tectonics Flashcards
(40 cards)
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
Identify secondary hazards of earthquakes
Liquefaction
Landslides
Tsunamis
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?
he 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
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
paleomagnetism
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.
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 the difference between magnitude and intensity?
Magnitude measures energy released at the source of the earthquake.
Determined from measurements on seismographs.
Intensity measures the strength of shaking produced by the earthquake/explosivity of a volcano at a certain location.
Determined from effects on people, human structures, and the natural environment.
For a disaster to be entered into the database of the UN’s International Strategy for Disaster Reduction, at least one specific criteria must be met. Do you know any?
Any of the following:
- A report of 10 or more people killed
- A report of 100 people affected
- A declaration of a state of emergency by the relevant government
- A request by the national government for international assistance
What is the definition of ‘Risk’?
The probability of a hazard event occurring and creating loss of life
What is the definition of ‘Vulnerability’?
A high risk combined with an inability of individuals and communities to cope.
What is the definition of ‘Capacity to Cope’?
The ability of affected communities to cope with a given hazard.
The Hazard Risk Formula/Equation measures the level of risk in an area. What components do you think make up this?
hazard x exposure x (vulnerability/manageability)
