Earth Structure and Physical Properties Questions and Answers

Answer: A

There is no change in chemical composition across the boundary between the upper mantle and lower mantle.

In the standard pyrolite model the upper and lower mantle have essentially the same bulk composition. The seismic discontinuity at about 660 km is a phase change: ringwoodite breaks down to bridgmanite (silicate perovskite) and ferropericlase, which are denser structures of the same elements.

The other boundaries are compositional. The Moho separates silica-rich crust from ultramafic peridotite; the core-mantle boundary separates silicate rock from iron-nickel metal. Option D does not name a recognised boundary within the Earth.

Answer: D

The measure that needs no instrumental record is intensity.

Intensity, on scales such as Modified Mercalli or MSK, is assigned from observed effects: how people felt the shaking, the damage to buildings and changes to the ground. It can be estimated from eyewitness accounts and is how historical earthquakes are sized.

Richter (local) magnitude is calculated from the maximum amplitude on a seismogram, and the seismic moment and the moment magnitude Mw are calculated from the spectrum or waveform of recorded seismograms. All three need instruments.

Answer: A

The keyed answer is that A is 10 times more intense than B.

The Richter magnitude is the logarithm to base 10 of the maximum trace amplitude, corrected for distance. A difference of one unit of magnitude therefore means a tenfold difference in recorded ground-motion amplitude, which is the sense in which the paper uses 'intense'.

The energy released grows faster, by a factor of about \(10^{1.5}\), roughly 32, per unit of magnitude, but no option gives that figure. 28 and 5 times correspond to no standard rule, and 1000 times is the energy ratio for two units of magnitude, not one.

Answer: B

The phase is PKIKP.

In seismic phase notation, P is a compressional leg in the mantle, K a leg through the outer core and I a P leg through the inner core. PKIKP therefore travels down through the crust and mantle, through the outer and inner core, and back up through the outer core and mantle to the surface: a complete traverse of the Earth's interior.

The other forms are not standard notation. 'M' and 'C' are not used for mantle and core legs, and subscripted labels like those in options C and D belong to crustal refraction phases such as Pn, not to core phases.

Answer: C

This is Kepler's law, specifically his second law.

Kepler's second law states that the line joining a planet to the Sun sweeps out equal areas in equal times. A planet therefore moves fastest near perihelion and slowest near aphelion. Physically it expresses conservation of angular momentum.

Newton's law of gravitation explains why Kepler's laws hold but is not this statement. Bode's (Titius-Bode) law is an empirical rule for the spacing of planetary distances, and the Doppler effect is the shift in frequency of waves from a moving source.

Answer: A

The average density is derived from measurement of acceleration due to gravity.

Surface gravity gives the Earth's mass once the gravitational constant is known:

\[ M = \frac{g R^{2}}{G} \approx 5.97 \times 10^{24}\ \text{kg} \]

Dividing by the volume of a sphere of radius R gives a mean density of about 5,515 kg per cubic metre. Cavendish's experiment, which measured G, was in effect the first 'weighing of the Earth'.

Surface rocks average only about 2,700-3,000 kg per cubic metre, so their density would give the wrong value; and planetary accretion and 'matter inside the earth' are not measurements at all.

Answer: A

The crust of Mars is out of hydrostatic equilibrium.

Mars carries enormous loads that are not isostatically compensated, above all the Tharsis rise with Olympus Mons and the other giant volcanoes, which stand many kilometres high. Its gravity field shows large positive anomalies over them and its shape departs strongly from that expected for a rotating fluid body, which shows that a thick, strong lithosphere supports these loads.

The same data also show that the crust is not of uniform thickness: it is thick under the southern highlands and thin under the northern lowlands, the so-called crustal dichotomy. That rules out the last two options as well as hydrostatic equilibrium.

Answer: A

Compression and rarefaction are associated with P-waves.

P (primary) waves are longitudinal: particles move back and forth along the direction of travel, so the medium is alternately squeezed together and pulled apart as the wave passes. They are the seismic equivalent of sound waves and can travel through solids, liquids and gases.

S-waves are transverse; particles move perpendicular to the direction of travel, so there is shear but no change of volume. Core waves are not a type of motion but waves that pass through the core, and 'K' is the notation for a P leg in the outer core, not a separate type of wave.

Answer: D

P-wave speed is highest in peridotite.

P-wave velocity depends on the elastic moduli divided by density, and dense, stiff ultramafic rock made of olivine and pyroxene has the highest moduli of the four. Peridotite, the rock of the upper mantle, carries P-waves at about 8 km/s, which is why velocity jumps to about 8 km/s at the Moho.

Basalt is slower, about 5.5 to 6.5 km/s. Shale, porous and weakly cemented, is slower again at roughly 2 to 4 km/s, and water carries P-waves at only about 1.5 km/s.

Answer: B

The dominant mechanism in a hot spring is Convection.

A hot spring is fed by groundwater that circulates deep into the crust, is heated by hot rock or a magma body, becomes less dense and rises back to the surface. Heat is carried by the moving water itself, which is convection (advection of heat by fluid flow).

Conduction through rock is far too slow to bring water to the surface hot, and radiation is negligible underground. 'Transmission' is not a mechanism of heat transfer.