GL23C IGNEOUS & METAMORPHIC PETROLOGY LAB 3 |
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STUDENT NAME ____________________________________________________________ You have been provided with a number of volcanic rock hand specimens by your instructor. You are to give a detailed description of each hand specimen, by describing its colour, texture, mineralogy and any other significant features. Attempt to classify each rock specimen using the IUGS Field Classification nomenclature. In describing the colour of volcanic rocks it is advisable to base the colour index on the groundmass, assuming it is crystalline and not glassy. If the groundmass is glassy then you will have to follow the nomenclature recommended by the IUGS for glass-bearing and glassy rocks. Textural descriptions should state whether the rock is porphyritic-aphanitic, aphanitic etc. The mineralogy should be divided into those minerals that occur as phenocrysts (assuming the rock is porphyritic) and those (if identifiable) that occur as groundmass. You should try to estimate the % phenocrysts to % groundmass, once you have a rock that is porphyritic. The shape, size and colour of the individual minerals should form part of your decription. You should also observe the overall homogeneity of the rock, i.e. whether it is uniform, banded, contains inclusions, is amygduloidal, vesicular etc. Finally, and as best you can, estimate the amount of felsic (light coloured) minerals in order to apply the IUGS classification. Pyroclastic rocks When describing pyroclastic rocks estimate the % of fragments (glass, pumice,lithic,crystal etc.) to the % matrix/groundmass in the rock, and the mean diameter of the fragments, as well as the composition and colour. If you are describing an individual fragment eg. a bomb, you should give its size, shape, mineral composition, vesicularity, colour index, etc. N.B. You are advised to read CHAPTERS 4 & 6-THE FIELD DESCRIPTION OF IGNEOUS ROCKS by THORPE & BROWN. List of Volcanic Rocks to be Examined in Lab 3
Use of TAS Classificatioin You are to apply the TAS classification to the following analyses of volcanic rocks which were classified prior to the new IUGS classification. Determine the names for the rocks using the TAS (and any other) classification, and note whether or not their nomenclature remains the same as before: Indicate the new name of the rock if it differs to the previous name. From the normative mineralogy establish which rocks are silica oversaturated, silica saturated, and silica undersaturated.
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