Spec S1 Flashcards
Structure of the Earth
Inner Core
- Solid
- Made of iron and nickel
- Extends out to 5100km below Earth’s surface
Outer Core
- Liquid
- Made of iron and nickel
- Extends out to 2900km below Earth’s surface
Moho Discontinuity
- Boundary between core and mantle
- Depth of around 2900km
Mantle
- Extends from edge of outer core to crust
- Very slowly flows, but closer to solid than nickel
Asthenosphere
- Section of the mantle
- Semi-molten, is capable of flowing slowly
- Extends 100-300km below surface on average
Lithosphere
- Rigid layer between crust and asthenosphere
- Varies in thickness and its boundary with asthenosphere is difficult define precisely as it starts to melt and becomes incorporated into the asthenosphere
Crust
- Outermost layer
- Solid rock
- Continental - 3.0kg/m^3 density, mainly granitic, silicon, aluminium
- Oceanic - 2.7kg/m^3 density, mainly basaltic, silicon and magnesium
Role of Convection Currents
- It is thought that within the asthenosphere, convection currents exist, which are cased by vast amounts of heat generated deep in the mantle
- As a result, the semi-molten asthenosphere flows, carrying with it, the solid lithosphere and crust
Evidence for sea-floor spreading
- Palaeomagnetism
- Age of sea-floor rocks
Paleomagnetism
- technology used to track submarines measured very small variations in Earth’s magnetic field
- igneous rocks that form oceanic crust and ocean floor originate as lava flows containing lava particles
- as lava erupts then cools, magnetic orientation of the iron particles is locked into the rock depending on the Earth’s polarity at the time
- polarity not constant, changes every 400,000 to 500,000 years, this is recorded in rocks on the ocean floor
- width of each strip of ocean bed with same magnetic orientation was found to correspond with the time scale of each magnetic reversal
- symmetrical pattern of geomagnetic reversals on either side of mid-ocean ridges indicated that as fresh molten rock from asthenosphere reached ocean bed, older rock was ‘pushed’ away from ridge
- sea-floor spreading moves material across ocean floors on ‘conveyor belt’, eventually sea floor reaches an ocean trench where material is subducted into asthenosphere and becomes semi-molten
Age of sea floor rocks
- during 1960s ocean drilling programme established which investigated ocean sediments and crustal rocks on deep ocean floor
- drilling recovered cores in water up to 7000m deep revealed spatial pattern of sediments that supported theory of sea floor spreading
- thickest and oldest sediments were found nearest to continents, cores also showed that nowhere in oceans was rock older than 200 million years, confirmed that ocean crust was constantly recycled over this period
Geological evidence of continental drift
- Fit of continents such as South America and Africa on either side of the Atlantic
- Evidence from around 290 million years ago of the effects of contemporaneous glaciation across southern Africa, Australia, South America, India and Antarctica, suggesting that these land masses were joined at this time, close to the South Pole
- Mountain chains and some rock sequences on either side of oceans show great similarity (eg. north-east Canada and northern Scotland)
Biological evidence of continental drift
- Similar fossil brachiopods (marine shellfish) found in Australian and indian limestones
- Similar fossil reptiles found in South America and South Africa
- Fossils from rocks younger than the Carboniferous period, in places such as Australia and India, showing fewer similarities, suggesting that they followed different evolutionary paths
What factors are linked to the global distribution of plates and plate boundaries?
- Science used for military purposes revealed invaluable data - from mid-1960s, nuclear tests were conducted underground
- These released vast amounts of energy which were picked up on seismometers
- Detailed maps produced from seismic data worldwide showed that most earthquakes, especially high-magnitude ones, were specially concentrated in narrow bands, in between were relatively large areas that generated few earthquakes
- This suggested that the rigid lithosphere and crust were broken up into tectonic places
- Plates were not static, in some places they were moving apart and in others they were converging
What features and processes are linked to divergent (constructive) plate boundaries?
- Plates moving apart leads to magma rising through the asthenosphere and forcing its way to the surface, mostly takes place at mid-ocean ridges
- Mid-ocean ridges have an average depth of 2.5km below the ocean surface, consist of very long chains of mountains (rise 3000m above the sea bed in some places)
- Mid-ocean ridges are not continuous, at frequent intervals they are broken into segments by transform faults, which displace the ridge sideways by tens or in some places, hundreds of kilometres
- Volcanic activity absent along transform faults, but as they slip, energy is released in the form of earthquakes
- Mid-ocean ridges vary in shape depending on the rate of spreading, which is determined by the amount and rate of magma brought to the surface by convection currents
- Eruption of magma along divergent boundaries occurs mostly underwater, magma erupting directly onto the sea bed is cooled rapidly, forming rounded mounds called pillow lavas
- As magma rises to surface, the overlying rocks can be forced up into a dome, the rigid lithosphere is placed under great stress and eventually fractures along parallel faults, this produces underwater rift valleys found along mid-ocean ridges
- At mid-ocean ridges, sea water seeps into rifts and is superheated, as it rises towards the surface it causes chemical changes in the basaltic rocks, superheated jets of water sometimes reemerge on the ocean floor containing metal sulphides - black smokers
What features and processes are linked to conservative plate boundaries?
- Plates move side-by-side, either in different directions or in the same direction at different speeds
- Volcanic activity is absent
- Frictional resistance to movement along the plate boundaries often causes build up of pressure
- Occasionally, these pressures cause rocks to fracture, releasing enormous amounts of energy as earthquakes
- Possible to discern active plate boundaries extending through the landscape like a giant tear, where rocks are exposed at the surface, the extent of movement between the strata may be visible
- Drainage is also modified as river courses are deflected by movements along the faults
What features and processes are liked to convergent (destructive) plate boundaries at continent-ocean locations?
- Denser oceanic plate is forced under the continental plate in a process known as subduction
- The angle at which the oceanic plate is subducted is between 30-70 degrees, as it descends, the oceanic plate comes under intense pressure and friction, faulting and fracturing occur in the Benioff zone, where the descending plate is at an angle close to 45 degrees, this process releases vast amounts of energy in the form of earthquakes
- Subduction also causes the oceanic plate to melt, because melted material is less dense than surroundings, it rises towards the surface as plutons of magma
- Huge intrusions of magma create further uplift of fold mountains, where rising magma reaches the surface it forms volcanoes
- Pressure from plates interacting with one another creates deep ocean trenches (long, narrow depressions with depths of 6000-11,000m), these mark the zone of subduction where the oceanic crust descends into the asthenosphere
- As oceanic plate converges on a continental, these sediments and rocks crumple, fold and are uplifted along the leading edge of the continental plate (fold mountains), continental crust is buckled and uplifted and vast amounts of molten material are injected into it, the result is mountain chains such as the Andes
What features and processes are linked to convergent (destructive) plate boundaries at ocean-ocean locations?
- Slightly denser plate will subduction under the other, creating a trench
- As descending plate melts, magma rises to the surface and forms chains of volcanic islands known as island arcs
- Earthquakes can occur due to pressure build up and explosive, composite volcanoes can be formed on island arcs
What features and processes are linked to convergent (destructive) plate boundaries at continent-continent locations?
- When two continental plates converge, little if any subduction takes place
- This is because the two plates have similar densities, in Europe, the collision of the African and Eurasian plates over the past 40 million years has created the Alps (fold mountains)
- Earthquakes do occur as the land buckling creates large tremors
What are the features of explosive volcanic eruptions?
- Convergent plate boundaries
- Lava is made of rhyolite and andesite, acidic (high silica content), has high viscosity, lower temperature at eruption
- Violent bursting of gas bubbles when magma reaches surface, highly explosive, vent and top of cone often shattered
- Gas, dust, ash, lava bombs and tephra erupted
- Tend to have long periods with no activity
Volcanoes are steed-sided, strato-volcanoes, with caldera usually more than 2km in diameter
What are the features of effusive volcanic eruptions?
- Divergent plate boundaries
- Lava made of basalt and is basic (low silica), low viscosity, higher temperature at eruption
- Gas bubbles expand freely, limited explosive force
- Gas and lava flows erupted
- Tend to be more frequent, an eruption can continue for many months
- Volcanoes have gentle sloping sides, shield volcanoes, lava plateaux when eruption from multiple fissures
Why do some volcanoes occur away from plate boundaries (e.g. hotspots and rift valleys)?
Hotspots
- Small areas of the crust with an unusually high heat flow, found away from plate boundaries
- Slowly rising mantle plumes are believed to create volcanic activity at the surface
- As the crust moves over the hotspot it creates a chain of islands, featuring mainly extinct volcanoes as they have moved away from the mantle plume
- The Hawaiian Islands are a good example of this - eight main islands that are progressively older and less volcanic with distance from the main hotspots plume which have developed as the Pacific plate has moved across the hotspot over time
- Volcanoes at hotspots often have the same characteristics as shield volcanoes but not always
Rift Valley
- Can occur when crust is being pulled apart and stretched, but there may be no plate boundary
- Stretching creates low valleys compared to the surrounding continental landscape and where the crust thins magma may rise and create volcanoes
- These are often shield volcanoes in a line along the rift
Example is the East African Rift valley which is creating a 4000km long rift creating several active or dormant volcanoes, such as Erte Ale, which is an active volcano in Ethiopia
