[Page no. 3.3.3]

[Title: Formation of the Earth's Core]

[Image: Caption: perhaps a section cut showing the internal constitution of the earth?]

[Text:

Once the proto-Earth had aggregated, internal heat from radioactive decay, combined with gravitational energy and collisional energy from planetesimal bombardment kept the planet molten. As the proto-Earth cooled, reduction reactions within the convecting system resulted in production of a metal-rich core and silicate-rich crust-mantle structure.

The original materials from which the Earth accumulated were iron, magnesium silicates (or iron oxide mixed with silicon and magnesium) plus carbon and sulphur. These last elements acted as reducing agents.

The core melts

The Earth heated up; the heat source was partly gravitational heating, a result of the size of the planet; partly impact heating through collisions; and partly, indeed mostly, from heat generated by the decay of radioactive isotopes. In this natural smelting process, the iron silicates were reduced to iron metal, which sank to the centre of the Earth forming a metal core.

We cannot reach the core of the Earth directly; study of iron meteorites helps us to understand planetary formation and differentiation processes and sets boundaries as to when and how the Earth's core formed.

The timescale over which core formation occurred can be deduced using several radiometric decay schemes, one of the most telling of which is the newly-established ¹⁸²Hf-¹⁸²W (Hafnium-Wolfram) chronometer. The strongly lithophile ¹⁸²Hf is partitioned into silicates, relative to the more siderophile W during differentiation, and subsequent variations in Hf/W are caused by decay of ¹⁸²Hf to ¹⁸²W (T_1/2 ~ 9 Myr).

Models based on the ¹⁸²Hf-¹⁸²W chronometer indicate that formation of the Earth's core took place gradually, some 50 Myr or so after the differentiation of iron meteorite parent-bodies.