Tectonics Flashcards
Tectonic processes, hazards, trends and modification
When was Tohoku earthquake?
March 2011
Magnitude of Tohoku earthquake
9.0 Richter Scale
Plates involved in Tohoku earthquake
Subduction zone between the Pacific and Eurasian plates
Height of the Tohoku tsunami
10-20m
Deaths from the Tohoku earthquake
16,000
Number of people missing from the Tohoku earthquake
3,000
Number of injured people from the Tohoku earthquake
6,000
Amount of debris from Tohoku earthquake
26m tonnes
Economic losses from Tohoku earthquake
$300bn
Why was the Fukushima Power Plant a major issue in the Tohoku earthquake?
It was flooded during the tsunami and had the potential to explode, causing a nuclear disaster. The area has since been permanently evacuated due to the high levels of radiation.
What did the government in Japan do following the Tohoku earthquake?
Produced a report on how to avoid future disasters. It suggested:
- More safety measures, such as higher tsunami walls
- Better planning, such as avoiding flat coastal areas
- Long-term support for those who had lost relatives or been affected
Features of destructive plate margins
- Oceanic plate slides under the continental plate
- Oceanic crust destroyed
- Friction between the plates can cause earthquakes
Features of constructive plate margins
- Plates move apart
- Pushed by convection currents
- Magma rises to fill the gap that is created
- New crust generated
Features of conservative plate margins
- Plates slide past each other horizontally
- No crust destroyed or produced
- Friction between the plates builds until they can’t take the stress and they slip past each other
Features of oceanic crust
- High density
- Made of basaltic rock
- Only 7-10km thick
- Can subduct
Features of continental crust
- Thicker (25-75km)
- Less dense than oceanic plates
- Made of granitic rock
- Do NOT subduct
Tectonic hazard definition
A natural event that has the potential to threaten both life and property
At what type of plate boundaries do tectonic hazards occur?
All 3 of them!
What is a geological hotspot?
A hot mass of rising heat under a weakness in a plate
Magma rises to fill this weakness
Example of a geological hotspot?
The Hawaiian islands formed as a result of a mid-Pacific hotspot
How do intra-plate volcanic islands form?
- Isolated plumes of convection heat rise towards the surface of the Earth
- Basaltic volcanoes created
- The plume remains stationary as the tectonic plate moves over it
- Continuing plate movement over time produces a chain of volcanic islands
How do intra-plate earthquakes occur?
Ancient fault lines are re-activated by tectonic stresses and this causes minor earthquakes.
What is the name given to the impact of the Indo-Australian plate with the Eurasian plate?
A collision zone
Features of the Earth’s core
- Central part of the Earth
- Made up of an inner core and an outer core
- Inner core = solid; Outer core = liquid
- A source of radioactive heat
What is the Earth’s mantle?
A semi-molten body of rock between the Earth’s crust and its core
Features of the asthenosphere?
- Part of the mantle
- Semi molten
- Below the lithosphere, which floats on top of it
Features of the lithosphere?
- The crust and upper mantle which form tectonic plates
- Around 80-90km thick, but this changes depending on whether the crust is oceanic or continental
Order of the layers of the Earth, starting in the centre
Core, outer core, mantle (including asthenosphere), lithosphere (including crust)
Which is more solid: the inner core or the outer core?
The inner core is more solid
Explain sea-floor spreading theory
In the 1960s, there was a discovery of magnetic stripes in the oceanic crust of the seabed.
Palaeomagnetic signals from the past reversal of the Earth’s magnetic field prove that new crust is created through the process of sea-floor spreading at mid-ocean ridges.
What is gravitational sliding?
Elevated altitudes of crust at ridges at destructive plate boundaries.
Creates a ‘slope’ down where oceanic plates slide
What was Holmes’ hypothesis?
Holmes (1930s) said that the Earth’s internal radioactive heat was the driving force of convection currents in the mantle, which move tectonic plates
Explain Wegener’s theory of continental drift
His hypothesis from 1912 suggested that our continents had once been joined together as supercontinents (i.e. Pangea).
What is slab pull?
At destructive boundaries, the high density ocean floor is being dragged down by a downward gravitational force beneath the adjoining continental crust
What are volcanic eruptions at constructive plate boundaries like?
- Basaltic eruptions
- Produce low viscosity lava
What are volcanic eruptions at destructive plate boundaries like?
Violent due to high viscosity lava
What are earthquakes at constructive plate boundaries like?
- Low in magnitude
- Shallow focus (less than 70km deep)
What are earthquakes at destructive plate boundaries like?
- High magnitude
- Deep focus (up to 700km deep)
- Frequent
Example of a place with destructive plate boundaries?
Western South America
At what plate boundary do deep-sea trenches and fold mountains form?
Destructive
What is a collision plate boundary?
The meeting of two continental plates resulting in the formation of a fold mountain belt
Example of a mountain range caused by a collision plate boundary
The Indo-Australian plate and Eurasian plates have created the Himalayan mountain range
What is volcanic activity like at collision plate boundaries?
There is no volcanic activity
What are earthquakes at collision plate boundaries like?
- Rare
- High magnitude
- Shallow focus (less than 70km deep)
What is volcanic activity at conservative plate boundaries like?
There is no volcanic activity
What are earthquakes at conservative plate boundaries like?
- High magnitude
- Shallow focus (less than 70km deep)
- Common
Example of a conservative plate boundary
The San Andreas Fault, California
What is the focus of an earthquake?
Where the pressure is released underground and where the energy radiates out from.
This is the place with the strongest waves that cause the most damage.
What is the epicentre of an earthquake?
The point directly above the centre of an earthquake on the Earth’s surface
What are seismic waves?
When energy is released from the focus in shock waves
True/false - The Earth’s plates are always moving
True
Tensions build up between them over time
How are earthquakes caused?
- Tension builds up between two plates
- The stress can suddenly be released when the strain overcomes the elasticity of rock
- This release of stress can cause parts of the surface to experience an intense shaking motion
Where is energy transferred in an earthquake?
Mostly transferred vertically to the surface, then moves outwards from the epicentre as seismic waves
How deep are shallow focus earthquakes?
0-70km under the Earth’s surface
How deep are deep focus earthquakes?
70-700km under the Earth’s surface
What causes deep focus earthquakes?
- Previously subducted crust moving towards the Earth’s core
- Previously subducted crust heating up
- Previously subducted crust decomposing
Why are deep focus earthquakes less damaging than shallow focus ones?
The shock waves have had to travel further and so cause less shaking at the surface
What are P waves?
Primary waves
Features of P waves
- Fastest
- Cause the least damage
What are L waves?
Love waves
Features of L waves
- Create significant damage, including crustal fracturing
- Only travel across the surface
- Have a large amplitude
What are S waves?
Secondary waves
Features of S waves
- Slower
- Make the ground shake violently
What do we call series of enormous waves caused by underwater earthquakes?
Tsunamis
What is crustal fracturing?
The Earth visibly separating and deep fissures in the ground are created
What are some primary hazards of earthquakes?
- Ground shaking
- Crustal fracturing
What are some secondary hazards of earthquakes?
- Landslides (in mountainous areas)
- Liquefaction
- Tsunamis (in submarine earthquakes)
What is liquefaction?
When earthquakes shake loose sediment (silt, sand and gravel) that is waterlogged. The compact ground forces water to rise to the surface. Damage to buildings and infrastructure is common.
Example of a tsunami
Boxing Day Tsunami, 2004
What is an ash fall?
A solid material of varying grain size ejected into the atmosphere (known as tephra)
How do ash falls damage buildings?
Buildings often collapse under the weight of ash falling onto their roofs
How can ash falls harm people?
Air thick with ash causes breathing difficulties
Example of an ash fall
The eruption of Mount Merapi in Indonesia in 2010
What are some examples of volcanic gases?
- Carbon dioxide
- Carbon monoxide
- Hydrogen sulphide
- Sulphur dioxide
- Chlorine
Why are volcanic gases dangerous?
They can be poisonous
Example of an eruption which produced lots of volcanic gases
1986 eruption of Lake Nyos, Cameroon
Features of pyroclastic flows
- Very hot (800 degrees)
- High velocity (200km/h)
- A mixture of gases and tephra
- Destroy everything in their path
Ancient example of a pyroclastic flow
AD79 eruption of Mount Vesuvius in Pompeii, Italy