OCR A-Level Geography exam with correctly answered questions(Hazardous earth)

What are the 3 layers of the Earth? Crust Thinnest layer (varies from a few km thick up to 80km) Lighter elements present here (magnesium, silica, aluminium) Mantle The thickest layer (2900km thick) Made up of the lithosphere, the asthenosphere and the inner mantle Outer Core Liquid The Gutenburg discontinuity is the boundary between the mantle and the outer core Inner Core Solid Most dense layer (about 1200km thick) containing iron and nickel What 2 layers does the upper mantle consist of? Lithosphere (semi-core) The top part of the mantle Rigid and stuck to the underside of the crust Varies in thickness Boundary with asthenosphere difficult to define (melts and becomes incorporated in asthenosphere) Asthenosphere extends 100-300km Semi-molten/viscous - Allows rock to move due to high pressure in mantle. Flowing slowly 00:02 01:24 What are continental and oceanic plates made up of? Lithosphere and crust Where do convection currents exist? What do these do? Asthenosphere Caused by vast amounts of heat generated in mantle. They pull on the underside of the lithosphere causing it to move, resulting in plate movement What are the properties of the continental crust? Thickness: 35km average (<30-70km) Density: 2.6-2.7 g/cm^3 Mineral composition: Mainly granitic, silicon, aluminium Much older than oceanic crust (up to 4 billion years old) What are the properties of the oceanic crust? Thickness: 5-10km Density: 3.0 g/cm^3 Mineral composition: Mainly basaltic, silicon and magnesium Much younger than continental crust (up to 260 million years old) Explain the movement of the crust caused by convection currents in the mantle. Hot rock rises from lower to upper asthenosphere. Hot rock spreads and cools, pushing plates apart. Cool rock sinks back down towards core. As (oceanic) plate subducts at ocean trenches, gravity pulls it under Rising mantle pushes crust upwards at mid-ocean trenches, while gravity pulls it back down What was Alfred Wegener's theory? 250 million years ago (Carboniferous period), all the Earth's continents fit together (Pangea). Over time, continents have moved apart through continental drift. Explain 3 pieces of (Wegener's) geological (rocks) evidence for continental drift. Mountain chains and rock sequences on opposite sides of oceans show close similarities (e.g. northeast Canada and northern Scotland). - These mountains are likely to have been created together and then split apart as the continents moved. Continents seem to fit together (particularly South America and Africa). - Suggests that continents once fit all together and continental drift has separated them. - (Erosion wouldn't've made them this shape). Evidence of glaciations 290 million years ago in southern Africa, Australia, South America, India and Antarctica. - Suggests these land masses were joined during this time, located close to the South Pole. - (India has a tropical climate now, and due to it's current location it wouldn't have been glaciated in an ice age. - Suggests India was one at a higher latitude further from the equator). Explain 3 pieces of (Wegener's) biological (living) evidence for continental drift. Similar fossils of marine shellfish (e.g. brachiopods) were found in Australian and Indian limestones. - Brachiopods are small shellfish that would be unlikely to cross open stretches of ocean between Australia and India. - Suggests India and Australia were once much closer together meaning the brachiopods have been separated by continents moving apart. Similar reptile fossils found in South American and South Africa. - Reptiles wouldn't've been able to cross the ocean suggesting all land was once joined together. - They are unlikely to have evolved the same in separate areas. Fossil from rocks younger than Carboniferous period show fewer similarities between animal, suggesting they followed different evolutionary paths. - These animals originally evolved together (during carboniferous period). - Once the continents split the animals were separated. - They evolved as their environments changed, becoming more different. What is sea-floor spreading? Lateral movement of new oceanic crust away from a mid-ocean ridge (constructive plate boundary). How does sea floor spreading provide evidence for continental drift? Sea-floor spreading moves material across ocean floors. Newest rock is found in the middle (at mid-ocean ridge), therefore new rock is being created here - these palaeomagnetic stripes show that each one is mirrored either side of the mid-ocean ridge, showing that new crust is being created here before it moves apart Rocks are not the same age as the earth (4.5 billion years) meaning oldest rock is being destroyed and replaced by newer rocks - also shows new crust is being created Indicates that, as fresh molten rock from asthenosphere reaches ocean bed, 'older' rock is pushed away from the mid-ocean ridge. Oldest rock would be located furthest from the point it is created (nearer land). Eventually, the sea floor reaches an ocean trench where material is subjected into the asthenosphere and becomes semi-molten (caused by convection currents). 00:02 01:24 How does the global patterns of plate boundaries provide evidence for continental drift? Earthquakes are concentrated in narrow bands at plate boundaries (rigid lithosphere and crust broken up into tectonic plates that are moving) - there is an obvious pattern that most volcanic eruptions and earthquakes occur around the edges of plates, e.g. the Pacific Ring of Fire, where the most movement occurs (evidence for continental drift) Although there are some in the centre of plates, there is an obvious pattern How do subduction zones provide evidence for continental drift? Where plates collide, fold mountains and ocean trenches prove that the plates are moving towards each other The Benioff zone in ocean trenches, where oceanic crust is being subducted, experiences earthquakes occurring below sea level - this proves that plates are subducted Volcanic activity also occurs on these boundaries due to the melting of the subducted plates - volcanic activity proves plate movement