L8 - Plate Tectonics

What is the evidence for continental drift?

• Mountain belts of similar rock are present in Africa and South Africa. Alfred Wagner suggested they were once connected, explaining their similar shapes and rock types.

• Fossils were present on the African and South American continents

• Evidence of glaciation on land masses along the equator.

What was the new evidence found for seafloor structure?

• 1960s = initiative to map the ocean surface and oceanic crust

• Mid-Atlantic Ridge was confirmed

What does Palaeomganetism explain?

• The Earth’s magnetic poles had reversed directions. When lava solidifies, it preserves the magnetic field at the time which records the direction of the Earth’s magnetic field.

• It was found that closer to the ridge reversals were younger than the material further away, and that this material is flowing away from the ridge.

What did the Ocean Drilling Programme reveal?

• The further from the ocean ridge, the age and thickness of the sediment increases

• New ocean crust was being formed at these ridges through the upwelling of the mantle.

Why is there melting at the asthenosphere at tectonic plates?

• Melting occurs due to the incorporation of water in the oceanic crust.

• At the spreading centre, this material is so hot that, when in contact with seawater, the water reacts with solidifying magma, creating new minerals with water in their structure

• During subduction, water is carried into the mantle

What drives convection?

• For convection to occur, there needs to be a variation in density with temperature

• The mantle has roughly no density at the top but relatively high density at the bottom.

• There is a temperature gradient, increasing with depth, towards the centre of the Earth because of the core (undergoing radioactive decay)

• The material in the mantle has a density that increases with decreasing temperature.

• The lower material is hot, with low density, causing it to rise until it cools and sinks

How does upwelling and ridge push and plate pull work?

• Ridge push and plate pull are the main forces driving plate tectonics.

• At mid-ocean ridges, oceanic plates move apart, creating space that draws mantle material upward—it's the space that pulls magma up, not magma pushing plates.

• Reduced pressure on the mantle lowers its melting point, causing it to melt and form basaltic magma, which creates new oceanic crust.

• The elevated ridge (positive relief) from upwelling causes gravitational sliding—a ridge push force moving plates away.

• As plates move away, they cool, thicken, and become denser, eventually sinking under their own weight—this is plate pull.

• Plate tectonics is a self-sustaining process, aided by mantle convection and the ability of the mantle to flow, partly due to water.

What volcanoes form at mid ocean ridges?

• There is an uplift of the asthenosphere and peridotite.

• Partial melting occurs due to decomprression, material rises up, and a basaltic volcano occurs

• Silica form of magma with relatively low viscosity

What volcanoes are associated with mantle plumes?

• Will generate volcanic islands; the silica-poor, viscous magma causes the islands to be quite shallow.

• Creates wide, low, shield volcanoes

What volcanoes form at subduction zones?

• Stratovolcanoes

• Subduction zones, where oceanic sediments and basaltic crust are melted, granitic melts are generated.

• Silica-rich and very viscous

• The body of magma can build up so generates high, pointed granitic volcanoes.

What features are associated with stratovolcanoes?

• Lots of ash is throw into the atmosphere and gets carried around the planet.

• Heavier, larger particle ash will separate from the ash cloud and will flow as pyroclastic flow.

• Often they will have snow on top. If ash melts snow, a lahar will be formed (massive mud flow causing destruction and death)

What did the Pinatubo eruption (1991) cause?

• Caused the planet’s temperature to drop because it put a lot of sulphuric acid in the atmosphere, causing aerosols, which increased albedo and reflected the sun’s rays.

What are the features of shield volcanoes?

• Produced above mantle plumes after initial period of eruption has passed.

• Magma has very low viscosity and does not build up

What are the features of cinder cones?

• Ash cones that are relatively small (~500 m in diameter)

• Magma is released as lava flow

• Can’t predict when they are going to happen

• Won’t kill many people but is very destructive for property

What happened at Lake Nyos, Cameroon (1986)?

• Volcanoes have craters, which can accumulate water

• Although the volcano is not erupting, magma moves to the surface, where it releases pressure and CO2 is released.

• For Lake Nyos, this CO2 accumulated in the bottom of this lake and then volcanic activity disturbed the lake.

• CO2 flowed down the hillside and killed animals and people.