ELECTRONS IN ATOMS - Chemical Fundamentals of Geology and Environmental Geoscience - page 82

5

ELECTRONS IN ATOMS

Why does a geologist need
to understand atoms?

of basalt flows, such as the Hawaiian islands, are
therefore characterized by very gentle slopes. Dacite or
rhyolite lava, on the other hand, has a much higher
viscosity and tends to form a bulbous, steep-sided lava
dome directly above the vent. Lava viscosity is
determined by the bonding structure linking individual
atoms together, and as we shall see in Chapter 7 this
varies according to the dominant types of atom present.
For example, the atomic characteristics of silicon (Si)
are such that melts rich in SiO 2 (dacite and rhyolite,
65-75% SiO 2 ) are much more viscous than silica-poor
lavas (basalt, 45-52% SiO 2 ). Atomic interactions on the
subnanometre scale thus exert a direct influence on the
shape of geological structures some 10 13 times larger.
The properties that geological materials inherit
from their atomic constitution also have important
economic applications. One geophysical approach
to mineral exploration is to measure the electrical
conductivity of the ground at a series of points in a

We now turn to examine the behaviour of matter on
the much smaller scale of individual atoms and
molecules, which are about 10 -10 m in size (Box 5.1).
The sub-microscopic atomic world seems at first sight
to have little bearing on everyday geology, which
people usually associate with events on a much
grander scale, such as earthquakes and volcanic
eruptions. However, as the next few chapters show,
many important properties of geological materials can
be related to the types of atoms of which they are made
and the chemical bonds that hold these atoms together.
The geometry of a lava flow, for instance, depends
upon the viscosity of the lava: basaltic lava of low
viscosity tends to spread out (if erupted on land) to
form tabular flows which may flow a long distance
and become quite thin; volcanoes constructed chiefly