Geology Reference
In-Depth Information
3
IGNEOUS TEXTURES AND CLASSIFICATION
When describing an outcrop or sequence of rocks that are in part or wholly made
up of igneous rocks it will be necessary to make a detailed examination of the
rock-type(s) with a view to visiting other outcrops, mapping the shape and form
of the igneous rocks and considering their mode of occurrence (for example,
intrusive, extrusive, pyroclastic, and so on). It will also be an important step to
start to classify the types of rock present in terms of their mineral contents and
approximate position on accepted igneous classification schemes. In this chapter
we introduce the basic methods used to describe and classify igneous rocks in
field outcrop/hand specimen.
NOTE: This is directed at identifying the primary igneous components and
is tailored specifically to textural and mineral characteristics of the rocks they
formed from, that is magmas of specific composition, so will be of particular use
to intrusive and volcanic rocks containing visible crystals. This would equate
to the primary magmas (juvenile component) of pyroclastic rocks, however
the textural relationships such as grain size, sorting and other key features of
pyroclastic/volcaniclastic rocks will be dealt with in Chapter 5.
3.1 Describing Rock Types
To what extent can igneous rocks be named/classified in the field? The business
of assigning field names to fresh samples may be surprisingly straightforward,
because igneous rocks vary widely in their physical characteristics. Even when
making a detailed study of a small region in which, for example successive lava
flows may appear remarkably similar at first sight, it is possible with practice
and by adopting a systematic approach to develop a keen eye for small changes
in the appearance of your samples. The main features of igneous rock samples
that should be noted in the field as an aid to identification are:
1.
Colour
, as a preliminary guide to the approximate chemical and mineral
composition of the sample.
2.
Texture, grain size and fabric
which are related to the rate and sequence
of crystal growth in intrusive and extrusive lava flows, and to eruption and
emplacement in pyroclastic/volcaniclastic rocks.
