Hazards 1- Geophysical systems Flashcards
Explain continental drift
-The Earth’s crust is made of a number of large and smaller plates.
-These plates are in constant motion, a process called continental drift.
-The plates move at very slow speeds (2-5cm a yr).
-Over millions of years the Earth’s continents come together & separate.
-There are 3 key factors that drive plate movement.
What are the three key factors that drive plate movement?
-Internal heating and convection currents
-Subduction and rifting at plate margins
-Mantle plumes and hot spots
What is the inner core of the Earth made of?
Iron and nickel
What is the temperature in the inner core?
It ranges from 5,000-7,000 °C
How has the heat from the inner core been generated?
-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.
Explain convection currents
-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.
Diagram of internal heating and convection currents
Explain 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.
Explain 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.
Diagram of ridge push and slab pull
How many massive mantle plumes are there in the Earth’s mantle?
Two
What are the two massive mantle plumes in the Earth’s mantle layer
One is centred under the Pacific plate and the other under the African plate.
Explain how hot spots are created
-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.
What is a flood basalt?
The result of a giant volcanic eruption or a series of eruptions which covers large stretches of land or ocean floor with basalt lava.
Diagram of a hot spot and flood basalts
Explain what evidence of mantle dynamics has revealed about how plates move
-Evidence of mantle dynamics from sources such as seismic tomography have allowed us to develop our understanding of the mechanisms that move plates.
-It seems, rather than being carried passively like rafts on huge convections cells in the mantle, that the plates themselves appear to be the main cause of the convection system in our plantet.
-Mantle convection not only includes the plates, but is primarily driven by them.
Explain how plates move by ridge push and slab pull
-The loss of heat from the lithosphere over time causes it to eventually become cold and dense enough to have negative bouyancy.
-This allows it to sink into the warmer and more ductile asthenosphere.
-This drags the surface plates by slab pull, which appears to be the main force responsible for plate movement.
-This, in turn, creates tension forces elsewhere on the plate (e.g. the Mid-Atlantic Ridge), thinning it and causing passive convection upwelling of the hotter mantle rock in response to the movement of the plates above (the convection seems to be an induced upwelling).
-This forms the ridge that 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.
-This solid but ductile rock of the atmosphere does flow, but the movements there seem to be mostly induced by the movements of the plates (like how a paddle pushes water around as it moves through the water).
Diagram explaining slab pull and ridge push
Diagram explaining mantle dynamics
Diagram of how islands move
How many different types of plate margins are there?
Four
What are the four types of plate margins?
-Constructive
-Conservative
-Destructive
-Collision
Explain how constructive (divergent) plate margin works
-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 create shield volcanoes.
-Landforms at a divergent plate boundary includeocean ridges,rift valleys, and shield volcanoes.
What landforms form at a constructive plate boundary?
-Ocean ridges
-Rift valleys
-Shield volcanoes
Give examples of constructive plate margins
-Ocean 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
Explain how conservative boundaries work
-Atconservative plate margins,fault lineswill be formed.
-These are plate boundaries where two plates 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.
Give an example of a conservative plate margin
TheSan Andreas Fault,where the Pacific and the North American plates are sliding past each other.
Explain how destructive plate margins work
-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.
Explain how collision boundaries work
-Collision zones form when two continental plates move towards each other and collide.
-The land between the plates is forced upwards to form fold mountains, eg The Alps and Himalayas.
Give examples of collision boundaries
The Alps and Himalayas
Diagram of a constructive plate margin
Diagram of a conservative plate margin
Diagram of the San Andreas Fault
Diagram of a destructive plate margin
Diagram of a collision plate margin
What are the different types of volcanoes?
-Composite
-Shield
-Cinder cone
Where do volcanoes form?
-Destructive and constructive plate margins
-Above hot spots
Where are composite volcanoes found
Ondestructive plate margins, where the oceanic crust sinks beneath the continental crust.
Explain how composite volcanoes form
-At destructive plate margins, the oceanic crust sinks beneath the continental crust.
-Magma rising from the deep boundary has plenty of time to pick up silica-rich components.
What are the characteristics of composite volcanoes?
-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.
Diagram of a composite volcano
Where are shield volcanoes found?
-Onconstructive plate margins, where two plates move away from one another.
-They also often form in oceans, where the oceanic crust is thinner.
Explain how shield volcanoes form
-Onconstructive plate margins, 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.
What are the characteristics of shield volcanoes?
-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
Diagram of a shield volcano
What type of volcano is Mauna Loa (Hawaii) and where was it formed?
It is a shield volcano, but was formed over ahot spot, rather than at a constructive plate margin, like other volcanoes.
Explain how cinder cone volcanoes form
-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.
Diagram of a cinder cone volcano
Diagram of the three types of volcanoes
What are the different types of volcanic eruptions?
-Effusive
-Explosive
What does the explosivity of an eruption depend on?
Two main factors:
-How much silica (SiO2) and how many bubbles of gas there are in the magma.
What is the effect of silica on the viscosity of magma?
It increases the viscosity (makes it less runny), trapping bubbles of gas in the magma, which increases pressure and leads to a more explosive eruption.
Explain how effusive eruptions occur
-These occur when magma has a low silica content and therefore low viscosity.
-They usually cause few hazards the gas emissions from larger eruptions can affect climate (?)
-Effusive eruptions form shield volcanoes, which are typically large, round, and low-lying structures.
-Effusive volcanoes do not produce ash, so the volcano is just made of lava.
-These eruptions take place at divergent plate boundaries and in the middle of plates.
Where do effusive eruptions occur?
At constructive plate boundaries and in the middle of plates.
What do effusive eruptions form?
Shield volcanoes
Explain how explosive eruptions occur
-Volcanoes with a high silica and gas content are explosive.
-They are very hazardous, producing ash (small fragments of broken rock), pyroclastic density currents (hot clouds of gas and rocks that flow down the sides of the volcano), and lahars (cement-like mixtures of water and hot ash.
-Explosive eruptions create composite volcanoes, build up of repeated layers of ash and lava over multiple eruptions.
-These eruptions take place at convergent plate boundaries.
Where do explosive eruptions occur?
At destructive plate boundaries.
What do explosive eruptions create?
Composite volcanoes.
Table comparing the type of plate margin (constructive vs. destructive) to the nature of the activity
What is a secondary hazard?
The indirect effects that result from the main (primary) impact of a hazard.
Give examples of secondary hazards
-Lahars
-Flooding
What are lahars?
Fast mud flows that occur when the rain mobilises deposits of volcanic ash.
What is tephra?
-Fragmented magma produced during a volcanic eruption.
-It includes ash, cinders, and volcanic bombs
How do lahars form and why are they dangerous?
-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 mudflows called lahars.
-These flow very fast and can have temperatures close to boiling.
-They can also travel 1os of km away from a volcano.
-Their consistency is like wet cement and they can bury infrastructure.
-There can be a risk of secondary lahars months and years after an eruption has occurred, as deposited tephra may mix with subsequent rainfalls, triggering mudflows.
Why are gases from a volcano dangerous?
-If volcanic gases build up to dangerous levels, they can be a risk to people if they are suddenly released in what is called a limnic eruption.
-There are fears of such as release from Lake Kivu beside Nyiragongo should an earthquake stir up the lake waters.
Explain how lava flows form
-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.
-This means that few lives are lost from lava at these volcanoes.
-However, where lava is less viscous at shield or fissure eruptions, it can travel many kilometers from its source.
-Although people generally have time to evacuate, any immovable infrastructure will be destroyed and perhaps buried under the lava flows.
Explain how pyroclastic flows occur and why it’s dangerous
-If the tephra is denser than air, it will flow downhill as a pyroclastic density current, forming a hot (hundreds of degrees), fast flow of volcanic ashes.
-When it hits buildings, it can destroy them due to its heat and force.
-People and animals caught within it can be severely burned and even killed.
-As PDCs flow downhill, they send to follow river valleys and are therefore more directional in their impacts.
-However, less dense PDCs can overtop topographical barriers.
-They are typically contained within 5-10 km of the volcano, but this can reach some of the villages that may be on the lower slopes of volcanoes.
Explain how ash plumes form (check)
-An ash column may rise many kilometers 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 cround.
What are the negative impacts of ash falling to the ground as it cools?
-It covers infrastructure and can cause buildings to collapse (especially if rain follows, increasing its weight).
-Vegetation and crops get destroyed.
-Water supplies can be contaminated.
-Breathing difficulties for locals (e.g. those with asthma).
-Major challenges with clean-up post-eruption.
What are the types of lava?
-Pahoehoe- smooth, billowy, has a low viscosity, and travels slowly.
-A’A lava – rougher in appearance, has a higher viscosity, and carries lava blocks called clinkers.
Why do risks from hazards vary?
-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, whereas constructive basaltic & runny.
-Secondary hazards – tsunami, lahar.
-Rural or urban location
-Magnitude & frequency – active or dormant.
Describe the earthquakes at constructive margins
-Small earthquakes
-Shallow
-Not much friction or strain as plate diverge
Describe the earthquakes at conservative (transform) margins
Have a large amount of friction and when the strain is released the shockwaves can be strong.
Describe the earthquakes at destructive margins
-Create largest amount of friction and strain, with earthquake focal points following the Benioff zone to considerable depths.
-Low frequency.
Explain how earthquakes occur
-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 then 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 types of seismic waves?
-P waves (primary)
-S waves (secondary)
-L waves (love)
Describe P (primary) seismic waves
-Fastest, though they cause the least damage.
-Pushes & pulls in the direction of travel ( compresses).
-Can move through solid rock & liquids.
Describe S (secondary) seismic waves
-Shakes the ground violently.
-Up and down movement.
-Only moves through solid rock.
Describe L (love) seismic waves
-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.
Diagram of the different types of seismic waves (seismogram)
What are the different types of stress and fault? (earthquakes)
-Tensional stress
-Compressional stress
-Shear stress
Where does tensional stress occur?
At constructive plate boundaries
Where does compressional stress occur?
At destructive plate boundaries
Where does shear stress occur?
At conservative plate boundaries
Diagram of the types of faulting
What is an earthquake?
An intense ground-shaking motion that lasts for a few seconds.
What is the epicenter?
The point on the Earth’s surface directly above the focus of an earthquake
What is the focus?
The point below the Earth’s surface where an earthquake occurs.
What is the depth of focus for shallow and deep earthquakes?
-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.
The human causes of earthquakes tend to cause ___ earthquakes
Low-magnitude
What are the human causes of earthquakes?
-Mining activities
-Fracking
-Dam-building
How can mining activities cause earthquakes?
They disturb the rock structure and the use of dynamite and heavy machinery can cause earthquakes.
How can fracking cause earthquakes?
Fracking involves the removal of gas from shale rock and injection of fluids that loads stress into faults.
How can dam-building cause earthquakes?
This adds significant weight and load to faults.
What is the difference between primary and secondary hazards (earthquakes)?
A primary hazard is directly linked to seismic wave energy; a secondary hazard is a consequence of the ground shaking.
What are the primary hazards associated with earthquakes?
-Crustal fracturing
-Ground shaking
Explain how crustal fracking is a primary hazard associated with earthquakes
-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.
Explain how the ground shaking is a primary hazard associated with earthquakes
-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 associated with earthquakes?
-Landslides
-Liquefaction
Explain how landslides are a secondary hazard associated with earthquakes
-This happens especially in mountainous areas e.g. Nepal.
-It loosens rock and causes it to move down a slope.
-It can hit settlements and block transport.
Explain how liquefaction is a secondary hazard associated with earthquakes
-This is 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.
Diagram of soil liquefaction
Diagram of a tsunami in the deep ocean and approaching shore
How are most tsunamis generated?
By submarine earthquakes at subduction zones.
Explain how tsunamis form and what their key features are
-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.
-They have a long wavelength when they are out at sea ( over 100km)
-They are short in amplitude at sea, 1m in height, and barely noticeable.
-Travel quickly at 700kph
-When they reach land – rapidly increase in height +25m
-Often preceded by a localized drop in sea level (drawback), as water is drawn back and then up.
-Hit the coastline as a series of waves – more like a flood.
What is mass movement?
The downwards movement of material down a slope.
What is unconsolidated material?
Rock fall, avalanche.
What is consolidated material?
Rotational slumps/landslides
What factors affect the stability of a slope? (check this)
-Gravity – the 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.
When does mass movement happen?
If shear stress is greater than shear structure & gravity & the angle of the slope is sufficient.
Diagram of how mass movement happens
What physical factors affect the speed of mass movement?
-The gradient/slope
-Prolonged rainfall
How does the gradient/slope affect the speed of mass movement?
It influences the effect of gravity & how fast the material moves.
How does prolonged rainfall affect the speed of mass movement?
-This 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.
Diagram of dry, moist, and saturated soil
How can human activity impact shear stress? (check whether increase or decrease)
-E.g. deforestation, house & road construction
-Tree roots bind the soil together and increase shear strength
-They also reduce pore water pressure through infiltration.
-Infrastructure can add weight to a slope, causing shear stress.
-Water infiltration may also be disturbed.
Annotated pictures of the effect of human activity on shear strength
Table showing what factors contribute to shear strength
Triangle diagram of the types of mass movement (check(
Diagram showing how mass movements differ with wetness and speed
Explain how fast mass movement occurs
-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 are unstable in their alignment to the angle of slope.
Diagram showing falls and topples in mass movement
Diagram showing flows in mass movement
Diagram showing slides in mass movement
Explain what debris flows are
-A geological phenomenon in which water-laden masses ofsoiland fragmentedrockrush down mountainsides, 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 due to widespread sedimentliquefactioncaused by highpore-fluid pressures, they can flow almost as fluidly as water.
Explain what slides are (mass movement)
-These are a very common form of mass movement and can be divided intorotationalandtransnational 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.
