Hazard 1: Geographical systems Flashcards
What is the earth’s crust made out of?
The Earth’s crust is made of a number of large and smaller plates.
What are the movements of plates?
These plates are in constant motion, a process called continental drift. The plates move at very slow speeds (2-5cm a yr).
What happened over a millions of years to the earth’s?
Over millions of years the Earth’s continents come together & separate.
What are the 3 key factors that drives the movement of plates?
- Internal Heating & Convection currents
- Subduction & rifting at plate margins
- Mantle plumes & hots spots
What is the inner core made of? And what is it’s temperature range?
The inner core is made up of iron and nickel. It’s temperature ranges from 5,000-7,000 °C.
Why does internal heating and convection currents cause movement of tectonic plate?
The heat from the inner core has been generated from the great frictional forced that occurred through the Earth’s formation, as well as through radio-active decay of elements. It generates 44 trillion watts of heat which flows away from the core to the mantle. In the mantle is behaves like a viscous liquid ( it has phases of liquid & solid consistency under pressure). When the convection cells make contact with the base of the crust, they cause friction & drag the crust along in the general direction of the convection.
What is ridge push?
At rift zones ( constructive/divergent boundaries) the crust is moving apart under tensional stresses. This reduces pressure on the asthenosphere ( lower part of mantle), allowing magma to rise into the lithosphere ( crust & upper mantle- tectonic plates) creating new oceanic crust. As it moves away from the ridge it cools, solidifies and sinks creating a slope, enabling further movement.
What is subduction and slab pull?
At destructive plate boundaries, denser oceanic plate is subducted under the less dense continental plate e.g. at deep ocean trenches, volcanic activity 100km from the plate boundary on the continental side, and earthquake focal points as deep as 700km along the Benioff zone. As a slab of oceanic plate descends, it pulls ( perhaps with some suction) the rest of the plate with it.
How does mantle plumes and hot spots cause plate movement?
- There are two massive mantle plumes in the Earth’s mantle layer; one centred under the Pacific plate and the other under the African plate.
- Molten viscous silicate material rises from the outer core/mantle boundary to about 700km beneath the lithosphere ( crust).
- From these mantle plumes small hot spots are created where magma rises through the asthenosphere and sometimes breaks through the crust. Isolated hotspots occur where there is an upselling of molten material directly from the outer core/mantle boundary to the surface e.g. Hawaiian islands.
- A flood basalt – result of a giant volcanic eruption or a series of eruptions which covers large stretches of land or ocean floor with basalt lava.
What are the steps to slab pull in the movement of plates?
The loss of heat from the lithosphere over time causes it eventually to become cold and dense enough to have negative buoyancy. This allows it to sink into the warmer and more ductile asthenosphere. This drags the surface plate by slab pull, which appears to be the main force responsible for plate movement.
What are the steps to ridge push in the movement of plates?
The slab pull motion in turn creates tension forces elsewhere on the plate (e.g. Mid-Atlantic Ridge), thinning it and causing passive convectional upwelling of the hotter mantle rock in response to the movement of the plates above (the convection seems to be induced upwelling. This forms the ridge which slides away laterally as it cools, pushing the plates apart by ridge push and contributing to the process of slab pull to move the plates.
What are the steps to hotspots in the movement of plates?
- Mantle plume - hotter, more buoyant mantle rock rises from the core-mantle boundary to the lithosphere.
- The plume head spreads out under the lithosphere and is dragged outwards in the direction of the plate movement.
- Decompression partial melting –> magma
- Magma rises quite quickly through the lithosphere and can erupt at the surface forming shield volcanoes.
- The older islands move away from the magma source and their volcanoes become extinct.
- Over time, the older islands are eroded more and more, first forming atolls, before being eroded below sea level forming relatively flat topped seamounts.
What occurs at a divergent (constructive) plate boundaries?
At adivergent plate boundary- also known as a constructive plate boundary, the plates move apart from one another. When this happens the magma from the mantle rises up to make (or construct) new crust. The movement of the plates over the mantle can cause earthquakes. Rising magma can also createshield volcanoes. Landforms at a divergent plate boundary includeocean ridges, eg the Mid-Atlantic ridge (where the Eurasian plate and the North Atlantic plate are movingapartfrom each other under the Atlantic Ocean),rift valleyseg the East African Rift Valley andshield volcanoes.
What occurs at a destructive (subduction) plate boundaries?
As the plates collide, the oceanic plate is forced beneath the continental plate. This is known assubductionand results in the formation of anocean trench. This happens because the oceanic plate is denser (heavier) than the continental plate.
When the plate sinks into the mantle it melts to form magma. The pressure of the magma builds up beneath the Earth’s surface. The magma escapes through weaknesses in the rock and rises up through acomposite volcano, also known as astratovolcano. The volcanic eruptions are often violent, with lots of steam, gas and ash.
What occurs at a conservative plate boundaries?
Atconservative plate margins,fault lineswill be formed.
These are plate boundaries where two plate are either slipping past each other in opposite directions or at different rates in the same direction.
The plates do not move past each other smoothly.
There will often be a build-up of friction or energy that is ultimately released as an earthquake.
A good example of this is along theSan Andreas Faultwhere the Pacific and the North American plates are sliding past each other.
What is a composite volcano?
Composite volcanoes are found ondestructive plate margins, where the oceanic crust sinks beneath the continental crust. Magma rising from the deep boundary has plenty of time to pick up silica rich components.
Composite volcanoes have the following characteristics:
Acidic lava, which is very viscous (sticky).
Steep sides as the lava doesn’t flow very far before it solidifies.
Alternate layers of ash and lava. For this reason, they’re also known asstratovolcanoes. Strato means layers.
Violent eruptions.
Longer periods between eruptions.
What is shield volcanoes?
Shield volcanoes are found onconstructive plate margins, where two plates move away from one another. They also often form in oceans, where the oceanic crust is thinner. The magma thus doesn’t have far to travel and has less time to change its composition to become silica rich
Shield volcanoes have the following characteristics:
basic lava, which is non-acidic and very runny
gentle sides as the lava flows for long distances before it solidifies
no layers, as the volcano just consists of lava
less violent eruptions
shorter periods between eruptions
What is cinder volcanoes?
They form when runny, low silica magma contains a lot of dissolved gas.
The gassy eruptions explode and sputter like soda spraying from a can. They issue loose volcanic fragments called cinders which settle and pile in conical shapes.
What is a secondary hazard?
The indirect effects that result from the main primary impact of the hazard.
What is a Lahars?
When tephra mixes with water (either as it melts snow that might be on top of a volcano or as it mixes with rain water) it produces deadly mud called lahars. They flow very fast and can have temperatures close to boiling. They also can travel 1os of km away from a volcano. Their consistency is like wet cement and they can bury infrastructure.
Why can lahars be a secondary hazard?
Fast mud flows that occur when the rain mobilises deposits of volcanic ash. There can by a risk of secondary lahars months and years after an eruption as deposited tephra may mix with subsequent rainfall, triggering mud flows.
What is tephra?
fragmented magma produced during a volcanic eruption. It includes:
- Ash (<2mm)
- Cinders (2-64mm)
- Volcanic bombs
What is ash plumes?
- Ash column can rise many km into the atmosphere due to its force and buoyancy.
- Ash can then spread far (10s to 100s of km, following the prevailing wind direction). At height, it can disrupt air travel as ash is very dangerous to aircraft engines.
- As the ash cools, it can fall to the ground.
- Covers infrastructure & can -> building collapse (especially if rain follows, increasing its weight).
- Vegetation and crops destroyed.
- Water supplies can be contaminated.
- Breathing difficulties for come locals (e.g. those with asthma).
-Major challenges in clear-up post eruption.
What are pyroclastic flows?
If the tephra is denser than air, it will flow downhill as a pyroclastic density current, forming a hot (hundreds of degrees) fast (averaging 60 mph, but can be much faster) flow full of volcanic gases.
What are the impacts of pyroclastic flows?
When it hits buildings, due to its forces and heat it can destroy them. Ppl & animals caught within it will be severely burned & many killed. PDCs flow downhill, they tend to follow river valleys and therefore are more directional in their impacts. However, less dense PDCs can overtop topographical barriers. They typically are contained within 5-10km of the volcano; but this can reach some of the villages that may be on the lower slopes of volcanoes.
What are lava flows?
The more viscous lava of composite volcanoes tends not to flow too far or too fast, so it is typically confined to within 5km of the volcano.
What are the impacts of lava flows?
This means that few lives are lost from lava at these volcanoes. However, where lava less viscous at shield of fissure eruptions, it can travel many km from its source. Although ppl generally have time to evacuate, any immovable infrastructure will be destroyed & perhaps buried under the lava flows.
What is volcanic gas?
If volcanic gases build up to dangerous levels, they can be a risk to people if they are suddenly released in a limnic eruption.
What are the different types of lava?
Pahoehoe = smooth & billowy. Has a low viscosity & travels slowly.
A’A lava = rougher in appearance. It has a higher viscosity & carries lava blocks called clinkers.
Why do volcanic risks vary?
- Geophysical location= hotspots are localized, so the risk is over a smaller area, whereas at plate margins the risks are over a larger area, Destructive (violent & explosive) in contrast constructive (basaltic & runny)
- Secondary hazards= tsunami, lahars
- Rural or Urban location
- Magnitude & frequency (i.e. active or dormant)
Define: earthquake
Earthquake= an intense ground-shaking motion that lasts for a few seconds.
Define: Epicentre
Epicentre= the point on the Earth’s surface directly above the focus of an earthquake.
Define: Focus
Focus= The point below the Earth’s surface when an earthquake occurs.
Why do earthquake intensities vary?
Constructive margins-small earthquakes, shallow, not much friction or strain as plate as plate diverge.
Conservative (transform) margins have a large amount of friction and when the strain is released the shockwaves can be strong.
Destructive margins- create largest amount of friction and strain, with earthquake focal points following the Benioff zone to considerable depths. Low frequency.
What key features of an earthquake?
- Earth’s crust is mobile, a build-up of stress within the rocks can take place.
- When this stress is suddenly released (where the strain overcomes the elasticity of the rock), parts of the surface experience an earthquake.
- Much of the energy is transferred vertically to the surface and the moves outwards from the epicentre as seismic waves.
- At the moment of fracture, rocks may regain their original shape but in a new position
What are the different types of waves?
- P waves ( primary): fastest, though cause least damage. Pushes & pulls in the direction of travel ( compresses). Can move through solid rock & liquids.
- S Waves ( secondary): shakes the ground violently. Up and down movement. Only moves through solid rock.
- L waves ( Love): travel only across the surface; have a large amplitude and cause significant damage, including fracturing the ground. Moves from side to side ( horizontal) as moves forward.
What are the different types of stress and faults?
- Tensional stress – constructive plate boundaries.
- Compressional stress – destructive plate boundaries.
- Shear stress – conservative plate boundaries.
How does the depth of the focus change the impact of the earthquake?
- Shallow earthquakes (0-70km) (75% of all energy released) cause the most damage.
- Deep ( 70-300km) and very deep ( 300-700km) earthquakes have much less effect.
How can humans cause earthquakes?
Tend to be low magnitude.
- Mining activities disturb the rock structure & the use of dynamite and heavy machinery has been known to cause earthquakes.
- Fracking which involves the injection removal of gas from shale rock and injection of fluids that loads stress into faults.
- Dam building add significant weight & load to faults.
What are the primary hazards of earthquakes?
Crustal fracturing – shockwaves travel fast through solid rock and can increase stress in it, especially if rock types of different. Stress causes strain until rock fractures.
Faults can rupture the surface & cause subsidence or uplift.
Ground shaking – 3 types of waves. Can happen in less than 60 seconds. Buildings may only be able to survive some types of shaking & will be damaged.
What are the secondary hazards of earthquakes?
- Landslides – especially in mountainous areas e.g. Nepal. Loosens rock and causes it to move down slope. Can hit settlements and block transport.
- Liquefaction – where the ground consists of loose sediments of silts and sands and gravels which are waterlogged. Earthquakes compact the sediments and force the water to the surface, undermining buildings & roads.
How are earthquakes and tsunamis related?
- Most generated by submarine earthquakes at subduction zones.
- The sea bed is displaced vertically – this motion displaces a large volume of water in the ocean column, which then moves outwards from the point of displacement.
- Have a long wavelength when they are out at sea ( over 100km)
- Short in amplitude, 1m in height and barely noticeable.
- Travel quickly 700kph
Reach land – rapidly increase in height +25m
Ofen preceded by localised drop in sea level ( drawback), as water drawn back and then up.
Hit coastline as series of waves – more like a flood.
Define: Mass Movement
Mass Movement – downward movement of material down a slope.
Define: Unconsolidated material
Unconsolidated material – rock fall, avalanche.
Define: Consolidated material
Consolidated material – rotational slumps/ landslides.
What effects slope stability?
- Gravity – steepness of the slope
- Shear structure – structure & stability of the slope, preventing movement.
- Shear stress – forces acting to reduce slope stability & lead to movement.
If shear stress is greater than shear structure & gravity & the angle of the slope is sufficient = mass movement.
What are the physical factors affecting the speed of mass movement?
- The Gradient/slope influences the effect of gravity & how fast the material moves.
- Prolonged rainfall will saturate the soil and accelerate soil movement on a
Slope. Water acts as a lubricant causing greater instability. Water enters the pores between the particles forcing them apart and preventing them from binding.
What are factors that contribute to shear stress?
- Removal of underlying support -undercutting by rivers & waves.
- Loading of the slope-weight of water, vegetation, accumulation of debris
- lateral pressure - water in cracks, freezing in cracks, pressure release
- transient stresses - earthquakes, movement of trees in wind
- slope steepening - wave action, erosion by rivers…
What are factors that contribute to shear strength?
- Organic effects - burrowing animals, decay of roots
- Change of structure - creation of fissures in clays
- Change in pore water - saturation, softening of material pressure
- Weathering effects - Disintegration of granular rocks, hydration of clay minerals, chemical solution of minerals - limestone
How does human activity increase shear stress?
E.G. Deforestation, house & road construction.
Tree roots bind the soil together & increase shear strength. They also reduce pore water pressure through infiltration.
Infrastructure can add weight to a slope = shear stress. Water infiltration may also be disturbed.
What is debris flow?
Debris flowsare a geological phenomena in which water-laden masses ofsoiland fragmentedrockrush down mountain sides, funnel intostreamchannels, entrain objects in their paths, and form thick, muddy deposits on valley floors. They generally havebulk densitiescomparable to those ofrock avalanchesand other types oflandslides(roughly 2,000 kilograms per cubic meter), but owing to widespread sedimentliquefactioncaused by highpore-fluid pressures, they can flow almost as fluidly as water.
What are slides?
Slides are a very common form of mass movement and can be divided inrotationalandtransnational slides, which can be subdivided further depending on the structure of the soil and rock and whether it is consolidated or not. Rotational slumps occur when a slump block, composed generally of loosely consolidated sediments, slides along aslip plane. Translational slumps follow a more linear axis of slip. Rotational slumps have several characteristic features. The slip plane isconcavein its shape.
How are faster mass movements caused?
Faster Movement - A rock fall begins as a rock becomes separated from the bedrock. As it falls it can set in motion an unconsolidated movement, as one rock knocks into another. Topples occur where the strata of the rock is unstable in its alignment to the angle of slope.
