Chapter 1: Locations-risk-tectonic hazards Flashcards
Where are earthquakes found?
Earthquakes are found along plate boundaries.
What boundaries are associated with the most powerful earthquakes?
Convergent and Conservative plate boundaries
What three things are in the tectonic activity pattern?
- Ocean fracture zone
- Continential fracture zone
- Scattered earthquakes in continential interiors.
What is the Ocean fracture zone?
Belt of earthquake activity through oceans along mid ocean ridges.
What is the Continential fracture zone?
Its a belt of earthquake activity following the mountain ranges from Spain to the Middle East.
What are scattered earthquakes in continential interiors?
Give an example.
The reactivasion of earhtquakes in weaknesses on old fault lines
Church Stretton fault in Shropshire.
What makes Earthquakes dangerous?
They are primary hazards (grund movement) that can cause secondary hazards e.g. landslides.
Define primary hazards.
Hazards that cause the ground to move.
Define Secondary hazards.
Give an example
Hazards caused by primary hazards,
e.g. Landslides, Tsunamis
Define: Volcano
A landform that develops around a weakness in the earth’s crust from which molten magma, volcanic rock and gases are ejected or extruded.
How is the violence of a volcanic eruption determined?
Amount of dissolved gases in magma and how easily the gases can escape.
What else can you call Constructive plate boundaries?
Divergent plate boundaries.
True or false: Constructive plate boundaries are low magnitude.
True
Do you find shallow focus events at constructive plate boundaries?
Yes.
In what way do plates move at constructive plate boundaries?
Plates move apart. < I >
In what way do plates move at convergent plate boundaries?
Plates move together. > I <
How do convergent plate boundaries cause frequent earthquakes and volcanoes?
They deform collision locations causing frequent earthquakes and volcanoes.
Do you find shallow focus earthquakes at conservative plate boundaries?
yes
How strong are magnitudes usually from conservative plate boundaries?
Very strong.
In which way do plates move at conservative plate boundaries.
Both directions against each other.
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What are the characteristics of earthquakes caused by constructive plate boundaries?
Earthquakes are frequent, small and a low hazard risk.
What do constructive plate boundaries create?
Creates new oceanic crust that is thinner and denser than continential crust.
What do conservative plate boundaries create?
Create friction which risks snagging.
E.g. Pacific plate and North American plate at San Andreas.
Convergent plate boundaries are the most damaging. Explain why.
Oceanic plate slides under continential plate as its denser, creating a subduction zone. Friction causes snags which are eventually overcome. This sudden jolt creates Tsunamis
What determins volcanic activity and distrabution.
Plate tectonics.
How do volcanoes form on convergent plate margins.
Volcanoes form at convergent plate margins when the mantle burns through the subducted oceanic plate picking up minerals as it goes. It then burns through the continential plate eventually forming a volcano.
Constructive boundaries create which volcanoes?
Rift volcanoes.
What are characteristics of Rift volcanoes.
Weaker eruption, very effusive as magma burns through thin layer of oceanic plate.
What are Hotspot volcanoes? And what are their characteristics?
- Found in the middle of tectonic plates
- Fed by underlying mantle plumes
- Very hot compared to surrounding mantle
- E.g. Hawaiin volcanoes within Pacific plate.
True of False: Magma picks up minerals under Hotspot valcanoes by burning through the plate during hot thermal plumes.
True
What are the two types of crust?
True or False: Oceanic crust is thicker than Continential crust.
False
What does each plate underline?
Oceanic crust underlines Ocean basins.
Continential crust underlines the continents.
What is each plate composed of?
Oceanic crust = Basalt primarily.
Continential crust = Granite primarily.
Whats the difference between rocks in the upper and lower mantle?
Upper mantle rocks are cool and brittle so cause earthquakes.
Lower mantle rocks are soft and weak so flow with forces.
What causes plates to move?
Heat from the earth’s core rises in the mantle to cause convection currents that move tectonic plates.
What is radioactive decay?
Heat from the earth’s core
What can plate movements be translated into?
Plate movements can be translated into Hazard risks.
How do we date the age of new tectonic crust?
Reconstructing paleomagnetic reversals
What is the Benioff zone?
An area of seismicity corresponding with the oceanic slab in a subduction zone. It is the site of deep focused earthquakes and so theoretically it is good at earthquake magnitude as it determines the position and depth of the hypocentre.
How do people work out the distribution of earthquakes?
Plot plate boundaries that have caused primary hazards in the past.
What is the process of an earthquake?
- At faults in earth’s crust there is a gradual build up of tectonic strain.
- When the pressure exceeds the strength of the fault the rock fractures.
- Produces a sudden release of energy creatign seismic waves.
- Brittle ground rebounds either side of fracture which is ground shaking.
Define: Hypocentre
The point of rupture underground.
What does a Seismometer measure?
Measures the horizontal and vertical movements of the ground.
What are the three types of seismic waves earthquakes produce?
- P waves, vibrations caused by compression
- S waves, vibrate at right angles to direction of travel
- L waves, surface waves on a horizontal plain
What does earthquake severity depend on?
The strength of it’s seismic waves.
What is Soil liquefication?
A side effect of earthquakes,
Water saturated material loses normal strength and behaves like a liquid due to strong shaking.
What can soil liquefacation cause?
Buildings to collapse and sloping ground.
Soil liquefacation causes sliding as it reduces friction. What is this called?
Lateral spreading.
What can lateral spreading from soil liquefacation cause?
Fissures in the ground.
What are some consequences of lateral spreading?
Damage to roads, bridges, and communications.
What is a short term and long term impact of lateral spreading?
Short term- delivery of aid.
Long term- rebuilding costs
What causes landslides?
When slopes weaken and fail.
What are landslides impacts of?
Where do they occur?
secondary impacts of destructive earthquakes.
Mostly occur in mountain areas.
In what scale of earthquakes do landslides rarely occur?
In 1-4 magnitude earthquakes do landslides rarely occur.
What are landslides particularly hazardous to?
People with property.
Give an example of a place landslides often occur?
Nepal.
Whats unusual about Tsunami’s wavelengths?
They are much longer than average.
Where are Tsunamis the most dangerous and why?
Mostly dangerous at coastlines as they vary in height.
What causes the series of waves in Tsunamis?
Seabed displacement.
Why are Tsunamis dangerously unpredictable?
Time between the waves vary.
From where are most Tsunamis generated? Example?
Subduction zones.
E.g. Japan-Taiwan island arc.
What are the six factors that impact Tsunami damage?
- Duration of event
- Wave amplitude, Water collumn displacement and distance travelled.
- Physical geography of coast, especially water depth and gradient of shoreline.
- Degree of coastal ecosystem buffer, e.g. protection by mangroves.
- Timing of event and quality of the warning systems.
- Degree of coastal development and its proximity from the coast.
When is a tsunami at its most dangerous?
When its influenced by both physical and human factors.
What are the primary hazards caused by volcanoes?
And what do they all have in common?
Pyroclastic flows, Tephra, Lava flows and Volcanic gases.
They all have very long geographical reach.
What is in pyroclastic flows?
Hot gas, volcanic debri, glass, pumice, crystals and ash.
Why can pyroclastic flows flow upwards?
They are not dense.
What makes pyroclastic flows incredibly dangerous?
They are extremerly fast and hot.
E.g. can reach 700 km/h and 1000°C
What is volcanic tephra?
Rock fragments ejected into atmosphere by valconoes.
That can vary from bombs to fine dust.
What negative impacts can volcanic tephra cause?
Can casue fires and buildings to collapse.
Dust can reduce visibility and effect air travel.
When is lava at its most threatening and what are it’s propeties when it is?
Lava is the most threatening when it flows fast.
It flows fast from low viscocity, when it has little silicon dioxide.
What determines lava’s viscocity?
How much silicon dioxide it contains.
What eruptions are volcanic gases associated with?
Explosive ones.
What does volcanic gas normally include?
Water vapour, Sulphur dioxide, hydrogen and Carbon monoxide.
Why is Carbon monoxide from volcanic gas especially dangerous?
It can’t be detected as it is colourless and odorless.
What are the two most significant secondary impacts of volcanic eruptions?
Volcanic mudflows (Lahars)
Catastrophic floods (Jökulhlaups)
What are Lahars composed of?
Fine sand and silt.
What mainly causes Lahars?
Heavy rainfall.
What influences the degree of the Lahar hazard?
- Steepness of slope
- Volume of material and partical size
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What are Jökulhlaups?
A type of glacial outburst flood.
What are the main impacts of Jökulhlaups?
Can modify landscape via erosion and deposition.
Can cause a lot of property damage.
What happens during a Jökulhlaup?
A large rapid discharge of water, ice, and debris come from a glacial source.
Where do Jökulhlaups occur?
Anywhere where water accumulates in a subglacial lake beneath a glacier.
What causes Jökulhlaups?
A failure in ice in glacier or Moraine dam.
How many have died in volcanic eruptions in in the last 300 years?
Over 250,000 people.
