Processes that Produce the Surfaces of Planetary Bodies

There are many processes that have produced the surfaces of planetary bodies. Here we shall concentrate on those that are widespread and of particular importance. Most of the examples will be for rocky materials, though most of the processes act on icy materials too. From the effect of these processes we can learn much about the surface that has been exposed to them. This, in turn, can tell us much about the interior.

First, we shall see how surfaces are emplaced. Then we shall see how they are subsequently modified.

6.2.1 Differentiation, Melting, Fractional Crystallisation, and Partial Melting

Differentiation of a planetary body was discussed in Section 4.5.1. Here we concentrate on the formation of the crust from the upper mantle. The composition of the crust depends on the composition of the materials from which the planetary body formed.

Consider first those planetary bodies dominated by silicates plus iron or iron-rich compounds. Either the body will have an iron-rich core from the start, or one will start to grow by differentiation as the interior temperatures rise. In either case there will be a silicate mantle more or less rich in iron. In Section 5.1.1 you saw that in the case of the Earth the mantle is predominantly peridotite, which consists of the minerals olivine, (Mg, Fe)2SiO4, and pyroxene, (Ca, Fe, Mg)2Si2O6, where the pyroxene metals can less commonly be Na, Al, or Ti. Table 6.1 lists these minerals, along with some others we shall shortly encounter.

Melting and fractional crystallisation

Consider a peridotite mantle in the late stage of accretion of a planetary body. It is possible that during this stage the uppermost mantle becomes completely molten. A magma ocean is then said to have formed, magma being wholly molten material (rocky here, but it could be icy). The ocean will consist of a mixture of substances and, as long as no crystals form, the

Table 6.1 Important igneous rocks and minerals, with their locations in the Earth and Moon as examples

Rock

Mineral content (major

Where found in the Earth and

0 0

Post a comment