tectonics Flashcards
(80 cards)
What is the general distrubution of volcanoes?
There are around 1500 active volcanoes on earth, most occuring along plate boundaries. About 50 of them erupt every year.
What is the global distrubtion of tsunami events?
90% of all events have occured in the pacific basin, especially off the Japan-Taiwan Island arc. They normally occure along convergent plate boundaries.
what causes an earthquake?
Subduction zones are broad areas where two plates are moving together, often with the thinner, more dense oceanic plate descending beneath a continental plate. The contact between the plates is sometimes called a thrust or megathrust fault. Where the plates are locked together, frictional stress builds. When that stress exceeds a given threshold, a sudden failure occurs along the fault plane that can result in a ‘mega-thrust’ earthquake, releasing strain energy and radiating seismic waves. It is common for the leading edge to lock under high friction. The locked fault can hold for hundreds of years, building up enormous stress before releasing. The process of strain, stress and failure is referred to as the elastic-rebound theory.
The 2004 Indian Ocean tsunami was the result of a mega-thrust locked fault (subducting Indian Plate) with strain building up at around 20 mm per year. It generated huge seismic waves and the devastating tsunami.
What are the three major types of plate boundary?
Convergent, divergent and conservative
What happens at a convergent plate boundary where an oceanic and continental plate meet?
As oceanic plates are denser than continental plates, the oceanic plate gets subducted underneath the continental plate when they collide. The oceanic plate begins to melt as it enters the mantle, which forces itself to the surface in the form of magmatic plutons through faults in the lithosphere to form explosive volcanic eruptions due to the high silica content in the oceanic plate. Deep ocean trenches mark where the oceanic plate was subducted. The subduction leads to the formation of fold mountains as the continental plate is folded and pushed upwards. Friction in the subduction causes deep earthquakes in an area known as the benioff zone. E.g the Nazca plate under the South American plate, forming the Andes.
What happens at a convergent plate boundary where two oceanic plates meet?
The oceanic plate which is moving faster or is denser tends to be subducted, forming oceanic trenches. Magma is created from the subduction, which escapes from the mantle to the ocean floor through faults in the crust. The creates underwater volcanoes which overtime will reach the surface to form island arcs such as Hawaii. Subduction also causes earthquakes (this was responsible for the 2004 Indian ocean tsunami)
What happens at a convergent plate boundary where two continental plates meet?
No subduction occurs at first as the two plates have very similar properties. Instead, they collide and crumple upwards, forming fold mountains. Overtime some subduction may occur when sediment is compressed and becomes denser. No volcanic activity as no magma is created, but shallow focus earthquakes which increases severity as energy is not dissipated through the crust.
What happens at a divergent plate boundary?
In the ocean, two plates separating depressurizes the asthenosphere, causing partial melting and magma to escape and cool, forming underwater volcanoes the Mid-Atlantic ridge - new crust is created to ‘fill in the gaps’. Islands can form like Iceland. When plates move apart on land, the crust cracks and forms faults, before it eventually collapses, forming rift valleys
What happens at a conservative plate boundary?
Two plates move past each other, often snagging as they move causing major cracks in the crust known as faults. If these continue to grow, they are known as transform faults. No magma is created, nor crust made or destroyed. However, powerful, shallow focused earthquakes occur. EG the san-andreas fault in California.
What tends to be the most severe plate boundary?
convergant, due to th existence of mega thrusts, the stress and tension built up tends to be far more severe than conservative and divergent boundaries
What causes a intra-plate earthquake?
The reactivation of ancient fault lines, caused by changes in the stress pattern across the lithosphere.
What is the general distribution of earthquakes?
Around 95% occure along plate boundaries, with 70% occuring in the ‘Ring of Fire’ in the pacific. Arounnd 5% are intra-plate earthquakes.
What causes intra plate volcanoes?
Mantle plumes below faults in the crust.
What is the structure of the earth?
What are the main processes that move plates?
Mantle convection - radioactive decay in the earths core generates heat, which heats the lower mantle, and creates convection currents within the mantle, causing plates to move (disputed)
Sea floor spreading - the spreading of sea floors creates mid ocean ridges, which become denser as they cool. Overtime, they will sink into the lithosphere, dragging the plate down with it. This is known as slab pull
subduction - gravity causes dense oceanic plates to sink when colision occures.This pulls the rest of the plate down with it.
What are the pieces of evidence that proved plate techtonics?
Studies during the 1950s highlighted paleomagnetism along mid ocean ridges. When scientists surveyed the mid ocean ridges, they found symetrical bands of magnetic stripes. This is due to the fact that every 400,000 years the earths magnetic field swaps, and the iron in newly fomed magma alligns itself to the earths polarity. The existence of symetrical bands of rock proved that new crust was being created through sea floor spreading.
Further analysis showed that the coninents use to fit together but became distant due to continetal drift. Further more, fossils of animals match across the continents, proving the existence of pangea.
- What is the Benioff zone?
- What is the hypocentre/focus?
- what is the epicentre?
- The planar zone of seismic activity which corresponds with the subduction of a plate
- The hypocentre/focus is the point within the point where an earthquake rupture starts - this is the point where seismic waves are emmited from
- The epicentre is the point ton land directly above where the hypocentre/focus is
What is signifcant about how deep/shallow the benioff zone and focus are?
The deeper the focus and benioff zone, the longer the energy from the earthquake can disipate before reaching the surface. Therefore, a shallow earthquake, even if it releases the same amount of energy as a deep earthquake, will have more adverse effects on a population.
Explain each of the 3 different waves emmited from an earthquake
- (P)rimary waves - These are compressional waves, and travel at 8km/sec, and therefore are the first to arrive. They cause the least damage and travel through solid and liquids.
- (S)econdary waves - These are slower, travelling at 4km/sec. They osicilate at right angles to the direction of travel. They only travel through solids. They cause more damage than P waves
- (L)ove waves - only travel through the surface, and shake the ground from side to side. They are the slowest, but cause the most damage as all their energy is focussed on the surface, and they have a greater amplitude.
What are the primary hazards caused by an earthquake?
- Ground shaking - when the release of seismic waves, mainly L waves, causes the ground to shake, destroying buildings
- Crustal fracturing - When Seismic waves create faults in the earths crust, causing gaps to appear
What are the secondary effects of an earthquake?
Liquifaction - the violent shaking of the earth causes saturated materials to behave like a fluid, which destablises infrastructure. In the 1995 Kobe earthquake, the container terminal was destroyed by liquifaction due to the destabloised ground.
Landslides - Shaking of the ground destablises sediment on an incline. In the 2015 Nepal earthquake, The Langtang village was covered by a landslide, killing 178 people.
Tsunamis (get to later)
What are the three forms of lava? What type of volcano do they create?
- Basaltic - low silica (50%) and gas content, therefore has a low viscosity meaning gasses can escape. This means eruptions are fairly gentle, and this type of magma creates sheild volcanoes
- Andestic - Medium silica (60%) and gas content
- Rhylotic - High silica (70%) and gas content
These types of lava trap gasses, and therefore eruptions are far more explosive and dangerous. These tend to be found at convergant plate boundaries
What are the primary affects of a volcano?
- Lava flows - streams of lava erupted from the volcano. These tend not to be too dangerous as most people can outrun them, however can cause devistating affects on buildings and agriculture
- Pyroclastic flow - the most dangerous affect of a volcano. These when a mixture of hot gasses pyroclastic materials like ash and glass flow down volcanoes in a cloud. These can be 1000c and travel at 700km/h
- Tephra - the eejection of fine material (ash) into the atmosphere. This can build up on roofs and cause themm to colla[se, and can also reduce visibility and air travel.
- Volcanic gasses - emit fromvolcanoes, and can be undetected by people becuase they are colourless and odourless. in 1986, emmisions of Carbond dioxide from Lake Nyos in Cameroon killed 1700 people, as it sat over there village.
What are the secondary affects of a volcanic eruption?
Lahars - When tephra deposites mix with heavy rainfall to form mudflows
Jökulhlaups - A devestating flood caused by glacial meltwater. Here, a volcanic eruption melts the bottom of a glacier, causing meltwater to form underneath the glacier. When enough meltwater is formed, it will lift away the ice block and escape from the volcanic crater, causing serious flooding in the surrounding area