The earth is made up of the inner core (solid), outer core (liquid), mantle (Liquid) then crust (solid). Within the mantle, there is radioactive decay which produces a lot of heat, this causes the mantle to flow in convection currents.
It is important to remember that in GCSE physics curriculum, tectonic plates don't stay in one place because convection currents move them. When two plates slide pass each other, they don't do it smoothly. The large friction force involves mean that strain builds up and the plate move suddenly when the strain is too much.
Subduction is when the oceanic plate which is thinner and more dense subducts under the less dense continental plate.
Sea floor spreading is when the convection currents move apart the two oceanic plate and magma rises up to solidifies to form a new oceanic plate.
Evidence of Continental Drift
There are three evidences in GCSE Physics curriculum:
- The shape of the continents fit like a jigsaw
- Rocks strata on separate continents are identical
- Fossil records from different continents are identical
Focus: a point where the rock moves. This is where seismic waves begin.
Epicentre: directly above the focus on the Earth's surface where the seismic wave meets the surface.
- Pressure or primary waves
- Longitudinal wave
- Make objects and buildings vibrate vertically.
- Travel through solids and liquids
- Secondary or Shear wave
- Transverse wave
- Make objects and building vibrate horizontally
- Only travel through solids
When seismic waves reach a boundary between different layers of the earth, some waves are reflected and they change speed as the properties e.g. density of the mantle and core change. They change is speed results in a change in direction which is refraction. The waves change speed gradually which results in a curved path, however when the properties change suddenly, the wave speed change and the path kinks. Observing the refraction and reflection of the wave scientist are able to discover where the properties of the Earth have changed.
In GCSE physics, seismometers readings are used to work out the distance to an earthquakes epicentre. P-waves and S-waves travel at different speeds, therefore you see two distinct tremors, the first is a P-waves and the second is S-waves. Using the time difference, it used to calculate how far away the earthquake was. You can also draw a circle on a map centred on the location of the seismometers with the distance you calculated above it's radius. This is called a distance arc.
The distance arcs from three or more seismometers will cross at one place where the epicentre is, this is called triangulation. This will only work with three or more, two cross at two places.
End of this topic!
Drafted by Gina (Physics)