Studying the Interior of the Earth


Sesimic Methods
	Vp = (4/3 (B+G)/Density)^1/2
	Vs = (G/Density)^1/2
	If density increases with increasing depth, predict how Vp 
and Vs should vary with increasing depth


Interior of the Earth


B, G and Density tend to increase with increasing depth 
within the Earth
However, B and G tend to increase at a greater rate than 
Density
Therefore, P and S waves tend to increase in velocity with 
increasing depth.


Discontinuities


When waves propagate across a boundary between rocks 
with contrasting properties [crust and mantle, for example] :
	Some of the energy is Reflected back towards the surface
	Some of the energy is Refracted across the boundary

Discontinuities Within the Earth


Moho : Crust - Mantle : marked increase in both P and S 
velocities


Lithosphere - Asthenosphere : Pwaves increase but at a slower rate : S 
waves are reduced due to the presence of a small quantity of liquid in the 
asthenosphere



Mantle - Outer Core : change in state - Solid to liquid : no S 
waves

Magnetism


The Earth has a magnetic field : result of currents set up in 
the outer liquid core

We sometimes describe the field as if there was a dipole (a N 
end and a S end) : as the temperature increases the strength of a 
dipole magnet decreases.  At its Curie point, the magnetic 
properties disappear : ~670 degrees C 

Variations in the Earth's Magnetic Field


A compass points toward Magnetic North - If you are on a line 
of longitude that passes through both the Magnetic N pole and 
the Rotational N pole, the needle points toward both poles - O 
Declination


Declination - the angular difference magnetic North and 
rotational North : varies over time.

Variations in Polarity


At the present time the North magnetic pole is in the Northern 
Hemisphere
At other times in the past the South magnetic pole has been in 
the Northern Hemisphere
Normal Polarity - as it is today
Reversed Polarity - 


Gravity


Attraction between an object and the Earth : the closer the 
object is to the center of the Earth, the greater the attraction.
The attraction is also influenced by the densities of the 
materials between the object and the center of the Earth
Anomalies : Negative - Less Dense and Positive - More Dense

Studying Magnetic Variations


Depositional Remnant Magnetism -- Magnetite (a naturally 
ocurring magnetic mineral) orients itself as it settles out of the 
water column.
Thermal Remnant Magnetism -- At temperatures above Curie 
Point magnetite is not magnetic.  Below the Curie Point it 
becomes magnetic and the field aligns parallel to the Earth's 
field.