Theoretical Frameworks
Geography: Edexcel A Level, topic 1.1.3
The Earth's Internal Structure
The Earth can be broken down into segments such as the core, mantle, asthenosphere and lithosphere.
The core
- The core is the central part of the Earth.
- It is made up of an inner core and outer core.
- The inner core is solid, the outer core is liquid. Both are made from iron and nickel.
- The core is a source of radioactive heat.
The mantle
- The mantle is a semi-molten body of rock between the Earth’s crust and its core.
The asthenosphere
- The asthenosphere is part of the mantle.
- The asthenosphere is semi-molten.
- It is below the lithosphere, which floats on top of it.
The lithosphere
- The lithosphere is the crust and upper mantle which form the tectonic plates.
- Together they are 80-90km thick
- The crust changes in thickness under oceans and continents.
- Under oceans, the crust is 6-10km thick.
- Under continents, the crust is 30-40km thick.
Diagram
The Theory of Plate Tectonics
Theoretical frameworks attempt to explain plate movements. There have been a variety of theories and discoveries over recent times. For example:
Wegener’s Continental Drift
- Wegener’s Continental Drift hypothesis (1912) said that our now separate continents had once been joined together as supercontinents e.g. Pangea.
Holmes' Hypothesis
- Holmes (1930s) said that the Earth’s internal radioactive heat was the driving force of convection currents in the mantle that could move tectonic plates.
Sea floor spreading
- In the 1960s, there was a discovery of magnetic stripes in the oceanic crust of the seabed.
- Palaeomagnetic signals from past reversals of the Earth’s magnetic field prove that new crust is created by the process of seafloor spreading at mid-ocean ridges.
Gravitational sliding
- Elevated altitudes of oceanic crust at ridges at divergent plate boundaries – create a ‘slope’ down which oceanic plates slide (gravitational sliding).
Slab pull
- At convergent boundaries, the high-density ocean floor is being dragged down by a downward gravitational force (slab pull) beneath the adjoining continental crust.
Processes at Different Plate Margins
There are four main types of process that happen at plate margins:
Processes at divergent margins
- Volcanic eruptions at divergent plate margins are basaltic and produce low viscosity lava.
- Earthquakes at divergent plate margins are low magnitude and shallow focus (less than 70 km deep).
Processes at convergent margins
- Deep-sea trenches and fold mountain ranges often form at convergent plate margins.
- Volcanic activity at convergent plate margins is often violent due to the high viscosity lava.
- Earthquakes at convergent plate margins are high magnitude, deep-focus (up to 700 km deep) and frequent.
- An example of this type of plate margin can be seen in western South America.
Processes at collision margins
- Collision plate margins are characterised by the meeting of two continental landmasses resulting in the formation of a fold mountain belt.
- E.g. The collision between the Indo-Australian and Eurasian plates has created the Himalayan mountain range.
- There is no volcanic activity at collision plate margins.
- Earthquakes at collision plate margins are rare - when they do happen, they are high magnitude and shallow-focus (less than 70 km deep).
Processes at conservative margins
- There is no volcanic activity at conservative plate margins.
- Earthquakes at conservative plate margins are high magnitude, shallow-focus (less than 70 km deep) and a common occurrence.
- An example of this type of plate margin is the San Andreas fault in California.
Test yourself on Theoretical Frameworks
Earth's Structure
What makes up the lithosphere?
What is true of the Earth's structure?
What did Holmes say was the driving force of convection currents in the mantle that could move tectonic plates?
At divergent plate boundaries, what do higher altitudes of oceanic crust at ridges create?