EARTH'S INTERIOR AND ACTIVITY

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THE EARTH'S DIFFERENTIATED INTERIOR

CRUST: Igneous, Sedimentary, Metamorphic rocks;

mostly SiO _2 , Al _2 O _3 (granite-like)

Oceanic Crust: Covers 3/5 of earth, averages 7 km thick,
rho = 3.0 g cm^{-3}

Continental Crust: Averages 36 km thick,
rho = 2.8 g cm^{-3}

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MANTLE: `plastic' rocks; mostly Fe, Mg, Si, O (basalt-like)

About 2900 km thick, contains most of earth's mass;
rho = 5 g cm^{-3} ; T = 1800 K

Heated mainly from below, convection currents are present.

SIDEBAR ON MODES OF HEAT TRANSPORT:

Radiation: microscopic (photon) mode
works best in vacuum, well in low density gases

Conduction: microscopic (molecules bumping) mode
works best in solids, then liquids, then gases

Convection: macroscopic (large-scale) motions of blobs
of matter: works in liquids, gases and (slowly) plastics

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CORE: metallic; mostly Fe, Ni, (S/O) In toto, about 3500 km radius or 1/6 of Earth's volume.

OUTER CORE: liquid, about 2200 km thick;
rho = 11 g cm^{-3} ; T = 4000 K ; Temperature `wins'

INNER CORE: solid, about 1300 km radius;
rho_c = 13 g cm^{-3} ; T_c = 5200 K; Pressure `wins'

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DIFFERENTIATED via MELTING

Separation into metallic core and rocky mantle could be explained either if:

Core formed first, mantle and crust added later, OR

Entire Earth formed together but was molten early on;
the denser (metallic) liquid sinks to the center.
This hypothesis fits all the evidence better.

COLLISIONAL HEATING plus RADIOACTIVE HEATING can nicely
explain current temperatures.

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THE AGE OF THE EARTH IS ABOUT 4.55 Gyr

We know this through RADIOACTIVE DATING

Some isotopes are unstable and decay:

PARENT NUCLEUS ---> DAUGHTER NUCLEI + X

Here X = proton, neutron, electron, positron,
alpha particle, gamma-ray

After ONE HALF-LIFE, t_1/2, ONE HALF OF THE ORIGINAL AMOUNT IS LEFT;

After two half-lives, one quarter is left;

After three half-lives, one eighth is left; -- etc.

Examples:

187^Re --> 187^Os : t_1/2 = 6.0x10^10 yr

238^U ---> 206^Pb : t_1/2 = 4.51x10^9 yr

235^U ---> 207^Pb : t_1/2 = 7.10x10^8 yr

40^K ----> 40^Ar : t_1/2 = 1.3x10^9 yr

14^C ----> 14^N : t_1/2 = 5.5x10^3 yr

The first four are used in dating old rocks, the last for
dating organic remains and artefacts up to 40,000 yr old.

Rocky Mountains roughly 60 Myr old but Appalchians over 200 Myr
The latter are lower and rounder: more ERODED over longer time.

Oldest rocks from Greenland are about 3.9 Gyr old

But oldest meteorites 4.5 Gyr and oldest moon rock 4.4 Gyr,

so we argue that oldest earth rocks were melted and
recycled so the earth's age is hard to find directly.
It must be older than 3.9 Gyr and allow for cooling down.

Also, the age of the Sun can be very successfully modeled to be 4.6 Gyr
and the Earth must be only slightly less old since the extra gas needed to form planets
would be lost from the proto-solar system very quickly.

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SEISMIC WAVES REVEAL INTERIOR

SEISMOGRAPHS detect EARTHQUAKES, VOLCANOS,
and EXPLOSIONS at varied distances.

Long surface waves can travel fastest, but are not very
useful as they don't probe the interior.

P-waves, PRELIMINARY, (push-pull waves) are
COMPRESSIONAL, LONGITUDINAL waves.

Propagate through liquids as well as solids.

V_P = function of (compressibility; composition, T, P)

S-waves, SECONDARY, (side-slip) are SHEAR, TRANSVERSE waves.

CANNOT propagate through liquids (OUTER core).

V_S = (a different) function of (rigidity; composition, T, P)

Thus, while both S and P waves can be detected from earthquakes on the same side of the earth,

ONLY P waves are detected on the opposite side of the earth

and there is a "shadow zone" band where neither S (absorbed) or P (refracted) waves are detected.