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 solidifieswith 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 as fiamme (right)and eutaxitic foliation (right) are both evidence of welding. Eutaxitic foliation can be seen throughout sample 36, with slight 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 axesof 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) / Y (mm) / Original Size (mm) / Change in X / Change in Y0.501 / 0.238 / 0.3695 / 35.58863329 / -35.58863329
0.815 / 0.194 / 0.5045 / 61.54608523 / -61.54608523
0.413 / 0.128 / 0.2705 / 52.68022181 / -52.68022181
0.388 / 0.235 / 0.3115 / 24.55858748 / -24.55858748
0.446 / 0.214 / 0.33 / 35.15151515 / -35.15151515
0.895 / 0.238 / 0.5665 / 57.98764342 / -57.98764342
0.692 / 0.242 / 0.467 / 48.17987152 / -48.17987152
0.447 / 0.096 / 0.2715 / 64.64088398 / -64.64088398
0.479 / 0.11 / 0.2945 / 62.64855688 / -62.64855688
0.476 / 0.154 / 0.315 / 51.11111111 / -51.11111111
0.401 / 0.098 / 0.2495 / 60.72144289 / -60.72144289
0.216 / 0.132 / 0.174 / 24.13793103 / -24.13793103
0.368 / 0.157 / 0.2625 / 40.19047619 / -40.19047619
0.496 / 0.119 / 0.3075 / 61.30081301 / -61.30081301
0.215 / 0.165 / 0.19 / 13.15789474 / -13.15789474
Average % Change: / 46.24011118 / -46.24011118