Tectonics - EQ1 - Why are some locations more at risk from tectonic hazards? Flashcards
1.1 - The global distribution of tectonic hazards can be explained by plate boundary and other tectonic processes. 1.2 - There are theoretical frameworks that attempt to explain plate movements. 1.3 - Physical processes explain the causes of tectonic hazards. (34 cards)
What is Crustal Fracturing?
Occurs when the earth’s crust causes rock to break and
fracture under stress and strain caused by seismic stresses
What is the Benioff Zone?
A zone of seismicity where earthquakes are produced by the interaction of a down going oceanic crustal plate in a subduction zone.
The deepest earthquakes recorded occur in this zone.
What is ‘Slab Pull’?
Slab Pull involves old oceanic crust being submerged into the
mantle. This pulling action drags the rest of the plate with it.
As a crustal plate moves further from an oceanic ridge, it cools and becomes increasingly dense. Eventually the dense oceanic slab of crust becomes heavier than the asthenosphere below it so it descends under its own weight. The weight of this sinking, cooling plate causes a major pulling action, which causes the rest of the plate to be pulled downwards as well.
This is thought to be a primary mechanism for plate movement.
Name the primary hazards of volcanoes
- Lava flows
- Pyroclastic flows
- Tephra and ash flows
- Volcanic gases
What are the characteristics of P (primary) waves?
- Fastest and first to reach the surface
- Travels through solids and liquids (travel forwards and backwards)
- Only damaging in the most powerful earthquakes
True or false?
Earthquakes can occur in the middle of plates
True -
They are called ‘intra-plate’
The causes of this are not fully understood but it is assumed that plates have
pre-existing weaknesses which become reactivated, forming seismic waves
What are the characteristics of L (love) waves?
The slowest and last waves to arrive
- Cause the most damage (ground shakes side to side)
- Larger and focus all energy on the surface
What are the 3 main types of seismic waves?
- P - Primary (or pressure)
- S - Secondary (or shear)
- L waves - Love
Identify secondary hazards of earthquakes
- Liquefaction
- Landslides
- Tsunamis
What are the characteristics of S (secondary) waves?
- Slower than P waves (by 60%)
- Only travel through solids (move in a sideways motion)
- Do more damage than P waves
Are Lahars and Jökulhlaups Primary or Secondary Hazards of volcanic events?
Secondary
What are Lahars?
Lahars are a combination of rock, mud and water which travel quickly down volcanoes. These can occur when an eruption coincides with heavy rainfall.
What are Jökulhlaups?
An Icelandic word - Snow and ice in glaciers melt after an eruption which causes sudden outburst floods which are dangerous .
What is a natural hazard?
A perceived natural/geophysical event that has the potential to threaten both life and property.
With regards to earthquakes this is due to the movement of tectonic plates
What is a natural disaster?
The realisation of a hazard, when it causes significant impact on a vulnerable population
What is the lithosphere?
Ridged outer part of Earth (crust and upper mantle), broken up into a number of plates, about 100km thick
What is the asthenosphere?
The part of the mantle, below the lithosphere, where rock is semi-molten (it is ductile - think like play dough!)
How are volcanic hotspots created?
A mantle plume is an upwelling of abnormally hot rock within the Earth’s mantle causing a hot spot on the surface (the lithosphere melts and allows magma to escape to the surface). The plume stays where it is as the crust slowly migrates over it leading to the formation of island arcs.
Can you explain the distribution of tectonic hazards?
An overview:
- Most hazards occur next to plate boundaries.
- Highest magnitude earthquakes appear to be on destructive and conservative boundaries.
- Concentration of Earthquakes around the Pacific (- Ring of Fire), Alpide belt and Mid-Atlantic Ridge.
- Most of the worlds active volcanoes are located around the Pacific Ring of Fire.
- Deepest earthquakes tend to occur at convergent zones where subduction is occurring (in Benioff Zone).
- Some hazards occur away from plate boundaries e.g. Hawaii (volcanic hotspots) (intra-plate)
- Low magnitude and shallow earthquakes occur near divergent boundaries.
Theory of Plate Tectonics:
Who hypothesised continents had once formed one land mass named Pangaea, before breaking apart and drifting to their current location? He called his theory ‘Continental Drift’.
Alfred Wegener, in 1912
Theory of Plate Tectonics:
What evidence did Alfred Wegener have?
- Continents fit like puzzle pieces
- Freshwater animal fossils on separate sides of the ocean
- Plant fossil findings in Antarctica
- Similar rock layers on separate continents
- Glacial deposits in now tropical/desert areas
Theory of Plate Tectonics:
In the 1960s Harry Hess proposed a new concept following on from Alfred Wegner.
What was his theory?
Seafloor spreading - a geologic process in which tectonic plates split apart from each other, creating ocean basins.
Theory of Plate Tectonics:
What is seafloor spreading?
Basaltic magma from the mantle rises to create new ocean floor at mid-ocean ridges (MORs).
On each side of the ridge the ocean floor moves away from the ridge. As the oceans became wider the continents drifted apart.
Sea floor spreading provides the mechanism that allows the continents to move.
What is Palaeomagnetism?
Resulting from some minerals cooling aligned with the magnetic polarity of planet earth.
The magnetic fields of Earth vary with time and sometimes flip completely. These changes can then be seen in the rocks as you move away from plate boundaries.
