Rock #36 Riccilee Keller. Hand Sample Gray, no noticeable discoloration Partially Welded Poorly Sorted Flattened pumices visible, Fiamme Small lithics.

IgnimbriteRock #36

Riccilee Keller

Hand SampleGray, no noticeable discolorationPartially WeldedPoorly SortedFlattened pumices visible, Fiamme Small lithics visibleSome phenocrysts are visible with a hand

lens

In Thin SectionMinerals Present:

> 1% Quartz 2% Plagioclase1% Sanidine> 1% Hornblende1% Orthopyroxene (including lithic contribution)

Orthopyroxene is Biaxial Positive ( + ) which infers Mg and Fe rich phenocrysts

Phenocrysts in the sample make up about 5% of the rock

Vesicals filled with clay minerals and some oxidesSome re-crystallization of pumices has occurred

•Discoloration of glass shard infers some welding has occurred.•Eutaxitic Foliation is defined by flattened glass shards. The squashed shards appear somewhat linear and have a tricuspate pattern.

•Fiamme are evidence of welding in the sample caused by heat and pressure. Easily seen with the naked eye. Dark portion of the pumice is caused by partial recrystallization

•Phenocrysts appear to be dominantly Orthopyroxene and Feldspar •Opaque oxides and Hematite also present in the sample

•Clay minerals rim the xenolith

•Eutaxitic Foliation can also be seen here

Textural RelationshipsAphanatic, PorphryticCompositionally similar

to a “Quartz Latite”Due to high temperatures

during emplacement and flow movement, the glass looks fluidal in thin section

Eutaxitic FoliationFiammePhenocryst compositions

appear to match those found in some xenoliths

HypothesisThis rock is an ignimbrite. It was

formed from the deposition of a pyroclastic flow. Pyroclastic flows occur when volcano’s release pressure, in the form of volatiles. These volatiles become mobile, mix with ash, pumice, and lithics, and flow much like an avalanche away from the vent. Heat and pressure cause deformation of pumices and glass during and after deposition. Degree of welding can be determined by examining changes in pumice shapes. Pumices are originally round but become elongate when exposed to high temperatures and pressures. This sample is moderately welded and was deposited away from the vent source.

Supporting DataX (mm) Y (mm)

Original Size (mm) Change in X Change in Y

0.501 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

This chart shows the average amount of deformation of 15 different pumice fragments in the sample. Percent average change describes the degree of welding and/or deformation that occurred during emplacement and formation of the ignimbrite.

ConclusionsThe sample, formed from a pyroclastic flow, does

not contain large amounts of lithics or phenocrysts. This implies the sample was deposited and lithified away from the vent source

Some glass has been devitrified, discoloration of glass is evidence

Moderately welded ash flow tuff, or IgnimbriteLong and short axes of 15 different pumices

measured to quantify percent of deformation 0r welding Average deformation: ~46%, meaning about 46%

welded

Referenceshttp://www.uwgb.edu/dutchs/EarthSC202Not

es/igneous.htmDr. Templeton, Volcanology Lecture, Spring

2011http://inapcache.boston.com/universal/

site_graphics/blogs/bigpicture/msh30_05_18/m22_mlip0113.jpg