4. Plate Tectonic Theory Flashcards
The core
- at the centre
- Inner core: at the very centre of the Earth and hottest part. It’s solid, mostly consists of iron
- Outer core: semi-molten and mostly consists of liquid iron and nickel
The mantle
- Surrounds the core and is the widest layer
- The upper part is solid but below it, the rock is semi-molten- forming the asthenosphere
The crust
- Forms the outer shell of the Earth
- Oceanic: thin, dense layer which lines the ocean floors
- Continental: older, thicker layer, less dense layer which makes up the Earth’s landmasses
What is the lithosphere?
The crust and upper mantle, the solid layer from which tectonic plates are formed
What is continental drift?
The gradual movement of the continents across the earth’s surface through geological time.
Who was Alfred Wegener?
- Creator of Plate Tectonic Theory
- Believed continents gradually began to drift apart around 300 million years ago
- Holmes supported the idea by creating convection current theory
What happened in 1912?
Wegener theory of continental drift
What happened in 1940s?
Harry Hess reveals shape of ocean floors , using sonar and radars
What happened in 1960s?
A drill ship collects crystal samples and dates them
What happened in 1963?
J Tuzo Wilson proposes theory of sea floor spreading
What happened in 1965?
J Tuzo Wilson and Harry Hess propose theory of plate tectonics
What is the geological evidence for the theory?
- the fit of South America and West Africa
- evidence of a late carboniferous glaciation deposits from which are found in South America, Antarctica and India. Formation of these deposits cannot be explained by their present position- must have formed together and moved
- rock sequences in northern Scotland closely agree with those found in eastern Canada
What is the biological evidence for the theory?
- fossil brachiopods found Indian limestone are comparable with similar fossils in Australia.
- fossil remains of Mesosaurus found in South and North Africa
- fossilised remains of a plant which existed when coal was being formed have been located only in India and Antarctica
What is paleomagnetism?
- In the 1950s, studies of paleomagnetism confirmed the sea floor was spreading.
- Every 400,000 years the Earth’s magnetic fields change direction- causing magnetic N+S poles to switch
- When lava cools to become rock, minerals line up with the Earth’s polarity at the same time. Scientists studying mid ocean ridges found the same pattern of magnetic direction at either side of the ridges (something that could only happen if new rock was being formed at the same time on both sides)
What is slab pull?
Denser oceanic plate sinks below less dense continental plate and the gravitational pull of the oceanic plate pulls the rest of the plate along with it.
What is ridge pull?
- Magma rises as the plate moves apart.
The magma cools to form new plate material; as it cools, it becomes denser and slides down away from the ridge which causes other plates to move away from each other.
How does an oceanic ridge form?
- Continuous features created when plates move apart in oceanic areas.
- The space between the separating plates is filled with basaltic lava upwelling form a ridge
- Volcanic activity occurs along the ridges forming submarine volcanoes which may eventually rise above sea level to create islands
Example of an oceanic ridge
Surtsey south of Iceland
How are deep sea trenches formed?
- Destructive boundary
- Oceanic and continental plates meet and the denser oceanic plate is forced underneath the lighter continental one. The downwarping of the oceanic plate forms a very deep part of the ocean known as a trench.
Example of a deep sea trench
Off the Western Coast of South America, the Nazca plate is subducted under the South American plate forming the Peni-Chile trench
How are Island Arcs formed?
- During subduction, the descending plate encounters hotter surroundings and this and heat is generated from friction begins to melt the plate.
- As this material is less dense than the surrounding asthenosphere, it begins to rise towards the surface as plutons of magma.
- Eventually, these reach the surface and form complex, composite and explosive volcanoes.
- If eruptions take place offshore, a line of volcanic islands forms.
Example of an Island Arc
Marianas Islands and Guam
How are rift valleys created?
- Form at constructive plate margins in continental areas
- Magma rises from below and heats the crust which expands and starts to rift
- As the brittle crust rifts and fractures, sections of it move apart.
- Areas of crust drop down between parallel faults to form rift valleys.
- Through the thinning and stretched crust, magma forces its way to the surface to form volcanoes
Example of a rift valley
Mount Nyragongo
How are young fold mountains created?
- Destructive plate boundary
- Continental plates have a lower density so there’s not much subduction when plates meet.
- When these plates moves towards each other, their edges and sediments are forced up into fold mountains
- As there’s little subduction, there’s no volcanic activity.
- Material is also forced down to form deep mountain roots.
Example of young fold mountains
The Himalayas
