Y12 MB - Volcanic Hazards Flashcards
What is the asthenosphere?
The semi-molten portion of the mantel
What are the differences between oceanic crust and continental crust?
The OC is thinner but also denser than the CC
The OC is made up of mainly basalt and formed beneath the ocean, at constructive plate boundaries. It is much younger than the CC as it is denser so is subducted and destroyed in the mantel
The CC is mainly made up of granite
Why is the Earth’s core so hot?
The primordial heat left from when the Earth was first formed and the radiogenic heat produced by the radioactive decay of isotopes
Why is the heat in the Earth’s core so important?
It helps drive mantel convection
What is subduction?
The process of large areas of oceanic crust on the ocean floor being pulled down into oceanic trenches and being destroyed
Describe the layers of the Earth
Crust - the thinnest layer, made up of solid rigid rock with a high strength. ranges from 5 - 70k thick
Mantel - the thickest layer of the Earth at around 2900km , made up of partially molten rock at between 500 - 2000 degrees C
Outer core - made up of molten iron and nickel with temperatures of 4000 - 6000 degrees C and is about 2300km thick
Inner core - mostly made up of solid iron due to the high pressure and is about 1200km thick. Temperatures are up to 7000 degrees C
What are the three parts of the mantel?
The lithosphere (rigid)
The asthenosphere (plastic, capable of flow)
The mesosphere (rigid)
Describe how convection currents occur
The inner core has temperatures of up to 7000 degrees C due to primordial and radiogenic heat
This heat is transferred through the core, to the mantel. The semi-molten rock in the asthenosphere is heated and becomes less dense so rises
When it reaches the crust, it cannot break through so spreads and moves sideways in the uppermost part of the mantel
It cools in the upper portion of the mantel so becomes denser and sinks to the asthenosphere where it is heated again and the cycle continues
This causes the tectonic plates to move in the same direction as the mantel due to friction
Name the reasons scientists believe cause tectonic plate movement
Convection currents
Ridge push
Slab pull
Describe the process of ridge push (gravitational sliding)
As the asthenosphere upwells at a mid-ocean ridge, the oceanic crust is raised 2-3 km above the ocean floor
As the new rock from the magma cools, it solidifies to become denser and therefore gravity causes it to slide down the slope, pushing the ocean floor
This is also known as gravitational sliding and is believed to contribute towards the movement of tectonic plates
Describe the process of slab pull
At a destructive plate boundary, the oceanic plate is denser than the continental plate so is subducted into the mantel
As the oceanic plate descends further into the mantel and is destroyed, it pulls the rest of the plate downwards with it due to gravity
Geologists think that this process alongside convection currents drives tectonic plate movement
What is the evidence for continental drift?
Continental fit - Theory produced by Alfred Werner where all continents were once joined in a large landmass called Pangea
Geological evidence - Some mountain ranges around the world are the same age and have the same structure so may have nice been connected
Climatological evidence - North America and the UK both have similar coal deposits with and similar age and were formed in a tropical climate. As they are no longer in tropical zones, they must have shifted
Biological evidence - The same animal fossils, such as the Mesosaurus, as found on different continents. This would only be possible if they could swim, which the majority of the animal fossils found could not
Paleomagnetism - Every 400,000 yers, the polarity of the Earth reverses. As the plates moves apart at constructive plate boundaries, a rock called magnetite is formed. This can be magnetised so shows the polarity changes of the Earth, evidencing sea floor spreading
What are the four different plate boundaries / margins?
Destructive subduction
Destructive collision (sometimes known as converging)
Constructive
Conservative
Describe a destructive subduction plate boundary
Where a continental and oceanic plate move towards each other
The denser oceanic plate is subducted beneath the continental plate in the Benioff zone into the mantel
The subducted plate melts and is destroyed due to friction and heat in the asthenosphere
The molten material from the melted oceanic plate can rise as andesitic lava and cause explosive volcanic eruptions. The friction of the plate movement can cause earthquakes
Landforms at this margin include composite volcanoes, fold mountains and oceanic trenches
Describe a destructive collision boundary
Two continental plates move towards each other
As they are light plates and have very similar densities, subduction does not occur when they meet
Instead, their edges and the sediment between them are forced upwards into fold mountains in orogenesis
Friction between the two plates can cause earthquakes and a secondary hazard is landslides
The landform at this boundary is fold mountains
Describe a constructive plate boundary
Two plates (usually oceanic) move away from each other
Basaltic lava rises and fills the gap created by the plates, solidifying and forming new crust
This new crust is continuously being created so is youngest at constructive boundaries
The rising magma fro the mantel can cause gentle volcanic eruptions and minor earthquakes can occur at transform faults
Landforms are ocean ridges, sub-marine shield volcanoes and rift valleys
