The Earth's Interior

To get some understanding of what happens below the surface, one might have to use indirect means.

Geophysics - a branch of geology that applies the laws & principles of physics to study of earth.

Seismic waves - Reflection (usually at a rock boundary); Refraction (changing of direction, bending as waves run from one layer to another layer of different density)

Minimum distance from epicenter for refracted wave to arrive before direct wave

The Earth's Internal Structure

1) Crust - (Granite, Schist, Gneiss, and Sedimentary rocks)

30-50 km thickness

density 2.7 gm/cm³

P wave travels at approximately 6 km/sec

Continental Sial

Oceanic Sima

(7 km) marginal

D = 3.0 gm/cm²

composition... Basalt (gabbros)

P-wave travels at approximately 7 km/sec

2) Mantle

P wave 8 km/sec

Ultramafic rocks (e.g. peridotitie)

Lithosphere - crust and part of upper mantle - make up the plates of the plate tectonic theory (70 - 125 km)

Asthenosphere with lower p- wave velocity is probably a partially melted rock zone. This may the zone of magma generation.

P-wave shadow zone in region between 103⁰ and 143⁰

3) Core
Composition of the core

Iron center? This is based on the average density of the earth which (which) is 5.5 gm/cm³

Crust and Mantle make up ~ 85% of earth's volume and here avg. density is lower than 5.5 gm/cm³

We know about the interior of the earth through
(1) Seismic activity

(2) density and assumptions

(3) based on meteorites composition

Isostasy - a balance or equilibrium between adjacent blocks of brittle crust floating on the plastic upper mantle

Gravity measurements

Force of gravity between A & B = constant

Gravity meter - measures the force of attraction of the earth to a mass over the meter. Also used to explore for ore or salt domes (positive and negative) gravity anomaly of ready is higher than the normal regional gravity

Existence of earth's magnetic field suggests metallic core

The Earth's Magnetic Field

Temp at the earth's center is too high for magnetism to hold as heat discharges magnetism

Magnetic field - a region of magnetic force dipolar - fluid metallic interior. Magnetic north and geographic north pole

Magnetic reversal - time interval during which the North magnetic pole and the south magnetic pole exchange positions

Magnetic meters - magnetic anomalies

Heat within the Earth

Geothermal gradient ~ 25C/km (75F/Mile) Problem with the gradient? This gradient cannot go on for ever. Core temp is between 4000⁰C to 6900⁰C