Inner model of earth Flashcards
Density
Average density of Earth (5.52x10^3 kg/m^3) is greater than the densities of surface rocks (2-3x10^3 kg/m^3). Therefore, mass must be concentrated at the centre of the Earth.
Moment of Inertia
The moment of inertia of Earth (0.33Ma^2) is less than the moment of inertia of a uniform sphere of the same radius (0.4 Ma^2). Therefore, mass must be concentrated at the centre of the Earth.
Seismology
Body waves:
P waves can travel through liquids, but S waves can’t as the shear modulus of liquids is 0. At internal surfaces, refraction and reflection occurs (Snell’s law).
Shadow zones:
Between 103 and 142 degrees from the focus, there is a P and S shadow zone. P waves are refracted away from this zone at the mantle/outer core boundary and S waves are stopped by the liquid outer core. Bewteen 142 and 180 degrees there is an S wave shadow zone as S waves are stopped by the liquid outer core.
Normal modes:
earthquakes with the greatest magnitudes excite the earth into free oscillation. These are fundamental oscillations of a ball in space. Their period is diagnostic of the interior density and velocity structure of Earth.
Conversions:
S-waves are present in the inner core, despite it being surrounded by molten outer core due to conversions between P and S waves at internal surfaces. The component of a P wave that oscillates perpendicularly to a liquid/solid interface will cause shearing and will generate an s wave. A P wave that remained a P wave will reach a point faster than one that was converted to an S wave and back.
Moho:
The boundary of the crust and the mantle. Above the Moho, the speed of P waves is c. 6 km/s. Below the Moho, the speed of P waves is c. 8 km/s. So, P waves after c. 250km from the focus arrive earlier than expected. The faster path is through the crust, along the Moho and up through the crust again. Change in P wave velocity shows a change in density and/or composition.
LVZ:
Partial melting of solid mantle at c. 125km depth.
Sudden increases in seismic velocities at c. 450km and c. 670km. Minerals change to denser forms without changing chemical composition.
Carbonaceous Chondrites (e.g Allende)
Missing some volatile elements, but otherwise have nearly exactly the same composition as the Sun (known from spectral lines in emitted light). From the asteroid belt i.e. the point where solid planets can be. So, when the solar system formed, the Sun was not the only thing to gain this composition. Therefore, we can assume it’s approximately the composition of the Earth. Since the mantle is mostly silicates, there is an iron deficiency. The amount of iron required (if it’s located in the core) satisfies the average density and moment of inertia that we have observed Earth is.
Iron meteorites
90% Iron with some Ni and S. Would be a good fit for the core composition
Earth’s magnetic field
Earth is a self-exciting dynamo. The core is too hot for it to be a permanent magnet (c. 4000 degrees celsius). The field changes with time, we know this as the magnetic pole has changed in recorded history (nautical maps). The convection currents in the outer core, fueled by the latent heat of crystallisation in the inner core, maintain the dynamo.
