tectonics Flashcards
how are earthquakes distributed
Most earthquake zones are found at, or close to, tectonic plate boundaries, often in clusters. About 70% of all earthquakes are found in the ‘Ring of Fire’ in the Pacific Ocean.
what are the most powerful earthquakes associated with
The most powerful earthquakes are associated with convergent or conservative boundaries.
what is the oceanic fracture zone (OFZ)
a belt of activity through the oceans along the mid-ocean ridges, coming ashore in Africa, the Red Sea, the Dead Sea rift and California.
what is the continental fracture zone (CFZ)
a belt of activity following the mountain ranges from Spain via the Alps, to the Middle East, the Himalayas to the East Indies and then circumscribing the Pacific.
how do earthquakes occur
Earthquakes are a sudden release of stored energy. As two plates move past each other they inevitably ‘stick’. This allows strain to build up over time and the plates are placed under increasing stress. Earthquakes are generated because of the sudden release of the stress
A pulse of energy radiates out in all directions from the earthquake focus. In some cases the earthquake motion displaces the surface, so a fault scarp can be seen.
how are volcanos distributed
they occur at, or close to tectonic plate boundaries, with the exception of hotspots, such as that by Hawaii.
what percentage of tsunamis occur in the pacific basin
90% of all events occurring in the Pacific Basin
This is due to the activity at the plate margins
how are tsunamis distributed
The global distribution of tsunami is fairly predictable in terms of source areas (90% Pacific basin)
Most are generated at subduction zones (convergent boundaries), particularly off the Japan-Taiwan island arc and South America
what are the causes of tsunamis
Tsunami are generated when a sub-marine earthquake displaces the sea bed vertically as a result of movement along a fault line at a subduction zone.
causes a bulge in water
The violent motion displaces a large volume of water, which then moves outwards in all directions from the point of displacement.
Sub-marine earthquakes that occur close to the shoreline can generate intense ground shaking damage, followed by the damage from the subsequent tsunami.
describe divergent (oceanic oceanic ) plate boundaries and what is found here
-Rising convection currents bring magma to the surface resulting in small, basaltic eruptions, creating new oceanic plate. (shield volcano- effusive eruptions VEI at 1-3)
-Minor, shallow earthquakes; low magnitude (5-6)
-can create an ocean ridge with central rift valley and volcanic islands.
describe divergent (continent continent) plate boundaries and what is found here
A geologically recent mantle plume splits a continental plate to create a new ocean basin. Mantle convection forces plates apart at constructive plate margins. Tensional forces open cracks and faults between the two planes (called a ‘fracture zone’) allowing for magma to move up and erupt
Basaltic volcanoes (shield)- effusive eruptions VEI at 1-3)
minor earthquakes- low magnitude (5-6)
describe convergent plate boundaries (oceanic oceanic) and what is found here
One oceanic plate is subducted beneath another, generating frequent earthquakes and a curving (arc) chain of volcanic islands, which erupt.
seismic activity: range of focal depths along Wadati benioff zone, moderate to high magnitude (7-9)
volcanic activity: explosive (moderate/high VEI scale 5-6)
describe convergent plate boundaries (continent continent) and what is found here
The collision of two continental landmasses, creating a mountain belt (fold mountains and plateaus) as the landmasses crumple, and magma is generated at depth. Infrequent major earthquakes distributed over a large area. Eruptions are possible, but as the magma cools and solidifies beneath the surface they are rare.
seismic activity: shallow to intermediate focal depth, moderate magnitude (6-8)
usually no volcanic activity
e.g. the Himalayas
describe convergent plate boundaries (continent oceanic) and what is found here
An oceanic plate is subducted under a continental plate, creating a volcanic mountain range, frequent large earthquakes and violent eruptions.
1.Mantle convection pulls plates towards subduction zones, whilst the other end of the plate is being pulled away at a divergent margin.
- Cold, dense oceanic plate is subducted beneath less dense continental plate; the density of the oceanic plate pulls itself into the mantle.
- 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.
-range of focal depths along the wadati benioff zone- often high magnitude (8-9)
-explosive, moderate to high VEI scale (5-6)
describe conservative (oceanic continent) and what is found here
Plates slide past each other, along zones known as transform faults.
Frequent earthquakes, with a shallow focal depth, so they can be very destructive if they are high magnitude. mag of 6-8
No volcanic activity.
ridges and scars on surface
E.g. San Andreas Fault, extends 1,200 km across California
what are intra-plate eruptions
intra plate eruptions are eruptions that take place away from a plate boundary at locations called mid-plate hotspots
-examples of these areas are Hawaii and the Galapagos islands
what are mantle plumes
mantle plumes are concentrated areas of heat convection. At plate boundaries, they are sheet-like, whereas at hot spots they are column like
at what speed to the earth plates move at per year
per year, the earths tectonic plates move at a speed of 2-5 cm per year
how thick is each tectonic plate
each plate is about 100km thick ( with a thinner oceanic/ continental crust on top)
why is the idea of plate tectonics still a theory
It’s still a theory because scientists have not yet directly observed the interior of the Earth.
what are key discoveries which confirm that plates move
- fossils confirm that places such Africa and South America have matching rock types. Fossils such as Cynognathus match across Africa and South America.
-In the 1950s, the ocean floor was surveyed showing the presence of mid ocean ridges where new crusts were formed.
-studies in the 1960s of magnetic strips in the oceanic crust confirm that tectonic plates move, these palaeomagnetic signals from past reversals of the Earth’s magnetic field and prove that new ocean crust is created by the process of sea-floor spreading at mid-ocean ridges. (seafloor spreading and palaeomagnetism occur at constructive margins, where new crust is being made) The striping due to palaeomagnetism revealed magnetic reversals in the Earth’s poles as oceanic crust moved away from the mid-oceanic ridges by seafloor spreading
-coal forms in tropical conditions however coal is found in the north sea
define subduction
the process of one plate sinking beneath another at a convergent plate boundary. Earthquakes at subduction zones occur at a range of focal depths from 10 km to 400 km, following the line of the subducting plate.
define slab pull
Cold, dense oceanic plate is subducted beneath less dense continental plate; the density of the the oceanic plate pulls itself into the mantle - this is slab pull. This occurs at destructive margins.
mantle convection: why is the interior of the Earth described as dynamic
Radioactive isotopes such as uranium-238 and thorium-232 in the Earth’s core and mantle generate huge amounts of heat which flow towards the Earth’s surface.
This heat flow generates convection currents in the plastic mantle.
All tectonic hazards are caused by the Earth’s internal heat engine.
what is the earths structure along with the states (g,l,s)
Crust
Mantle - a solid, but is deformable and capable of a very slow flow because of the very high temperatures.
Outer core - liquid
Inner core - solid
what are transform faults and give an example
transform faults are seen in conservative plate boundaries. They traverse the earths surface in a zig-zag pattern.
The most famous transform fault is the San Andreas fault which creates an area of frequent earthquake activity.
do earthquakes have shallow or deep focal depths and what does this mean for the earthquakes at conservative boundaries
Earthquakes along conservative boundaries often have shallow focal depths, meaning high magnitude earthquakes can be very destructive. Volcanic activity is absent.
what are all the characteristics of constructive margins
Eruptions are small and effusive, as the erupted basalt lava has a low gas content and high viscosity.
Earthquakes are shallow, less than 60 km deep, and have low magnitudes of under 5.0.
Oceanic-Oceanic: Minor, shallow earthquakes.
Continent-Continent: Basaltic volcanoes and minor earthquake
what are all the characteristics of destructive margins
Earthquakes at subduction zones occur at a range of focal depths from 10 km to 400 km, following the line of the subducting plate. This is called the Benioff Zone, and it can yield very large earthquakes up to magnitude 9.0.
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.
Oceanic-Oceanic: Frequent earthquakes, and violent eruptions from curving chain of volcanic islands.
Continent-Continent: Infrequent major earthquakes distributed over a wide area.
Oceanic-Continent: Frequent large earthquakes and violent eruptions.
what are the characteristics of conservative margins
Earthquakes along conservative boundaries often have shallow focal depths, meaning high magnitude earthquakes can be very destructive. Volcanic activity is absent.
Oceanic-Continent: Frequent, shallow earthquakes, but no volcanic activity.
what is the focus of an earthquake
the focus is the point at which a pulse of energy radiates out in all directions from the earthquake focus
what is the epicentre of an earthquake
The epicentre is the point on the Earth’s surface directly above the focus.
what is a fault scarp
it is a feature resultant from an earthquake and it displaces the surface
what are the 3 seismic waves that earthquakes generate
-Primary waves
-Secondary waves
-Love waves
The overall severity of an earthquake is linked to the amplitude and frequency of these wave types.
give the features of P waves including:
-speed
-how damaging
-extra info
The fastest, (rate of about 8 km/sec) so they arrive first
Cause the least damage
They are vibrations causes by compression
give the features of S waves including:
-speed
-how damaging
-extra info
Arrive next (about 4 km / sec)
Shake the ground violently, causing damage
give the features of L waves including:
-speed
-how damaging
-extra info
Arrive last, as they travel only on the surface- ‘vibration occurring in the horizontal plane’
They have a large amplitude and cause significant damage, including fracturing the ground surface.
what does magnitude measure
magnitude measures the amount of energy released at the epicentre
how is earthquake magnitude measured
earthquake magnitude is measured using the moment magnitude scale (MMS)
MMS measures the energy released during an earthquake, it uses a logarithmic scale
what does an earthquakes intensity measure
an earthquakes intensity is the effect on people, structures and the natural environment
how is earthquake intensity meaured
by using the modified marcalli intensity scale which takes observations from people who experienced the earthquake
what are the primary effects (direct results) of an earthquake
ground shaking- causes buildings, bridges, roads and infrastructure to collapse, killing or injuring those nearby
crustal fracturing- when energy is released during an earthquake causes the Earths crust to crack -leaving gaps
what are the secondary effects of an earthquake
liquefaction- the violent shaking during an earthquake causes surface rocks to lose strength and become more liquid than solid. The subsoil loses its ability to support buildings. It can make rescue efforts more difficult and also disrupt underground power and gas lines.
landslides and avalanches- The ground shaking places stress on slopes, so they may fail (landslides, rockslides, mudslides and avalanches). This can lead to damage and injuries
tsunami- some underwater earthquakes generate tsunami that cause major problems for costal areas.
what are landslides
landslides are where material loses cohesive strength and moves downwards under the influence of gravity- they occur when seismic waves loose shock
give examples of where landslides have caused devastation
El Salvador, Central America
case study: What happened in Christchurch?
In 2011, a 6.5Mw earthquake occurred which had a shallow focus and the shockwaves were amplified by nearby solid rock. An upper soft layer slapped back down on the layer underneath sending renewed vibrations back to the surface, amplifying liquefaction
