Geoscience Reference
In-Depth Information
worth making a preliminary assessment of mineralogy and
texture (to confi rm that you are indeed looking at what you
think you are!) before studying the fi eld relationships in
detail.
7.2 Field relationships of igneous rocks
7
This section is concerned with studying igneous rocks exposed
by natural erosion or quarrying. Active volcanoes and young
lava fl ows that retain exposed original surface morphology (up
to tens of thousands of years in arid regions) offer different
opportunities and are discussed in Section 7.4.
7.2.1 Relationships with surrounding rocks
The most fundamental attribute of a body of igneous rock, and
one that you should try to ascertain as soon as you can, is
whether it is intrusive (e.g. Figure 7.1) or extrusive. You may
wish to classify an intrusion according to the scheme given in
Appendix A7, Table A7.1, but more important than this is to
try to deduce how it was emplaced, based on as wide a variety
of evidence as you can assemble.
Figure 7.1
A dyke of Cenozoic
age cutting discordantly across
complexly folded wackes (Augrim
quarry, Co. Down, Northern
Ireland). The dyke is the 2-m-wide
reddish feature, passing nearly
vertically up the rock face behind
the person in the foreground.
(Richard Warner, British Geological
Survey and Donald Fay, Open
University Geological Society, UK.)
