Stop+6+STU+Granite

**St Thomas University Sign**
The red background with white lettered sign at the entrance to the St. Thomas University. Welcome to St Thomas University – a step above UNB!

The red background to the STU entrance sign is an igneous rock, as is the rock seen at Stop 1 (the boulder outside the geology building at UNB), though the grain size is larger and one of the principle minerals here is the pink-red mineral ‘orthoclase’, compared with the white ‘plagioclase’ seen dominating the glacial boulder. The abundant clear, glassy mineral here is quartz. It is the grain size and the relative proportions of these minerals that allow geologists to discern one rock type from another and assign them names.


 * EXERCISE**: Using the Tables below, your observations of the crystal size and the facts presented above, what name would you assign to the rock seen in the sign?

//**(NB. Coarse grained is assigned to rocks with crystals > 3mm diameter, Medium grained to crystal sizes of 1-3 mm, and < 1mm grain sizes are considered fine grained.)**//
 * ORTHOCLASE RICH ROCKS (GENERALLY PINK) - If the rock is pink, use this table. **
 * || ** No quartz ** || ** Minor quartz ** || ** Abundant quartz ** ||
 * ** Coarse grained ** || Alkali gabbro || Syenite || Granite ||
 * ** Medium grained ** || Alkali microgabbro || Microsyenite || Microgranite ||
 * ** Fine grained ** || Alkali basalt || Trachyte || Rhyolite ||
 * PLAGIOCLASE RICH ROCKS (GENERALLY WHITISH-GREY) - If the rock is grey-whitish, use this table. **
 * || ** No quartz ** || ** Minor quartz ** || ** Abundant quartz ** ||
 * ** Coarse grained ** || Gabbro  || Diorite  || Granodiorite  ||
 * ** Medium grained ** || Diabase || Microdiorite  || Microgranodiorite  ||
 * ** Fine grained ** || Basalt || Andesite  || Dacite  ||