hazardous earth theory Flashcards
(45 cards)
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
