Why are some locations more at risk from tectonic hazards? Flashcards
What are all tectonic hazards caused by?
The Earth’s internal heat engine
What does the heat, generated in the Earth’s core and mantle, which flows towards the Earth’s surface, generate?
Convection currents in the plastic mantle
Where do most tectonic hazards occur?
At or near tectonic plate boundaries
What two factors does plate boundary type depend on?
1 Motion
2 Plate type
How thick are oceanic plates?
7-10 km
How thick are continental plates?
Much thicker at 25-70 km
What happens at mid-plate hotspots?
At these locations:
- isolated plumes of convecting heat, called mantle plumes, rise towards the surface, generating basaltic volcanoes that tend to erupt continually.
- a mantle plume is stationary, but the tectonic plate moves slowly over it
- over millennia, this produces a chain of volcanic islands, with extinct ones most distant from the plume location.
Name the two plate types associated with a divergent plate boundary.
Oceanic-Oceanic
Continent-Continent
Give an example of a divergent plate boundary associated with an Oceanic-Oceanic plate type.
Mid-Atlantic ridge at Iceland
Describe the process occurring at a divergent plate boundary for an Oceanic-Oceanic plate type.
Rising convection currents bring magma to the surface resulting in small, basaltic eruptions, creating the new oceanic plate.
What type of eruption is common for a divergent plate boundary with an Oceanic-Oceanic plate type?
Minor, shallow earthquakes
Give an example of a divergent plate boundary associated with a Continent-Continent plate type.
African Rift Valley/Red Sea
Describe the process occurring at a divergent plate boundary for a Continent-Continent plate type.
Caused by a geologically recent mantle plume splitting a continental plate to create a new ocean basin.
What type of eruption is common for a divergent plate boundary with a Continent-Continent plate type?
Basaltic volcanoes and minor earthquakes
Name the three plate types associated with a convergent plate boundary.
Continent-Continent
Oceanic-Oceanic
Oceanic-Continent
Give an example of a convergent plate boundary associated with a Continent-Continent plate type.
Himalayas
Describe the process occurring at a convergent plate boundary for a Continent-Continent plate type.
The collision of two continental landmasses creating a mountain belt as the landmasses crumple.
What type of eruption is common for a convergent plate boundary with Continent-Continent plate type?
Infrequent major earthquakes distributed over a wide area
Give an example of a convergent plate boundary associated with an Oceanic-Oceanic plate type.
Aleutian Islands, Alaska
Describe the process occurring at a convergent plate boundary for an Oceanic-Oceanic plate type.
One oceanic plate is subducted beneath another, generating frequent earthquakes and a curving (arc) chain of volcanic islands.
What type of eruption is common for a convergent plate boundary with an Oceanic-Oceanic plate type?
Violent eruptions
Give an example of a convergent plate boundary associated with an Oceanic-Continent plate type.
Andean Mountains
Describe the process occurring at a convergent plate boundary for an Oceanic-Continent plate type.
An oceanic plate is subducted under a continental plate, creating a volcanic mountain range.
What type of eruption is common for a convergent plate boundary with an Oceanic-Continent plate type?
Frequent large earthquakes and violent eruptions
Name the plate type associated with a conservative plate boundary.
Oceanic-Continent
Give an example of a conservative plate boundary associated with an Oceanic-Continent plate type.
California, San Andreas fault zone
Describe the process occurring at a conservative plate boundary for an Oceanic-Continent plate type.
Plates slide past each other, along zones known as transform faults.
What type of eruption is common for a conservative plate boundary with an Oceanic-Continent plate type?
Frequent, shallow earthquakes but no volcanic activity
Where can earthquakes occur?
Earthquakes can occur in mid-plate settings, usually associated with major ancient fault lines being re-activated by tectonic stresses.
What speed do the Earth’s tectonic plates move at?
2-5 cm per year
Give one example of a key discovery used to explain the theory of plate tectonics.
The discovery in the 1960s of magnetic stripes in the oceanic crust of the seabed; these are palaeomagnetic (ancient magnetism) signals from past reversals of Earth’s magnetic field and prove that new oceanic crust is created by the process of seafloor spreading at mid-ocean ridges.
Why is plate tectonics still a theory rather than a proven fact?
It remains a theory because scientists have not yet directly observed the interior of the Earth.
What happens to tectonic plates at constructive margins?
Mantle convection forces plates apart at constructive plate margins. Tensional forces open cracks and faults between the two plates. These create pathways for magma to move towards the surface and erupt, creating the new oceanic plate.
Why are eruptions at constructive margins small and effusive in character?
The erupted basalt lava has a low gas content and high viscosity
What type of earthquake occurs at constructive margins?
Earthquakes are shallow, less than 60 km deep, and have low magnitudes of under 5.0.
What is the name of the margin where one plate is subducted beneath another?
Destructive margin
What happens to the tectonic plates at destructive margins and subduction zones?
- Mantle convection pulls oceanic plates apart, creating the fracture zones at constructive margins, and convection also pulls plates towards subduction zones.
- Constructive margins have elevated altitudes because of the rising heat beneath them, which creates a ‘slope’ down which oceanic plates slide (gravitational sliding).
- The cold, dense oceanic plate is subducted beneath less dense continental plate; the density of the oceanic plate pulls itself into the mantle (slab pull).
What is the name of the zone where convection pulls plates towards?
The Subduction Zone
Why do constructive margins have elevated altitudes?
Because of the rising heat beneath them
What happens to the cold, dense oceanic plate at destructive margins and subduction zones?
Cold, dense oceanic plate is subducted beneath less dense continental plate; the density of the oceanic plate pulls itself into the mantle (slab pull).
What is the name of the zone where earthquakes at subduction zones occur at a range of focal depths from 10 km to 400 km, following the line of the subducting plate?
The Benioff Zone
What happens to the descending plate at the Benioff Zone?
The descending plate begins to melt at depth by a process called wet partial melting. This generates magma with a high gas and silica content, which erupts with explosive force.
What are the names of the two continental plates in a collision which forms the Himalaya mountains?
The Indo-Australian and Eurasian plates
Why is subduction not possible for the two continental plates in a collision, forming the Himalaya mountains?
As both continental plates have the same low density, subduction is not possible.
Why are eruptions very rare at the Himalaya and Tibetan Plateau?
Magma is being generated at depth, but it cools and solidifies beneath the surface so eruptions are very rare.
What are collision zones cut by?
Huge thrust faults that generate shallow, high-magnitude earthquakes
What type of earthquakes are generated at collision zones?
Shallow, high-magnitude earthquakes
What do conservative plate boundaries consist of?
Transform faults
Give an example of a conservative plate boundary where long transform faults act like a boundary in their own right.
California where a fault zone - including the San Andreas fault - creates an area of frequent earthquake activity
What type of earthquake occurs along conservative boundaries?
Earthquakes along conservative boundaries often have shallow focal depths, meaning high-magnitude earthquakes can be very destructive. Volcanic activity is absent.
What is an earthquake?
Earthquakes are a sudden release of stored energy.
Why are earthquakes generated?
As tectonic plates attempt to move past each other along fault lines, they inevitably ‘stick’. This allows strain to build up over time and the plates are placed under increasing stress. Earthquakes are generated because of a sudden release of stress - so-called ‘stick-slip’ behaviour.
When can a fault scarp be seen?
In some cases the earthquake motion displaces the surface, so a fault scarp can be seen.
What are the three types of seismic waves that earthquakes generate?
- P-waves, or primary waves
- S-waves, or secondary waves
- L-waves, or Love waves
Why are S and L waves more destructive than P waves?
- S, or secondary waves, arrive next and shake the ground violently, causing damage
- L-waves, or Love waves, have a large amplitude and cause significant damage, including fracturing the ground surface.
What percentage of deaths did landslides account for in the 2008 Sichuan and 2005 Kashmir earthquakes?
30%
What is liquefaction?
Liquefaction is a particular hazard in areas where the ground consists of loose sediment such as silt, sand or gravel that is also waterlogged - often found in areas close to the sea or lakes.
How does intense shaking cause liquefaction?
Intense earthquake shaking compacts the loose sediment together, forcing water between the sediment out and upward. This undermines foundations and causes buildings to sink, tilt and often collapse.
Where do violent eruptions associated with volcanoes occur?
At destructive plate margins
What is a lava flow?
Extensive areas of solidified lava, which can extend several kilometres from volcanic vents if the lava is basaltic and low viscosity. It can flow at up to 40 kmh
Which type of volcano produces lava flows?
Subduction zone volcano (composite type) Hot-spot volcano (shield type)
What is a pyroclastic flow?
Very large, dense clouds of hot ash and gas at temperatures of up to 600 degrees C. They can flow down the flanks of volcanoes and devastate large areas
Which type of volcano produces pyroclastic flows?
Subduction zone volcano (composite type)
What is an ash fall?
Ash particles, and larger tephra particles, can blanket huge areas in ash, killing vegetation, collapsing buildings and poisoning water courses
Which type of volcano produces ash falls?
Constructive plate margin volcano (cinder cone, fissure eruption)
Subduction zone volcano (composite type)
What is a gas eruption?
The eruption of carbon dioxide and sulphur dioxide, which can poison people and animals in extreme cases
Which type of volcano produces gas eruptions?
Subduction zone volcano (composite type)
Hot-spot volcano (shield type)
What is a lahar?
Volcanic mudflows, which occur when rainfall mobilises volcanic ash. They travel at high speed down river systems and cause major destruction
Which type of volcano produces lahars?
Subduction zone volcano (composite type)
What is a Jokulhaup?
Devastating floods caused when volcanoes erupt beneath glaciers and ice caps, creating huge volumes of meltwater. They are common in Iceland
Which type of volcano produces Jokulhaups?
Construcitve plate margin volcano (cinder core, fissure eruption)
Why do in most cases, only large composite volcanoes found at destructive plate margins represent a significant tectonic hazard?
These eruptions often have lava flows, pyroclastic flows, lahars and extensive ash and tephra fall that can affect areas up to 30 km from the volcanic vent.
Which type of plate boundary produces the most hazardous volcanoes?
In most cases, only large composite volcanoes found at destructive plate margins represent a significant tectonic hazard.
What generates a tsunami?
A tsunami can be generated by landslides and even eruptions of volcanic islands.
Explain how a tsunami is generated.
Tsunamis are generated when a submarine earthquake displaces the seabed vertically (either up or down) as a result of movement along a fault line at a subduction zone.
What has to happen to the seabed during an earthquake for a tsunami to be generated?
The seabed has to be displaced vertically
Why are tsunami characteristics very different from those of wind-generated ocean waves?
- Wave heights are typically less than 1 m
- Wavelengths are usually more than 100 km
- Speeds are 500-950 kmh
What happens as tsunami waves approach shore?
As the waves approach the shore, they slow dramatically, wavelength drops but wave height increases.
