries
Earthquakes - shallow foci, generated by magma movement and tensional forces. Submarine and not very threatening, may cause tsunami
Volcano - gentle, non explosive eruptions due to low silica content, very runny, submarine eruption eventually can build high enough to create islands
What is found at constructive c/c boundaries?
Earthquakes: Continental rift valleys can be made, causing rifting and rift valleys, cause shallow foci earthquakes.
Volcanos: Horizontal forces drag plates apart, magma rises and cools, forming blocks in the gap which rise and fall, creating volcanos. An example is the Graben valley of blocks. Transform faults may also occur at right angles to plate boundaries due to different spreading rates
What is found at convergent c/c boundaries?
Earthquakes: fold mountains form and focus of earthquake at the point inside the crust where pressure is released
What is found and convergent boundaries?
Earthquakes: friction builds up at subduction zone, shallow foci earthquakes when pressure released. Compressional forces cause crustal stresses and deep earthquakes in the Benioff zone line, subject to major earthquakes deep at an angle to the subduction zone
Volcanos: friction causes partial melting, rises through fissures and cracks to form volcanos. Lava has high silica content and is andesitic but low temperature. It is viscous and gassy, building highly explosive eruptions.
What is found at conservative boundaries?
Earthquakes: friction builds and when released creates frequent shallow foci earthquake events
What are intraplate boundaries?
Areas where earthquakes and volcanos do not occur on plate boundaries
Earthquakes may occur due to human activity
Hawaiian islands are a good example of a series of volcanic islands not on a plate but instead of hotspots and mantle plumes where magma rises from the mantle.
What are the types of magma?
All magma contains oxygen and silicon
Basic lava: low silica content, low gas and low viscosity, high mean temperatures. Lava cools in basalt and is dark in colour due to iron and magnesium
Acidic lava: highest silica content, high gas content and viscosity, low mean temperatures. Traps gas bubbles in magma chambers causing explosive and destructive eruptions which violently eject lava into the air, cooling into rhyolitic and andesitic rock.
What causes earthquakes?
Result from movement along faults. Crustal stressed occur from the movement as tension builds up over time when rocks are locked together
When this becomes too great, there will be a sudden release of the stress as rocks shift along the fault.
As the fault moves, shock waves span out, known as seismic waves. The point of origin is called the focus and the epicentre is the point directly above the focus.
What are the primary and secondary hazards from earthquakes?
Primary: ground shaking, surface faulting
Secondary: liquefaction, tsunamis, injury, pollution, damage to property, flooding, fires, disease, loss of infrastructure - e.g. electricity, sewage, communications
What are the processes of an earthquake?
- May be preceded by small tremors known as foreshocks and followed by aftershocks, particularly damaging as damage buildings damaged by initial shock
- Seismic waves travel along surface of the earth and through the body, moving energy without moving material.
- The first waves are Primary and Secondary waves. Primary waves travel through compression and expansion, passing through solids, liquids and gases. S waves travel side to side, passing only through solids - take longer, have longer wavelength and cause more destruction than P
-When a free surface is present, they combine to form complex surface waves - Love and Rayleigh waves.
Love waves cause the earth to move at right angles to the direction of movement whereas Rayleigh waves are the slowest, causing it to move in a rolling motion causing large amounts of damage in an earthquake as they produce horizontal and vertical ground movement, causing huge building damage to foundations
How does liquefaction occur?
- Unconsolidated sediment under the surface shakes in a less predictable way, so the damage is far greater to foundations.
- P waves can turn solid sediment into fluids like quicksand by disruption sub-surface water conditions as unconsolidated sediment falls and water is forced up above ground, causing the water to mix with the sediment and form liquid like surfaces.
- The soil loses its cohesive strength and becomes weak and liquid like.
- It flows like a liquid causing structures to tilt and shake, leading them to fall and be unstable and structure failure and instability.
- This leads to buckling - piles are embedded into the group due to loss of soil support, spreading as soil moves down, loss of foundation support between liquefied and non liquefied soil, bridges lose support.
What are the resultant hazards of earthquakes?
- Damage to buildings, infrastructure and transport systems
- Collapsing stuff causes injuries and deaths as well as effect of being able to help
- More damage caused by aftershock as they shake already weak structures
- Surface displacement occurs along fault lines which fracture gas pipes and cause damage to communication lines
What are tsunamis?
- 90% of Tsunamis occur in the Pacific as they are associated with subduction zones
- Sudden uplift from the sea floor occurs during subduction
- Underwater earthquakes can snap the sea floor at a fault line, lifting a column of water above it. As gravity pulls the water down, waves fan outwards
- Travel 600-900km/h
- Tectonic up thrust causes vast quantities of water to be displaced by coastal uplift, creating large energy waves and a train of oscillatory waves over the ocean surface.
- Wavelengths continue to grow with the wave’s periods. As it approaches the coastline, they are slowed dramatically by friction, causing the amplitude to increase to large levels of damage
- This may also occur due to underwater volcanos and landslides, particularly along destructive plate margins where two plates move towards each other causing pressure
- Volcanic eruption - Tonga - huge displacement of water from eruption
- Landslides into water - La palma, Vajont dam or landslides underwater - Boxing Day
How are earthquakes classified?
The Richter Scale looks at the amount of energy released on a logarithmic scale, so an earthquake of 5.0 is 10x stronger than 4.0. It uses the momentum magnitude scale measuring the amount of energy released and produces figures from the richeter scale
The modified mercali scale relates ground movement to observations of damage done in the environment and around people. This may be advantageous as it allows ordinary eyewitnesses to provide information on the earthquake?
What factors affect earthquake damage?
- Strength and depth of earthquakes
- Number of aftershocks increases damage
- Population density - type of buildings - HICs tend to have better quality buildings
- time of day
- Distance from centre
- Type of rock and sediments - unconsolidated material damaged
- Secondary hazards such as mudslides, disease, tsunami, fires, hunger
- Level of economic development influences preparedness, technology and ability to recover
What can be measured to predict earthquakes?
- Ground surface changes
- Uplift of subsidence
- Ground tilt
- Changes in rock stress
- Micro-earthquake activity
- Anomalies in magnetic fields
- Radon gas concentration
- Strain meters
- Laser measures
- Magnetometers
What are the methods of predicting earthquakes?
-Satelitte surveying - topography, movement, shape, emf uses motion detectors - INSAR - stored digitally, mapped and analysed for risk mapping, accurate, large time interval, relies of land deformation, often not sensitive enough
Laser reflection - movement in earth and elevation, accurate, tiny variations may not be enough however
Seismometer - records ground motions with detectors, minor shocks precede a larger earthquake, graphed
Gravity meter - measures changes in gravity in rocks as stress increases in build up, rocks stressed
Radon gas - sensor as radon gas product of geothermal heat generated underground and released through cracks and breaks in the rock. Cracks allow radon gas to escape to surface, indicating earthquakes - unreliable, hard to position and may be due to other factors
Strain meter - measures stretching and compression of crust using silicon oil. Makes stresses visible, reliable gives evac time, hard to place, inaccessible once placed
Water table level - drops prior to earthquakes as when rocks crack create new spaces in crust for groundwater to flow. Easy to monitor but may occur for other reasons.
