plate tectonics and volcanoes Flashcards
what is the earth divided into?
- crust
- mantle
- inner core
- outer core
what is the crust?
- this is the outer layer of the earth which is mostly made of oxygen and silicon and is 0-100km thick
- it is split into a thin layer of dense basalt that is found under the oceans and a thicker layer of less dense granite rock that makes up the continents
what is the core and what is it split into?
- the core is made up of dense rocks containing iron and nickel alloys and is a solid because of pressure
- it is subdivided into a solid inner core and a very hot molten ( liquid ) outer core with a temperature of over 5000 degrees
what is the mantle?
- the thickest layer made of molten and semi-molten rock that is rich in iron and magnesium
- has a depth of 2900km and around 1000-3500 degrees
- the part of the mantle nearest the core is quite rigid and the very top is rigid
what is the asthenosphere?
- this is a semi-molten mantle that lies below the lithosphere, at depths between 100 and 700 km
- it acts plastically, meaning it can move and deform very slowly
what is the lithosphere?
the rigid uppermost part of the mantle and the crust together
what are the two types of crust?
- continental crust
- oceanic crust
what are the features of a continental crust?
- this is the thicker crust ( 30-70km )
- older ( 1500 millions years )
- lighter ( 2.6 g/cm3 )
- mainly made up of granite, silicon, aluminium and oxygen
what are the features of an oceanic crust?
- thinner crust ( 6-10 km )
- younger ( less than 200 million years old )
- heavier ( 3 g/cm3 )
- mainly made of basalt, silicon, magnesium and oxygen
what is the theory of continental drift?
this theory was introduced by Alfred Wegener and stated that at one point in time, all of the continents were joined together as one large mass of land, then spread apart and drifted apart into their current positions
what evidence did Alfred Wegener have for continental drift?
- the distribution of ancient fossils, rocks and mountain ranges matched across different continents e.g. South Africa and North America
- he used the location of ancient climate zones, such as glaciation, as he found grooves that when matched with different continents, fit perfectly e.g. South America, Antarctica and India
why wasn’t Alfred Wegener initially believed?
as Wegener couldn’t provide evidence for the physical processes of the continents, moving through the oceanic crust and mantle, as he stated they ‘could move’ but not how
define a tectonic plate
one of a series of rigid sections of the earths crust that float on the upper mantle and move relative to one another
what is plate tectonic theory and what evidence supports it?
- the theory that states that the earths crust is made up of several rigid plates moving relative to one another
- the mid-Atlantic ridge provides evidence for plate tectonics as the polarity of the rock either side of the ridge was alternated in a striped pattern- that was mirrored on each side of the ridge
what is paleomagnetism?
- paleomagnetism happens approximately every 100,000 years and the rock containing iron particles is made of liquid, this allowing the iron to orientate itself from north to south
- on the mid-oceanic ridge volcanoes will form providing lava
- as tectonic plates diverge, the magma rises which then cools and condenses to form new crust
- overtime the new crust is dragged apart and even more new crust forms between it pushing older crust further away from the mid-Atlantic ridge
what is sea floor spreading?
- this is when more crust is being created due to new volcanoes
- this means that the distance between continents is increasing at a rate of a few centimetres per year
what are the 3 main causes of plate movement?
- convection currents
- slab pull
- ridge push
what are convection currents?
- these are circular movements of semi-molten rock and occur when:
- the earths mantle is hottest close to the core, so lower parts of the asthenosphere heat up and become less dense and slowly rise
- as they move towards the top of the asthenosphere they cool down and become more dense and slowly sink
- these circular convection currents create drag on the base of the tectonic plates, causing them to move
what is slab pull?
- slab pull happens at destructive plate boundaries when the colder, denser oceanic plate sinks into the mantle
- gravity then pulls the whole plate into the mantle and this force that the sinking edge exerts is slab pull
what is ridge push?
- ridge push happens at the constructive plate margins when magma rises to the surface and forms new crust which is very hot, and heats the surrounding rocks which expands and rises above the surface
- the new crust cools and becomes denser
- gravity acts on this older, denser lithosphere causing it to slide away from the ridge, down the sloping, semi-molten asthenosphere below
- the occurrence of shallow earthquakes from the tearing apart of the newly formed crust, indicates that there is some frictional resistance to this force
what are the three main types of plate margins?
- constructive plate ( divergent )
- destructive plate ( convergent )
- conservative plate
what happens to the plates at a constructive margin?
at this margin the two plates move apart from each other ( diverging )
how are volcanoes formed at a constructive plate margin?
- the mantle is under pressure from the plates above, when they move apart the pressure is released at the margin
- the release of pressure causes the mantle to melt producing magma
- the magma is less dense than the plate above so it rises and can erupt to form a volcanoe
how are earthquakes formed at a constructive plate margin?
- the plates don’t move apart in a uniform way - some parts move faster than others
- this causes pressure to build up and when the pressure becomes too much the plate cracks making a fault line, causing an earthquake
what 2 landforms are created at the constructive plate margin?
- ocean ridge
- rift valley
how is an ocean ridge formed?
- at constructive plate margins, plates are moving away from one another and the lithosphere is created
- under the oceans, this has produced an extensive ocean ridge system comprising of underwater volcanoes that stretch for nearly 65,000 km and most of them lie at an average depth of 2,500 metres
- some examples are the mid Atlantic ridge and the east pacific rise
how is a ridge valley formed?
- where plates diverge beneath land, rising magma causes the continental crust to bulge and fracture, forming fault lines
- as the plates keep moving apart, the crust between parallel faults drop down to form a rift valley e.g. east African rift system that stretches from Mozambique to the red sea
- volcanoes are found around rift valleys e.g. mount Kilimanjaro and mount Kenya are volcanoes in the east African rift system
what happens to the plates at a destructive plate margin?
at this margin the two plates move towards each other ( converging )
what three plate movements happen at the destructive plate margin?
- oceanic continental
- oceanic oceanic
- continental continental
what landforms are created at the oceanic-continental?
- deep sea trench
- fold mountains
- volcanoes
- earthquakes
how are deep sea trenches formed?
- this is where the continental crust and oceanic crust converge, the more dense oceanic crust is forces under the less dense continental crust e.g. Mariana trench
how are fold mountains formed at the destructive plate margin?
- formed when the plates meet
- they’re made up of sediments that have accumulated on the continental crust, which are folded upwards along the edge of the continental crust
how are volcanoes formed at the destructive plate margin?
- the oceanic crust is heated by friction and contact with the upper mantle, which melts into magma
- the magma is less dense than the continental crust and will rise back to the surface to form volcanoes
how are earthquakes formed at the destructive plate margin?
- as one plate moves under the other they can get stuck
- this causes pressure to build up
- when the pressure becomes too much the pates jerk past each other, causing an earthquake
what happens at the oceanic oceanic crust?
- two plates of oceanic crust are moving towards each other, the denser of the two will be subducted forming a deep sea trench and triggering earthquakes and volcanic eruptions
- volcanic eruptions that take place underwater create island arcs, which are clusters of islands that sit in a curved line
what happens at the continental continental crust?
- where two plates of continental crust move towards each other, neither is subducted so there are not any volcanoes but the pressure build up between them can cause earthquakes
- fold mountains form when continental crusts converge e.g. the Himalayas
what happens to the plates at the conservative plate margin?
- at this margin the two plates move past each other
- the two plates get locked in place and pressure builds up, releasing the energy as an earthquake
- e.g. the pacific plate is moving past the north American plate causing many earthquakes along it’s fault line e.g. the San Andreas fault in California
what is a magma plume / hot spot?
a vertical column of extra-hot heat that rises up from the mantle, the magma is less dense than the surrounding rock, where volcanoes form above the magma plumes