Tectonics Flashcards
Types of plate margins
- conservative
- constructive/divergent
- convergent (collision and destructive)
Conservative plate boundaries
- plates slide past each other horizontally
- jagged edges can catch and snag against each other causing friction and pressure
- friction and pressure may result in earthquakes, but not volcanoes
Constructive/divergent plate boundaries
- plates move apart, causing a gap between plates
- magma rises up causing volcanoes and new crust is created (sea floor spreading)
- no earthquakes
destructive plate boundaries
- dense oceanic plate subducts beneath continental plate and crust is destroyed
- rocks catch, building pressure until plates slip past each other, causing earthquakes and tsunamis
- melted crust can rise up through faults as volcanoes
oceanic plates
- dense
- basaltic rock
- 7-10km thick
continental plates
- less dense
- granitic rock
- 25-75km thick
subduction
the oceanic plate is recycled into the earth’s mantle at a destructive plate boundary
mantle plume
a large column of magma rising through the mantle that creates a hot spot on the surface
Intra-plate
occurring within a tectonic plate e.g. an earthquake in Gujarat, 2001
Convection
the exchange of heat by the movement of a liquid. Convection currents in the mantle move tectonic plates
Seismic waves
When energy from an earthquake is released from the focus in shock waves and travels through the earth’s layers. Can be P-waves, S-waves and L-waves
Focus
Where an earthquake occurs. Where the pressure is released underground and where the energy radiates out from
Epicentre
The point directly above the center of the earthquake on the earth’s surface.
Hazard
A perceived natural event that has potential to threaten life and property
Disaster
The reality of a hazard happening and causing a significant impact on a vulnerable population
Hazard risk equation
risk = hazard * vulnerability / capacity to cope
Distribution of tectonic hazards
- Distribution is uneven, some areas are at a high risk, whereas other areas are at a low risk of tectonic hazards
- Hazards happen at specific spots, usually associated with plate boundaries
Hotspots
-Hot plumes of magma rising under a weakness in a plate causes the magma to rise through the surface through the weakness
- The magma plume stays where it is as the plate slowly moves over it,
- The magma dries, a chain of volcanic islands with extinct volcanoes are produced e.g. Hawaii
Intra-plate earthquakes
- Caused by stresses within a plate
- as flat plates move over a spherical surface, zones of weakness are created
- this can lead to earthquakes e.g. 2001 Gujarat in the centre of the Indian plate
Earth’s core
- central part of the earth consisting of the inner and outer core
- made up of iron and nickel
- inner core is solid and outer core is liquid
- source of radioactive heat
Mantle
- semi-molten rock between the earth’s crust and core
- has a temperature gradient that drives convection currents
Asthenosphere
- part of the semi-molten mantle
Lithosphere
- forms tectonic plates
- consists of the crust and upper mantle
- floats on top of the asthenosphere
Ridge push
elevated altitudes of oceanic crust and convection currents at ocean ridges causes plates to slide down
Slab pull
Dense oceanic plate is pulled downwards by convection currents and then sinks down into the mantle due to its own weight. pulling the rest of the plate with it
Sea floor spreading
Mid ocean ridges at divergent plate boundaries
- magma is forced up through the ridge
- hardens as it cools, creating new crust and pushes plates apart
- this forms new crust
- confirmed by palaeomagnetism
Palaeomagnetism
- confirmed the sea floor is spreading
- the earth’s magnetic field changes direction (magnetic N&S swap) every 400,000 years or so
- The iron particles in rising magma align themselves with the earth’s polarity as they harden to form new crust.
- scientists found symmetrical magnetic stripes along both sides of mid ocean ridges
Wegener’s continental drift hypothesis (1912)
- theory that our now separate continents were once joined together in supercontinents (Pangea)
Holme’s hypothesis
- the earth’s radioactive heat was the driving forces of convection currents in the mantle that could move tectonic plates
Benioff zone
a seismically active underground area within a subduction zone, where the slab is being thrust downwards. The different speeds ad movements of rock causes many intermediate/deep focused earthquakes
Why is the Benioff zone an important theoretical framework?
It is an important factor in determining earthquake magnitude, as the position and depth of the hypocentre can be determined
Locked fault
A fault that is not slipping because the frictional force is greater than the shear stress across the fault. Locked faults can be ‘stuck’ for hundreds of years, building up enormous stress before it releases, generating large magnitude earthquakes. ‘elastic rebound theory’
What release of a locked fault resulted in a disaster?
The ‘mega-thrust locked fault’ subducting Indian plate resulting in the 2004 Indian Ocean tsunami
What is an earthquake
The shaking of the ground caused by sudden motions along faults or fractures in the Earth’s crust
What is the sequence of events leading up to the generation of an earthquake?
- A gradual build up of tectonic strain, stored as elastic energy in crustal rocks
- The rock fractures when the pressure exceeds the strength of the fault
- this produces a sudden release of energy, creating seismic waves that radiate away from the point of fracture
- the brittle crust then rebounds either side of the fracture, creating ground shaking felt on the surface
Hypocentre
The ‘focus’ point within the ground where the strain energy of the earthquake stored in the rock is first released
The focal length
The distance between the hypocentre and the epicentre on he surface
Primary (P) waves
vibrations caused by compression (+dilation). Spread quickly from the fault (8km/sec)
Secondary (S) waves
Vibrate at right angles to the direction of travel and cannot move through liquids. Move more slowly (4km/sec)
Love (L/Q) waves
surface waves with the vibration occurring in the horizontal plain. High amplitude
Intensity
A measure if the ground shaking. This ground shaking causes the most building damage and loss of life.