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.