Geoscience Reference
In-Depth Information
7
(a)
(b)
Figure 7.15
Dyke chilled margins. (a) The chilled margin runs from top to bottom of this view parallel to the
pen. Its location is indicated by the arrow drawn in the deliberately positioned fi eld notebook. This exposure is
weathered and fragmented, but there is a slight darkening on fresh surfaces from left to right towards the chilled
margin. To the right of the chill, the intruded rock is the interior of another dyke. (b) Chilled margin of dyke
(right) against gabbro (left). Note the inclusion of the compass in the photo, to show the strike (Oman Ophiolite,
Arabia). (a and b: David A. Rothery, The Open University, UK.)
fragmented and transported as part of the eruptive process.
When interpreting a volcaniclastic rock look for contextual
clues to try to establish whether it is truly pyroclastic. (A fall
deposit from a volcano counts as pyroclastic, unless it has been
reworked by wind or water.)
Pyroclastic rocks may contain textural evidence of their nature
and mode of emplacement. Fragments of spatter from around a
vent (hitting the ground as 'cow-pat' bombs) may have become
welded together in such a way that their integrity can no longer
be determined, but scoria that was more chilled by the time it
accumulated may preserve its original texture. Fragments of
lava encased in ash, or ash or lava or both found in association
with blocks up to tens of metres in size of other rock types
(either related igneous lithologies or country rock), have
traditionally often been regarded as 'vent agglomerate',
denoting a deposit that fi lled a volcanic vent or fi ssure
following explosion or collapse. If you fi nd such an occurrence
this is a good working hypothesis to start with, but you would
do well to consider other possibilities too. Look for evidence to
help you choose between alternative explanations such as a
lahar or a debris avalanche deposit, or a proximal fall deposit
extending beyond the confi nes of a vent. Mapping out the fi eld
