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Ignimbrite

Ignimbrite. Rock #36 Riccilee Keller. Hand Sample. Gray, no noticeable discoloration Partially Welded Poorly Sorted Flattened pumices visible, Fiamme Small lithics visible Some phenocrysts are visible with a hand lens. In Thin Section. Minerals Present: > 1% Quartz

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Ignimbrite

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  1. Ignimbrite Rock #36 Riccilee Keller

  2. Hand Sample • Gray, no noticeable discoloration • Partially Welded • Poorly Sorted • Flattened pumices visible, Fiamme • Small lithics visible • Some phenocrysts are visible with a hand lens

  3. In Thin Section • Minerals Present: • > 1% Quartz • 2% Plagioclase • 1% Sanidine • > 1% Hornblende • 1% 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 oxides • Some re-crystallization of pumices has occurred

  4. 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

  5. Clay minerals rim the xenolith • Eutaxitic Foliation can also be seen here • Phenocrysts appear to be dominantly Orthopyroxene and Feldspar • Opaque oxides and Hematite also present in the sample

  6. Textural Relationships • Aphanatic, Porphrytic • Compositionally similar to a “Quartz Latite” • Due to high temperatures during emplacement and flow movement, the glass looks fluidal in thin section • Eutaxitic Foliation • Fiamme • Phenocryst compositions appear to match those found in some xenoliths

  7. Hypothesis • This 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.

  8. Supporting Data 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.

  9. Conclusions • The 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 Ignimbrite • Long and short axes of 15 different pumices measured to quantify percent of deformation 0r welding • Average deformation: ~46%, meaning about 46% welded

  10. References • http://www.uwgb.edu/dutchs/EarthSC202Notes/igneous.htm • Dr. Templeton, Volcanology Lecture, Spring 2011 • http://inapcache.boston.com/universal/site_graphics/blogs/bigpicture/msh30_05_18/m22_mlip0113.jpg

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