Petrographic Problem Assignment #1: Sample #36
Rock Sample number 36 is an ignimbrite, or ash flow tuff, formed from a pyroclastic flow
deposit. The rock was lithified away from the source of eruption and is partially welded. Careful
observations have provided evidence to support this hypothesis.
This rock is primarily composed of glass, otherwise known as ash. Ash forms when volatiles, in
the form of bubbles, rise out of the magma chamber. Magma adheres to the outside of the
bubbles and as pressure is released, the gas bubbles expand and eventually pop. Magma on the
outside of the bubbles solidifies with temperature and pressure reduction, and is thrown into
the air in the form of glass. The glass becomes mobilized due to high temperatures and a
density differential with the surrounding atmosphere. As energy continues to increase, due to
the release of pressure, the flow travels down the mountain face and away from the vent
source. As the flow travels, it picks of foreign lithics and mineral fragments along the Earth’s
surface.
Pyroclastic flow deposits have distinguishing features based on their locality within the deposit.
In a typical flow the largest, and most abundant deposit of lithics and phenocrysts, are found
closest to the vent. This occurs because as the flow moves, its flow front (head) gains the most
energy. The tail of the flow cannot support larger inclusions, because it is turbulent energy, so
they become deposited near the source. Farthest from the vent source, where the flow comes
to rest, lithic and crystal inclusions are typically small and more sparse. Based on this
information, it would appear that sample 36 is a pyroclastic flow deposit found away from the
volcanic vent.
Sample 36 is poorly sorted with an aphanatic-porphrytic texture. Approximately five percent of
this rock sample is composed of crystals including plagioclase, sanidine, and small amounts
orthopyroxene, hornblende, and quartz. Using a weighted average of crystal composition in the
sample and a QAP diagram, this rock is
compositionally similar to a quartz latite. Crystals
in the sample range in size from about 10 microns
to 0.5 mm. Pumices in the sample range from
coarse to fine grained ash (1/16-2mm). Small sizes
of the inclusions are evidence for deposition away
from the vent.
Lithics found in this sample came from the magma
chamber. Phenocryst composition in the sample
appears to mimic the composition of lithics found.
Small crystal inclusions are a result of a high
energy explosion that occurred as pressure was
being released from the magma chamber. This
high energy caused the existing crystals of the
system, at the time the pyroclastic flow began, to
fracture and break apart causing them to be very
small phenocrysts in the sample.
Ignimbrites, or ash flow tuffs, typically undergo
some degree of welding. Welding of ignimbrites
is a mechanism of temperature, composition,
and lithostatic load. Flattened pumices, known
Fiamme
as fiamme (right) and eutaxitic foliation (right)
are both evidence of welding. Eutaxitic foliation
can be seen throughout sample 36, with slight
Eutaxitic Foliation
orientation to the shards. Fiamme are also
present in this sample and are useful for
analyzing the degree of welding. When pumices are deposited they are round. After and/or
during emplacement, pumices are deformed due to heat and pressure acting upon them from
the flow deposit. By measuring the long(x) and short(y) axes of 15 different pumice fragments, I
was able to determine the degree of deformation each experienced. By taking an average of
the long and short axes of each pumice, the approximate size of the pumice before
deformation was quantified. A ratio of the original size to both the X and Y axes was used to
approximate the average change in shape for each pumice fragment. The data table below
shows the results and based on this information, sample 36 has been about 46% welded. This is
also evidence to support the hypothesis that my rock is in fact, an ignimbrite.
X (mm)
0.501
0.815
0.413
0.388
0.446
0.895
0.692
0.447
0.479
0.476
0.401
0.216
0.368
0.496
0.215
Original Size
Y (mm) (mm)
0.238
0.194
0.128
0.235
0.214
0.238
0.242
0.096
0.11
0.154
0.098
0.132
0.157
0.119
0.165
0.3695
0.5045
0.2705
0.3115
0.33
0.5665
0.467
0.2715
0.2945
0.315
0.2495
0.174
0.2625
0.3075
0.19
Average %
Change:
http://0.tqn.com/d/geology/1/0/N/N/1/600QAPvolcanic.jpg
Change in X
Change in Y
35.58863329
61.54608523
52.68022181
24.55858748
35.15151515
57.98764342
48.17987152
64.64088398
62.64855688
51.11111111
60.72144289
24.13793103
40.19047619
61.30081301
13.15789474
-35.58863329
-61.54608523
-52.68022181
-24.55858748
-35.15151515
-57.98764342
-48.17987152
-64.64088398
-62.64855688
-51.11111111
-60.72144289
-24.13793103
-40.19047619
-61.30081301
-13.15789474
46.24011118
-46.24011118