Geoscience Reference
In-Depth Information
7
Recording features of
igneous rocks
David A. Rothery
7
The study of igneous rocks in the fi eld relies on many of the
skills already established in this topic. Modern volcanic
eruptions are well documented and some individuals have
been privileged to have experienced one, but most igneous
processes lie further from the realm of commonplace
experience. Nobody has actually seen a granite intruding into
its country rock, or witnessed a pyroxene crystal growing
within a magma, so these processes are impossible to observe
directly. Thus, it is crucial to make fi eld observations in the
light of general understanding of igneous processes, backed up
by laboratory studies.
The main reasons to study igneous rocks are to: (1) understand
volcanoes and the hazards they pose to life on Earth; (2)
understand processes beneath the Earth's surface that drive
plate tectonics and that contributed to the evolution of Earth
and other planetary bodies; (3) understand the distribution of
ore minerals; and (4) for chronology.
7.1 Equipment, basic tips and safety
If igneous rocks that you intend to study are in contact with
other rock types, you should carry the basic equipment for
general geological fi eldwork listed in Table 2.1 (p. 4). For
igneous rocks a geological hammer is usually important so that
you can obtain a fresh surface to examine the mineralogy.
Additions and refi nements for working on active volcanoes and
young lava surfaces are discussed in Section 7.4.
Depending on the nature of your fi eld project, you may already
know (or suspect) that the rock at a particular locality is
igneous. When visiting an
unknown
exposure, you fi rst need to
form at least a working hypothesis as to which parts of it are
likely to be igneous, as outlined in Section 3.2. Confi rmation,
or revision, will come from observations of fi eld relationships
(Section 7.2) and also of mineralogy and texture (Section 7.3).
It is good to explore fi eld relationships early, but it is usually
