Exploring the Earth's Interior

Exploring the Interior with Seismic Waves- Velocity at which seismic waves travel depends on the material they pass through(e.g., P and S waves travel faster through basalt than granite). At the boundarybetween two materials, some seismic waves bounce off (reflect) and others are transmittedinto the second material. Waves bend or refract as they cross an interface becausetheir velocity is not the same in the second material as the first.

Paths of Seismic Waves in the Earth - Waves bend as they go from layer to layer, and as a result their paths through the interior are curved.

Shadow zones from wave refraction

P-waves that hit the core are refracted both as they enter the core and as they leave the core. As a result, no P-waves arrive at angular distances between 105° and 142°.

S-waves that hit the core do not propagate so there is a shadow zone of no S-wave arrivals from 105° to 180°.

The existance of the shadow zone implies the outer core is liquid both because S-waves do not propagate and because P-waves are bent away from the interface.

Waves Reflected in the Earth - At any boundary between materials seismic waves will reflect. Reflected seismic waves are used to find interfaces between the crust, mantle, and core as well as interfaces between sedimentary layers in oil exploration. Reflections from different interfaces can be distinguished by their amplitude and frequency.

Composition and Structure of the Interior- P and S wave velocities vary with depth.

Crust - is thin under oceans (5 km), thick under continents (40 km) and very thick under continental mountain ranges (65 km).

Moho (Mohorovicic discontinuity) - boundary between fast P-wave velocities of mantle (8 km/sec) and slower velocities of crust (continents - felsic, 6 km/s and oceans - mafic, 7 km/s)

Principle of Isostasy - buoyancy occurs under mountain ranges because the volume of crust displacing the mantle weighs less (just like an iceberg). Thus mountains float high because they are a thick layer of low density material. Continental crust floats higher than oceanic crust because it is thicker and less dense. Removal of a surface load (erosion of a mountain) will cause the crust to rebound. Melting of the continental glaciers is causing Canada and Scandinavia to rebound

Mantle - Below the lithosphere, S-wave velocity decreases and some energy is absorbed. This implies a small amount of partial melting (asthenosphere). At 400 and 660 km depth, S-wave velocity increases rapidly indicating repacking of atoms into denser forms because of high pressure

Core - Outer core is liquid. P-wave velocity increases at 5100 km depth indicating that the inner core is solid. Core is thought to be mostly Iron because iron is dense and it is cosmogenically abundant enough to make up the core (e.g., gold is dense enough but too rare).

Earth's Internal Heat - Early earthwas hot from impacts and radioactive decay, but it has been cooling ever since.

Conduction - Heat energy exists as vibration of atoms. Heat energy is transferred by conduction when atoms and molecules jostle each other mechanically transferring the vibrational motion from a hot region to a cold region. Rock is a very poor heat conductor, so heat transport by conduction is too slow to have significantly cooled the earth.

Topography at Mid-Ocean Ridges – decreases with age of the seafloor (due to cooling)

Convection - Heated fluid expands and rises because it is less dense. Convection is much more effecient than conduction because the heated material itself moves. The mantle is convecting and transferring heat from the interior towards the surface.

Temperatures in the Earth - In the upper 8 km or so of direct measurments, temperatures increase 2-3°C per 100 m. At that geotherm, most of the earth's interior would be molten. Based on constrains from the seismology (e.g., the outer core is liquid, the inner core is solid), it is generally believed that the earth's center is only 4000 to 5000°C

The Interior Revealed by Earth's Magnetic Field

The Earth's magnetic field can be approximated by a bar magnet at the center of the Earth and inclined at 11° from the rotational axis.

A compass needle that is free to swing aligns itself with the Earth's magnetic field (points North).

Earth's magnetic field cannot be a permanent magnetization because the interior is too hot.

Magnetic field is due to flow of material in the liquid outer core, which generates electrical currents and produces a magnetic field (a dynamo)

Paleomagnetism - Rocks canrecord the Earth's magnetic field from some point in the past by a perferential alignment(parallel to the earth's magnetic field) of iron atoms. This perferential alignmentcan occur either as rocks cool down below about 500°C (thermoremanent magnetization)or by small grains containing iron aligning themselves as they sink through water(depositional remanent magnetization)

Inclination of the permanent magnetization (angle magnetic field vector makes with the horizontal) indicates at what latitude the rock formed (e.g., rocks that make up much of Alaska have an inclination of near zero (horizontal) indicating that they formed near the equator).

Remanent magnetization shows that the Earth's magnetic field reverses polarity (compasses point South) on a regular basis (about every 500,000 years).

Epochs - Approximately 500,000 year intervals of either normal or reversed polarity

Events - shorter intervals of either normal or reversed polarity