Recording features of igneous rocks - Geological Field Techniques - page 161

• Examine lava fl ow morphology. Identify the individual lava
fl ows making up a lava fi eld. If the lava is basalt, look for
pahoehoe (which comes in many varieties; Figures 7.23 and
7.24) and ' a ' a (Figure 7.12a). See if you can fi nd a pahoehoe
fl ow that turns into ' a ' a where it passed onto a steeper
slope so that the fl ow rate was forced to increase. In a
pahoehoe fl ow fi eld, look for tumuli (mounds), where lava
was squeezed up from below. Lines of tumuli can indicate
the track of a lava tube. If the tube has emptied, you are
absolutely sure it is safe and you can fi nd a way in, it may
be worth exploring to look for 'tide marks' on the wall left
by the waning fl ow. Study a fl ow front; where would it
have gone next if the supply of lava had not run out?

7

• Can you fi nd any volcanic bombs? Were they completely
fl uid, or crusty, or fully solidifi ed when they hit the
ground? Do they represent magma, or wall-rock ripped out
of the vent?

• Has there been a recent pyroclastic fl ow? This may be a
good chance to see the damage that one can do. In the
proximal region the deposit may be vanishingly thin (and
so virtually unrecognizable in ancient examples), but it may
have scoured away soil and previous ash deposits.

• Is there fresh fall-out ash still in situ? Is it on steep slopes,
prone to being washed away during heavy rain to generate
a lahar? Which areas are at risk?

• Are any vents accessible? Features such as scoria cones,
tuff rings, maars and tuff cones that can be hard to interpret
when found in cross-section in ancient rocks are easier to
make sense of in a modern setting.

7.5 Further reading

Francis, P. W. and Oppenheimer, C. 2003. Volcanoes , Oxford
University Press, 536 pp. [A nice account of volcanism in general.]
Gill, R. 2010. Igneous Rocks and Processes , Wiley-Blackwell, 480 pp.
[A lengthy review aiming to give students the skills and confi dence
to identify igneous minerals and interpret igneous rocks.]
Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B.
and Bateman, P. (eds) 2008. Igneous Rocks, a Classifi cation and
Glossary of Terms: Recommendations of the International Union of
Geological Sciences Subcommission on the Systematics of Igneous
Rocks , Cambridge University Press, 256 pp. [Formal, detailed
account of the systematic classifi cation of igneous rock types.]
McPhie, J., Doyle, M. and Allen, R. 1993. Volcanic Textures , Centre of
Ore Deposit and Exploration Studies (CODES), University of
Tasmania, 198 pp. [Excellent photographic atlas of volcanic rock
textures. Reprinted in 2005.]
Rothery, D. A. 2010. Volcanoes, Earthquakes and Tsunamis , Hodder
Headline, Teach Yourself Books, 288 pp. [A basic introduction to