Composition of the Earth Questions and Answers

Answer: A

The second most abundant element in the bulk Earth is oxygen.

By mass the whole Earth is roughly 32 per cent iron, 30 per cent oxygen, 15 per cent silicon and 14 per cent magnesium. Iron comes first because the core is mostly iron, while oxygen, the most abundant element of the crust and mantle, comes second.

Iron is the most abundant, not the second, and silicon and magnesium come after oxygen. (In the crust alone the order is different: oxygen first and silicon second.)

Answer: A

The region near Neptune and beyond is the Kuiper belt.

The Kuiper belt is a disc of icy bodies, remnants of planet formation, extending from about Neptune's orbit (30 astronomical units) to about 50 astronomical units from the Sun. Pluto, Eris and many other dwarf planets and short-period comets belong to it or to its scattered disc.

The Oort cloud (not 'Oort nebula') is a far more distant spherical shell of comets, thousands of astronomical units away. There is no 'Kepler's belt', and 'planetary ocean' is not a region of the solar system.

Answer: C

Corundum has the highest condensation temperature of the four.

In equilibrium condensation calculations for a gas of solar composition, the first solids to appear are the refractory calcium-aluminium oxides and silicates. Corundum (Al₂O₃) condenses first, at roughly 1,700 K at the pressures assumed for the solar nebula, which is why such minerals are found in the calcium-aluminium-rich inclusions of chondrites.

Anorthite and the magnesium silicates such as forsterite and enstatite condense later, at lower temperatures of a few hundred kelvin less. Magnetite forms only at low temperature, by oxidation of metallic iron, near the end of the sequence.

Answer: A

The oxides are Al₂O₃, CaO and Na₂O.

The mantle is dominated by MgO, SiO₂ and FeO, which make olivine and pyroxene. The few percent of alumina, lime and soda decide which aluminous phase is present: plagioclase at shallow depth, spinel deeper, and garnet below about 80 km, and they also control the amount of clinopyroxene. They are minor constituents but they set the mineralogy of mantle peridotite.

K₂O and P₂O₅ are trace components; Pb₃O₄ (red lead) is not a mantle oxide; and option D lists formulas such as Al(OH) and Na₂O₃ that are not the mantle's oxides.

Answer: C

The most abundant element in the Earth's crust is oxygen.

Oxygen makes up about 46% of the crust by weight, almost all bound in silicate and oxide minerals. Silicon is second at about 28%, aluminium third at about 8%, and iron fourth at about 5%.

Aluminium is the most abundant metal, and iron is the most abundant element of the Earth as a whole, because of the core. Both are traps when the question asks only about the crust.

Answer: B

The sequence is Big Bang nucleosynthesis, then fusion in stars, then fusion in supernova explosions.

In the first few minutes after the Big Bang, nucleosynthesis produced only the lightest nuclei: hydrogen, helium and traces of lithium. Much later, fusion inside stars built carbon, oxygen and heavier elements up to iron. Elements beyond iron need extra energy and neutron capture, which happens mainly in supernova explosions (and neutron-star mergers).

The sequence starting with fusion in stars puts stars before the Big Bang. 'Big Bang fission' and 'galactic collapse' are not nucleosynthesis processes, and 'electroweak formation' refers to an early-universe phase transition, not to making elements, and it cannot come after star formation.