Geology Reference
In-Depth Information
4.2 A Compositional Divide for Lava Flows
Although there is a continuum in composition from ultra-mafic through mafic,
intermediate and felsic/silicic (acidic) compositions, two major types of lava
flows are recognised readily in the field; mafic/basaltic flows and felsic/silicic
flows. This reflects the nature of the predominance of one or other type
of flow in a particular volcanic setting (for example, Chapter 1), and the major
differences in viscosity from mafic/basaltic (low viscosity) to intermediate-
felsic/silicic (high viscosity). In this topic we will present the major lava flow
structures under these two main sub-sections, but one should bear in mind that
there is overlap in some instances.
When confronted with many lava flows in the field one can be somewhat
stuck to work out the composition and therefore classification. If the lava is fine
grained or a quenched glass with no crystals visible, a type of flow known as
aphyric, the colour index (as described in Chapter 3) is not of use to qualify the
composition (for example, Obsidian, a highly silicic glass is black in colour).
In such cases the morphological features of the deposit described below in this
chapter may help in determining an approximate 'type' for the lava. However, in
most cases lava flows erupt carrying a crystal cargos (within the magma) known
as phenocrysts, which usually represent the main phases crystallising from the
melt. These lava flows, termed porphyritic lavas (for example, Figure 4.3), are
very useful as the phenocryst population can be used to gauge the lava composi-
tion (for example, Table 4.1). In combination with features such as colour, field
relationships and mode of occurrence, Table 4.1 is a useful guide to naming
porphyritic rocks in the field. For example, olivine is most common in basalt
and basaltic andesite lavas; abundant feldspar (plagioclase) with hornblende is
characteristic of andesite; and quartz and feldspar (alkali feldspar) are diagnostic
of rhyolite.
Basic rocks composed of natural glass are termed
tachylite
and these form on
the skin of lava flows or along the marginal zones of basic sills or dykes. Vol-
canic glass of intermediate and acid composite is termed
obsidian or pitchstone
.
Obsidian is a black or dark-coloured glass, with a bright glassy lustre and well-
developed conchoidal fracture. Pitchstone has a dull,
resinous
or pitchy lustre,
rather than the glassy lustre of obsidian. (This reflects the high content of water
in pitchstone in comparison with obsidian.) Fragments of both types may occur
as pyroclastic materials, or form minor intrusions or lava flows. Glassy volcanic
rocks of intermediate and particularly acid composition commonly show flow
banding (cf. Figure 2.9) as a result of the drawing out of small inhomogeneities
during viscous flow.
Such natural glasses are unstable and start to hydrate and to crystallise or
devitrify
soon after formation. This process takes place progressively so that, in
general, there are hardly any unhydrated natural glasses older than about 20 Ma,
and, although glasses of Palaeozoic age are known, such glasses rarely exceed
