hazardous earth theory Flashcards
the structure of the earth chemically
-core is iron and nickel
-mantle rich in silica magnesium and iron
mechanical structure of the earth
-top layer is lithosphere (brittle solid)
-asthenonsphere (ductile (deform and flow placicity under high temperate and pressure allows rigid plates to move over it) )
-lower mantle (solid)
-outer core (liquid)
inner core (solid)
crust thickness
30-40km thick
convection currents and updates theories
Convection currents are still part of the movement of plates. This when heat rises from below in the mantle and causes convection currents which contribute to plate movement. However now it is believed that convection currents occur as patchy thermal plumes (it’s a more complex structure) rather than a pattern of strong, regular cells. Additionally geologists now believe the cooling from above drives mantle convection more than heat from below. Downwelling is when slabs of cold oceanic plate become dense enough to sink into the mantle. This causes the lithosphere to stretch and thinned elsewhere this reduces pressure on rock below and forms an upwelling response.
continental drift
Alfred Wegener produced his theory of continental drift that the continents are not fixed but they are actually moving. He proposed that all the continents were one land mass (Pangea)
evidence for Pangea/continental drift
-jigsaw fit: similarity in congenital coastlines either side of the Atlantic suggests they were joined
-fossil evidence: same fossils found either side of the oceans e.g the mesosaurus fossils found in south America and Africa
-geological fit: similar mountains ranges on either sides of the oceans e.g the Appalachians have the same rock type and age as mountains found in Britain and Scandinavia
-ancient glacials: glacial deposits from 300 million years ago suggest a single ice sheet covered Southern Africa, Australia, south America, India and Antartica
evidence for sea floor spreading
-theory that the sea floor is constantly being recycled sinking and being replaced with new lithosphere elsewhere
-Paleomagnetism: Iron- rich minerals in lava align with the earths magnetic field. Every few 100,000 years the earths magnetic field reverses its polarity and iron in rocks aligns with this
-age of sea floor rocks: they increase in age as you get further from the mid ocean ridge
evidence for plates
-most earthquakes and volcanoes occur in narrow bands leading to the idea that the earths rigid lithosphere is broken into plates
how many large lithospheric plates are there
7
convergent: destructive plate boundary
Oceanic - continental
-two plates move towards each other
-the denser oceanic plate subducts
-Ocean trench is formed because the dense plate can bend under pressure. An ocean trench is a narrow long deep depression in ocean floor
-thrust and fold mountains form due to the continental plate faulting
-an accretionary wedge is also formed with the sediment that is scrapped of the oceanic plate. adds material to continent resulting in lateral growth
-when subduction happens the oceanic plate gets more dense which causes an increase in slab pull
-less dense magma rises through the continental plate resulting in volcanoes
-earthquakes are also present
convergent destructive plate boundary
oceanic- oceanic plates
similar sorts of processes but the denser oceanic plate is the older and colder and that’s the one that subjects.
Magma generation often forms a island arc An island arc is a chain of curved islands made by volcanoes. They form when one piece of Earth’s outer layer sinks beneath another, creating volcanoes that rise up from the ocean floor to make islands. when they erupt creates more magma and more land
convergent- collision
continental-continental plate boundaries
none of them are dense enough to subduct. so they collide this forms fold mountains. alongside this violent earthquakes occur.
metamorphism can also occur: Metamorphism is a process in which pre-existing rocks undergo changes in mineralogy, texture, or chemical composition
Himalayas were formed because of thus (India colliding with Eurasia)
divergent (constructive plate boundaries)
-this where two plates are pulled apart
-sea floor spreading: two plates move away from each other this spreads lithosphere making it thinner reduces pressure on anthesopheric mantle meaning it can upwell and partially melt and new magma forms new ocean crust.
-Mid-ocean ridge: 60,000 km long chain of submarine volcanic mountains they are thin lithosphere over areas of upwelling. Volcanic eruptions may occur along the mid-ocean ridge, adding additional layers of lava and volcanic material to the seafloor. These volcanic features contribute to the growth and evolution of the mid-ocean ridge system. have central Rift Valley
- Transform Vaults: offset mid ocean ridges very 50-500km meters generating shallow earthquakes. normally distance between central rift
- ridge push: MOR have high elevation so plates slide down either side due to gravity pushing the plate away from the ridge
conservative/transform plate boundaries
when plates move alongside each other, frictional forces lock building up strain energy and when plates slip this releases energy is released. example is San Andreas fault between North and Pacific American plates
when do volcanoes form
when subduction occurs and pressure builds up gas built up in magma is the driving force of an eruption. the amount of gas in the magma determines the nature of the erruption
explosive volcanoes
-trapped gas bubbles burst violently when magma reaches the surface. vent at the top of the cone is often shattered
-silicic lava high viscosity
-temp 700-1000degrees celcius
-lots of lava, gases, pyroclastic material, dust, ash, lava bombs, and blocks
-long periods of no activity
-steep sided composite volcanoes
-usually at convergent plate boundaries
effusive volcano
-gas bubbles escape easily (less violent bursting), lava flows freely with limited explosive force
-mafic lava - low viscosity
-temp above 1000 degrees C
-lava and gas (water vapour, CO2,N, SO2) comes out
-regular eruptions that continue for many months or year
-shield volcanoes with gentle slopes and a lava plateau
-divergent boundaries and hot spot
eruptions not at plate boundaries
these result from mantle plumes reaching the base of the lithosphere
-this causes partial melting of the area and then a hot spot is formed
-the plate moves over the hotspot creating a chain of volcanoes
-examples are the Hawain chain and the East African Rift Valley
3 types of volcanoes
-composite volcanoes : alternating layer of tephra and viscous lava pile up near the vent, forming a distinctive conical shape with steep sides. Tend to be no more than 10km in diameter but may exceed 3000m in height.
-sheild volcanoes: broad rounded shape with gentle slopes because the lave flows far from the vent largest can be 100km wide
-super volcanoes: a volcano that has erupted over 1000km3 of material in a single event. These eruptions eject so much magma that the chamber is emptied and the volcano collapses n its self forming a caldera
the volcanic explosivity index
measures the magnitude of volcanic eruptions allowing them to be compared. It uses the volume of material ejected, eruption cloud plume height plus qualitative description to assign a value from 1-8. Each increase describes a ten fold increase in explosively with number 8 being a super volcano .
volcanic fissure
linear volcanic vents through which lava erupts effusively
hazards generated by volcanic eruptions
-lava flows (burns, burys and bulldozes)
-pyroclastic flows: extremely hot above 500degrees Celsius . Mixture of gas and rock destroying everything in there path can ignite fires melt snow damn streams
-lahars: pyroclastic floe material mixed with water that is often from ice melting at the summit. gets bigger as it flows down the volcano
-tsunamis: large volumes of ester displaced by eruptions at volcanic islands
-tephra: fragments that range from fine ash to volcanic bombs that can be unto 6 cm long.
-man gas that is released from volcanic eruptions is water (70% water vapour. chlorine is also released and can contaminate water sources.
strain energy
energy stored inside rocks from applies stress and its eventually released in the form of an earthquake. The lithosphere is constantly under stress due to plate movements.
how does earthquake happen
eventually the strain energy and stress overcomes the strength of the rock causing it to break along lines of weakness these are caused faults. and seismic waves are released which radiate from the focus in all directions the
epicentre
point on the surface above the focus
shallow-focus earthquake
0-70km
-at all plate boundaries and sometimes at interpolate locations
-provide evidence that lithosphere is divided into plates
-stronger ground shaking
-ground shaking concentrated in a smaller area
deep-focus earthquake
70-700km deep
-at destructive plate boundaries
-provide evidence for subduction
-weaker ground shaking at surface as strength of seismic waves decrease over distance
-shaking over a wide area because the seismic waves spread out far away from the source
shallow focus earth quake example
kashmir, pakistan 2005
focal depth 23km
Moment magnitude 7.5
deaths was over 87,000
deep focus earth quake example
Hindu Kush, Afganistan 2015
focal death 212 km
M 7.5
deaths was 399
seismic waves
energy waves caused by a sudden release of strain energy from the lithosphere
earthquake magnitude
is the amount of energy released by an earthquake. each earthquake only has one magnitude
earthquake intensity
the amount of ground shaking in a specific location
this depends on four factors:
earthquake magnitude
epicentre and focus
rock and soil characteristics
resistance of building and infrastructure on ground shaking. therefore intensity depends on where its measured
Richter scale
(1935) asses energy released using maximum amplitude of seismic wave measured on a seismogram
moment magnitude scale (1979)
-(1979)
-includes additional factors like size a fault rupture and rock strength
-more accurate main method used by seismologists
-each increase on the moment magnitude scale is 10 fold increase on shaking and 32 fold on energy released
-a negative is that its a logarithmic scale which is hard for the general public to understand so can effect preparedness
modified mecrali intensity scale
(1931)
-12 point scale based on qualitative descriptions of shaking felt and observed environment
-intensity of a single earthquake observed in many locations can be plotted of a map using isolines
-can be inaccurate as it depends on human perception which can vary and reduce accuracy . However more useful than magnitude when assessing impact
where do earthquakes normally occur
along faults which are brittle cracks in the lithosphere
earthquake effects on landscape
earthquake effects on landscape can be severe because the rock either side can be displaced by meters at a time. when the plates move away from each other it creates Rift Valley. Escarpment is a tilted upland area which has a short steep scrap slope and along gentle dip on the other side its formed where the fault breaks.
hazards generated by earthquakes
-ground shaking(pimrary)
: ground moves vertically, horizontally. shaking depends on distance from epicentre and magnitude and geology.
-liquefaction(secondary): unconsolidated deposits with high water content loose mechanical strength and become fluid (sand, and silt on a flood plain)
-landslides and avalanches (secondary): can trigger slope failure and mass movement
-Flooding(secondary): destroying and destabilises damns, destroying or lowering protective levees, causing dams and rivers to form quake lakes.
tsunami as a secondary impact
tsunami(secondary):series of giant waves that a caused by a sudden displacement of water caused by seabed uplift. In deep sea they have a low wave height and very long wave length and travel quickly. when it reaches shallow water it will gain height. If a wave trough reaches the shore first it was cause sea level to drop this is called drawdown then behind there should be a tall wave crest.
what has more deaths
data shows earthquakes do
change of risks over time
-the frequency of geophysical events hasn’t changed but the people living in tectonically active locations has increased. the number of geographical disasters has increased. Human activity may have increased risk eg. deforestation makes landslides more likely.
geophysical disasters
defied as an earthquake volcano or mass movement effecting over 100 people
the disaster risk equation
the risk posed by a hazard depends on the level of vulnerability of the people living there.
R= frequency of or magnitude of HAZARD x level of VULNERABILITY \ CAPACITY of population to cope and adapt
possible future strategies to cope with earthquakes
-deterministic predictions: stating when and where an earthquake of a certain magnitude will occur is not currently possible
-probabilistic forecasting:
