Topic 1b - Tectonics Flashcards
Describe the crust
- thinnest layer
- made of silicon-based rocks
- it has 2 types of crust: Oceanic and Continental
What is Oceanic crust like?
- thin - 5 to 8km
- made of basaltic rock
- denser than continental crust
What is continental crust like?
- thick - 30 to 40 km
- can reach 70km under mountain ranges
- lighter than oceanic crust
- made of granatic rock mostly
Describe the core
- hot and very dense
- inner core is a ball of solid
- outer core is a liquid made of iron and nickel
- temp bw 4400-6000*
- under high Pa so can’t melt
Describe the mantle
- thickest layer of Earth - 2900km
- temp bw 1000 to 3700
- upper mantle = mostly solid but some melting at plate boundaries and flows very slowly
- mantle becomes more depth with depth and high pa
- the very top bit of mantle is rigid and nearest to the core is rigid
made of silicon-based rocks
What is the upper mantle divided into?
- Lithosphere
- Asthenosphere
Describe the lithosphere:
- includes the crust and top layer of the upper mantle
- made of peridotite
- about 800-1000km
- broken into tectonic plates
Describe the asthenosphere:
- denser mobile layer of the upper part of the mantle
- 100-300km deep
- high temp = 1300*
- Pa is low enough so that the rocks can flow very slowly
What do tectonic plates float on?
on the mantle
- asthenosphere
How is heat generated in the mantle?
Through the radioactive decay of some elements in the mantle such as uranium
How do convection currents form?
- lower part of asthenosphere heat up, they become less dense and rise
- move towards the top, cool down and become more dense = sink
- circular movements of semi-molten rock create convection currents
Why do the tectonic plates move due to convection currents?
- convection currents create drag at the base of the tectonic plate as they are solid and rigid
- this causes them to move and creates earthquakes and volcano formation
What are the 3 types of plate boundaries?
- convergent
- divergent
- conservative
Describe convergent plate boundaries:
- plates move towards each other
- oceanic + continental plate = subduction and creates trenches and forms composite volcanoes
- cont + cont plate = collide and form mountains/ major earthquakes as high Pa = rocks snap along faults
What is subduction?
- when denser oceanic plate sinks under continental plate into the asthenosphere
How do composite volcanoes form?
- oceanic plate subduction = more temp and Pa released water and impurities into asthenosphere = which melts into the molten rock
- magma rises and breaks through the Earth’s surface as a composite volcano
Describe divergent plate boundaries:
- move apart from each other
- mostly under oceans
- plates break part = more heat and less Pa = sheild volcanoes
- very small earthquakes = if any
How do shield volcanoes form?
- plates breaking apart = high heat and low Pa = causes asthenosphere to melt - magma rises to fil in the gap/rift valley between the plates
- this cools to form new oceanic lithosphere but this may break through Earth’s surface = shield volcanoes
Give an example of where convergent plates are found?
- West Coast and South Africa
Give an example of where divergent plates are found?
- North American and African Plate
Describe conservative plate boundaries:
- slide past each other
- plates may move in same/ opp directions at diff speed and angles
- crust not created or destroyed
- Pa along the boundary builds up until one jerks = earthquakes
Give an example of where conservative plates are found?
- North American and Caribbean plate moving opp
- North American and Pacific Plate moving in the same direction
- West coast of USA
What are the volcanic hazards associated with convergent plates?
- subduction = melts and destroys mantle
- pool of magma forms and rises through the cracks in the vents
- magma erupts on the surface forming a volcano-
What are the volcanic hazards associated with divergent plates?
- magma rises up into the gap and forms a volcano
What comes out in a volcanic eruption?
- lava
- gases
- ash formed (can cover the land, block out the sun and form pyroclastic flows)
What are hotspots?
- where volcanoes form at not between plate boundaries
How are hotspots formed?
- formed by a plume of superheated rock/magma rising very slowly through the mantle
- causes asthenosphere and base of lithosphere to melt
- magma produced rises through crust’s weakness and erupts at earth’s surface
How do hotspots form chains of volcanic islands?
- hotspots are stationary but the crust above them moves to create chains of volcanic islands
- e.g. Hawaii - in the middle of the Pacific Ocean
What type of eruption happens in oceanic hotspots?
- erupts basaltic lava
- creates shield volcanoes
What type of eruption happens in continental hotspots?
- erupts viscous granitic lava
- this may erupt explosively
Where are composite volcanoes formed?
- formed alongside convergent plate boundaries
What do composite volcanoes look like?
- steep-sided cones rising up several thousand km
Why are composite volcanoes so eruptive?
- subducted oceanic crust has water which reacts with magma and creates gases causing the crust to erupt = explosive
- start with ashy explosions that deposit layers of ash
- then erupt andesitic lava with high silica content, making it viscous so it flows slowly and travels a short distance before cooling
When do composite volcanoes erupt?
- infrequent but violent
What are the primary and secondary hazards after an eruption from a composite volcano?
primary
- vent is blocked with solidified lava = build-up of Pa in next eruption
- lava may shatter into pieces = lava bombs
- hot flows of gas and ash (pyroclastic flows) hazard
secondary
- more deaths may be caused by mudflows (lahars)
Where are shield volcanoes formed?
- over hotspots and divergent plate boundaries
- not very explosive and made up only of lava (erupt from fissures and the crater)
What do shield volcanoes look like?
- low gentle sloping domes with a wide base
Why are shield volcanoes not eruptive?
- erupt basaltic lava which has low silica content so are runny and flows a long distance before cooling
- frequent but not explosive eruptions
What are the primary hazards after an eruption from a shield volcano?
- basaltic flows destroy property and crops but loves are rarely ost
What are the earthquake hazards associated with convergent boundaries?
- most occur here
- tension builds up when one plate gets stuck as its move past another into the mantle
What are the earthquake hazards associated with divergent boundaries?
- tension builds up along the cracks as they move past
What are the earthquake hazards associated with conservative boundaries?
- tension builds up when plates that are grinding past each other get stuck
- plates eventually jerk past each other = shock waves are sent out (vibrations) = earthquake
How are earthquakes measured?
- moment magnitude scale (Mw)
- it measures the amount of energy released
- also a seismometer
What is the focus?
- where it started below the Earth’s surface
- can be up to 700km below
What is the shallow focus?
- caused by tectonic plates moving at or near the surface
- focus is around 0-70 km
What is the deep focus?
- caused by a crust that has been subducted into the mantle
- focus is around 70-700 km
What is the epicentre?
- the point on the ground surface directly above the focus
Why do deeper earthquakes create less damage?
- less damage at the surface
- this is as shock waves travel through more rocks so thus they have less power when they reach the surface
How do earthquakes cause tsunamis?
- underwater earthquakes cause the seabed to move
- the water is displaced and waves spread out from the epicentre
- waves travel very fast in deep water and cab hit the shore without much warning = high death toll
Why do shallow focus earthquakes create bigger tsunamis?
- they can displace more water
- thus stronger and more powerful waves hitting the shore
- even higher death toll than deep focus
How can earthquakes be predicted?
not reliable but scientists can monitor signs:
- lasers detect movement of tectonic plates
- vibrations in Earth’s crust can be measured by seismometers (more vib = earthquake)
- gases measured (radon) - escapes from cracks in the crust before the earthquake (Eq)
- rocks crack and expand due to more Pa before eq
How can volcanic eruptions be predicted?
can be told if they’re well monitored:
- tiny earthquakes and changes in shape of a volcano (bulges in the land where magma built up under)
- thermal imaging camera - detects change in temp
- analyse gases escaping as they emit lots of sulfurous gases before an eruption
What long term planning can we do to prepare for tectonic hazards?
more dev countries = plan for impacts
- emergency services train and prepare for disasters
- buildings designed to withstand earthquakes
- educating people
- gov can plan evacuation routes = fewer people killed by mudflows, fires and pyroclastic flows
- emergency supplies
How can buildings be designed to prepare for tectonic hazards?
- reinforced concrete
- foundations that absorb earthquake’s energy
What emergency supplies can be stockpiled to help prepare for tectonic hazards?
- blankets
- clean water and canned food
- these stockpiles can be moved close to areas that are less likely to be affected
What short term relief is needed right after a disaster?
deals with immediate impacts of tectonic hazards so more dev countries can:
- provide food, drink and shelter to help evacuated people
- treat injured people = fewer deaths
- recover dead bodies = prevents diseases spreading
= rescue trapped people or people cut off by damage to roads and buildings
- provide temp supplies of gas, electricity, and communication if regular supplies damaged
