Tectonic Hazards Flashcards
Intraplate earthquake
These occur in the middle or interior of tectonic plates and are much rarer than boundary earthquakes
Volcanic hazards
Associated with eruption events
Volcano
A landform that develops around a weakness in the Earth’s crust from which molten magma, volcanic rock and gases are ejected and extruded
Seismic hazard
Generated when rocks within 700km of the Earth’s surface come under stress that they break and become displaced
Tectonic hazards
These include earthquakes and volcanic eruptions as well as secondary hazards such as tsunamis and represent a significant risk in some parts of the world in terms of loss of life, livelihoods and economic impacts
What percentage of earthquakes are found along plate boundaries?
95%
What percentage of volcanoes are found in the Pacific ‘Ring of Fire’?
Around 70%
The distribution of earthquakes reveals 3 patterns:
- The Oceanic Fracture Zone - activity found in mid-ocean ridges (e.g. Mid-Atlantic ridge)
- The Continental Fracture Zone - activity found in mountain ranges (e.g. the Himalayas)
- Scattered earthquakes in continental interiors (e.g. Church Stretton fault)
Hotspot volcanoes
- Hotspot volcanoes are found in the middle of tectonic plates. The mantle plumes give them life and these are thought to be much hotter than the mantle encasing them. The mantle itself can be found beneath the sea, under the ocean’s crust
- Hotspot volcanoes are formed when one of the Earth’s plates moves over the hottest parts of the Earth’s mantle
Example of a hotspot volcano?
Kīlauea, Hawaii
Crust
Solid rock layer, 0-60km thick, 200-400°
Mantle
Semi-molten rock (magma), liquid, 2900km thick, 1000-3700°c
Outer core
Liquid layer made up of iron and nickel, about 2200km thick, 4500-5500°c
Inner core
Solid, iron and nickel, 1220km thick, temperatures up to 5500°c
Causes of intraplate earthquakes
These earthquakes are due to stress building in ancient faults/weaknesses, no subduction takes place here
James Hutton theory
Theory of the Earth, 1785 - theory that the processes or erosion, deposition and uplift were connected and operated continuously - driven by the Earth’s internal heat
Alfred Wegner theory
1912 - published two articles about a concept called continental drift
Harry Hess theory
1962 - Proposed that ridges on the ocean floor were the result of molten rock rising from the asthenosphere
John Tuzo Wilson theory
1863 - proposed that volcanic island chains (e.g. Hawaii) are created by fixed ‘hotspots’
Asthenosphere
The part of the mantle below the lithosphere, where the rock is semi-molten
Slab pull
Newly formed oceanic crust at mid-ocean ridges becomes denser and thicker as it cools. This causes it to sink into the mantle under its own weight, pulling the rest of the plate down with it
Mantle convection
Heat produced by the decay of radioactive elements in the Earth’s core hearts the lower mantle - this creates convection currents (hot liquid magma currents). These move in circles in the asthenosphere, causing plates to move
Palaeomagnetism
1950s studies of this confirmed that the sea floor was spreading. Every 400,000 years, the Earth’s magnetic fields change direction, lava cools to become rock, and minerals line up with the Earth’s magnetic direction (polarity)
Subduction
- Process of a plate being destroyed
- Two oceanic plates/an oceanic and a continental plate move towards each other into the mantle
- This melts into an area called the subduction zone
Seafloor spreading
The process of new crust pushing tectonic plates apart. Hot magma is forced up from the asthenosphere
Plate margins
The areas adjacent to plate boundaries (where two tectonic plates meet)
What are convergent plate boundaries also called?
Destructive margins, collision margins
What are divergent plate boundaries also called?
Constructive margins
What are conservative plate boundaries also called?
Transform margins
How many cm do the Eurasian plate and the North American plate move apart per year?
2.5cm, separated by the Mid-Atlantic ridge (mid-ocean ridge)
What earthquakes and volcanoes are found at mid-ocean ridges?
Shallow focus earthquakes (less that 70km into the crust), and submarine volcanoes (less explosive/more effusive)
Which type of plate boundary are mid-ocean ridges and rift valleys found?
Divergent
What is a rift valley?
- When continental plates move apart, the crust stretches’ and breaks into sets of parallel cracks (faults)
- The land between these faults then collapses, forming steep-sided valleys called rift valleys
- e.g. East-African rift valley
What earthquakes are found at rift valleys?
Shallow and low-magnitude (no volcanoes)
Subduction zone
The broad area where two plates are moving together
Which plate is subducted: the oceanic or continental?
Often the thinner, more dense oceanic plate descends beneath the continental plate
Locked fault
In the subduction zone, as plates move together they can get stuck due to frictional resistance
- Such faults may store strain for extended periods, that is eventually released in a large magnitude earthquake
Benioff Zone
An area of seismicity corresponding with the slab and being thrust downwards in a subduction zone
What tectonic hazards occur at convergent boundaries (destructive margins)?
- Tectonic hazards that occur here: explosive volcanic eruptions, deep and intermediate earthquakes (in the Benioff zone)
- Fold mountains can also form here
What occurs at destructive plate margins (convergent boundaries)?
- When 2 oceanic plates collide, the one which is faster or denser is subducted beneath the other - deep ocean trenches occur at this boundary
- As the subducted plate melts, magma rises to form underwater volcanoes. Over millions of years, these volcanoes grow and island arcs form. These can form hotspot volcanoes in a chain
- Earthquakes occur due to locked faults
- e.g. 2004 Indian Ocean Boxing Day earthquake and tsunami
What occurs at collision plate margins (convergent boundaries)?
- No subduction because both have the same density (both continental crust) and are less dense than the asthenosphere beneath them
- Instead, they collide. Sediments between them are crumpled and forced up to form high fold mountains (e.g. the Himalayas)
- May be some subduction caused when they’re compressed (making them more dense). Sediments result in plate subduction beneath them
What earthquakes occur at collision plate margins?
- Shallow earthquakes - no Benioff zone, so they happen at the top where there’s less seismic activity
- Magma can’t come up (mantle has no gap), so no volcanoes
San Andreas fault factfile
- Found in California, USA
- Its roughly 1200km long
- Between the Pacific plate and North American Plate
- Direction/motion = right-lateral strike-slip
- Due to stress built up over time from tectonic plate movement, earthquakes can happen. This is problem; LA, San Francisco and San Diego sit along it. The earthquake could cause gas pipe explosions, leading to fires which can spread to woodlands and houses/infrastructure will be destroyed
- Scientists predict the next earthquake at this fault will be before 2032
What technology can be used to measure seismic activity at a fault?
GPS instruments, accelerometers and seismograms
What happens at a conservative plate margin?
- At a conservative plate margin, two plates are sliding past one another, resulting in a major break in the rust between them as they move
- The break itself is called a fault, and where it occurs on a large scale is known as a transform fault
Sinistral
Movement to the left
Dextral
Movement to the right
What earthquakes occur at conservative plate margins?
- Powerful, shallow earthquakes can occur here due to frictional resistance
- No volcanoes occur here
Explain the reasons why volcanoes are more likely along some plate margins than others (6)
Volcanoes are more likely to be distributed on convergent plate margins as new magma is forced. For example, the Pacific Ring of Fire is home to a good proportion of the world’s volcanoes, due to subduction of the Pacific plate under the Eurasian plate. Volcanoes form here from the subduction of the oceanic crust which, as it enters the mantle, undergoes melting. This newly formed magma pushes up through the faults in the continental crust which means that as it enters the surface, it forms a volcano.
Volcanoes are less likely to occur on conservative plate margins where there is no presence of magma. For example, the San Andreas fault in California is on a conservative plate boundary where only earthquakes occur. Volcanoes don’t form here as the plates, in this example, slide past each other which means that no crust is made of destroyed here, fundamentally resulting in no new magma being formed.
Focus/hypocenter
The point inside the crust from which the pressure is released
Epicenter
The point on the surface directly above the focus/hypocenter
Primary waves (P-waves)
- Body waves - they travel through the Earth’s body
- Fastest waves, first to reach the surface (8km/sec)
- Can travel through both solids and liquids
- Move in a backwards and forwards motion
- They are only damaging in the most powerful earthquakes
Secondary waves (S-waves)
- Body waves - they travel through the Earth’s body
- They move at 4km/sec
- Only travel though solids
- They move in an up and down motion, perpendicular to the direction of travel
- They do more damage than P-waves
Love waves (L-waves)
Surface waves - they only travel on the Earth’s surface
- They are the slowest of the 3 types of seismic waves
- They move in a side to side motion, perpendicular to the direction of travel
- They are larger and cause the most damage
Physical factors of earthquakes
- Type of fault
- How long ago an earthquake happened
- Whether the focus is shallow or not (depth)
- Magnitude (size of wave)
- Distance from the epicenter
- Geology (e.g. soft rock can amplify shaking)
Human factors of earthquakes
- Different types of infrastructure (if stronger, higher chance of withstanding earthquakes)
- Level of development
- Level of population
- Effectiveness of emergency response
- Impact of secondary hazards
Liquefaction
When surface rocks lose strength and become more liquid than solid. The subsoil loses its ability to support building/infrastructure foundations, so they sink or tilt
Ground shaking
This causes buildings, bridges, roads and infrastructure to collapse
Landslides/avalanches
Slopes fail as the shaking (of earthquakes) places stress on them, resulting in landslides, rock slides, mudslides and avalanches
Crustal fracturing
When the Earth’s crust cracks due to the energy that is released
Primary hazard
- Hazards caused by the event itself
- For an earthquake, these consist of ground movement and shaking
Secondary hazard
- Caused as a consequence of primary hazards
- These consist of liquefaction, rock falls, landslides, tsunamis, floods, fires, falling debris, ground settlement and lateral ground displacement
Aftershocks
- These occur in the general area of an earthquake
- These are a result of the readjusting along the part of the fault that slipped originally
- These can occur for weeks, months or years after an earthquake
- If an earthquake is larger, aftershocks are numerous and larger
- In 2011, a 6.3 magnitude aftershock struct Christchurch, which caused more damage and loss of life than the original
Over the past 300 years, approximately how many people have died as a result of volcanic eruptions?
260,000 people
Lava flows
- These are streams of lava that have erupted onto the Earth’s surface
- They can reach up to 1170°C and take years to cool completely
- Due to their, heat they destroy everything in their path
- However, they are generally not threatening to humans as most of them move slowly as people can move out of their way
Pyroclastic flows
- These are a mixture of dense hot rock, lava, ash and gases ejected from a volcano
- They can reach temperatures up to 700°C
- They can move very quickly, up to 100km per hour (you cannot outrun these)
- These destroy everything in their path
Tephra and ash falls
- Tephra are pieces of volcanic rock and ash that blast into the air during eruptions
- The large pieces tend to fall near the volcano where they can cause injury or death (as well as damage structures). The smaller pieces (ash) can travel for thousands of kilometers
- Ash falls can be very disruptive as it covers everything causing poor visibility and slippery roads. Roofs may collapse under the weight and engines may get clogged up and stop working
Gas eruptions
- Magma (lava that hasn’t reached the Earth’s surface) contains dissolved gases that are released into the atmosphere during eruptions
- These gases include water vapour (around 80%), carbon dioxide and sulfur dioxide
- Once the gases are in the air they can travel for thousands of kilometers
- Some gases can be potentially hazardous to people, animals and structures
- This is a natural cause of global warming!
Lahars
- These are masses of rock, mud and water that travel quickly down the sides (flanks) of a volcano
- They vary in size and speed. The largest can be hundreds of metres wide and can flow at tens of metres per second (cannot outrun these)
- They are caused when a eruption quickly melts snow and ice OR when heavy rainfall has occurred either during or after an eruption and has eroded rocks and soil, causing it to become lose and surge downslope
Jökulhlaup (glacial outburst floods)
- These are when the heat of a volcano melts the snow and ice in a glacier, causing heavy and sudden floods
- These can suddenly release large amounts of water, rock, gravel and ice that can be extremely dangerous as they can flood and damage land and structures
Tsunami
A series of large waves generated mostly by underwater earthquakes (a small number are generated by underwater landslides or meteor/asteroid strikes)
How does a tsunami occur?
Energy released during the event (e.g. earthquake) causes the seafloor to uplift, displacing the water column
Water column
The area of sea water from the surface to the sea floor
Characteristics of tsunamis
- They can move up to 500 miles/hour
- When a wave crest reaches the shore, it first produces a vacuum effect - it sucks water back out to sea, exposing a large amount of the sea floor (this can be used a warning sign)
Describe how seismic activity causes tsunamis
Oceanic and continental or two oceanic plates - the thinner, more dense one will subduct into the mantle. This pulls the other plate down with it, leading to the creation of a locked fault. Frictional resistance builds up overtime, causing the plate that is being pulled down to flick up, which displaces the water column above - this causes a tsunami
DART system
- Used to monitor the sea after seismic activity has occurred (as tsunamis are a secondary hazard)
- Recorder on seabed monitors changes in pressure. It can detect tsunamis as small as one centimetre → acoustic link transmits data to moored surface buoy (used to monitor changes in sea level and pressure) → satellite transmits data to ground stations
Pros and cons of computer modelling for monitoring tsunamis
+ Predict which areas were affected worst - saves lives!
+ Information can be reviewed regularly due to multiple sensors and sea surface buoys
- Expensive, specialist equipment to be used - specialist jobs + tsunamis tend to happen in poorer countries and they can’t afford the equipment
- Not accurate
- Equipment can become damaged in an earthquake
