Geology 222b - Basalts Basalt is the most common volcanic rock. It is the rock formed at mid-ocean ridges and, therefore, beneath a thin layer of sediment, is found nearly everywhere at the bottom of the ocean. Similar basalts erupt in rift zones on continents, such as in the Connecticut Valley. Basalt can be found on continents covering large areas as “flood basalts”, such as those in eastern Washington state (Columbia River Flood Basalts) and in India (Deccan Traps). It is the main rock of many ocean islands, such as Hawaii. Basalt is one of the rock types extruded by volcanoes above subduction zones, like the Cascade volcanoes in the western United States. This assignment is to look at some examples of basalt, to become familiar with their mineralogy and textures, and to think about the processes responsible for their variety and their similarity. Rocks S-35 and S-36 were collected near Spokane, Washington. Please describe fully one or

the other (mineralogy, mode, texture, name). Include at least one labeled photomicrograph (with scale) as part of your description. You can submit the lab report through Moodle to include color without paying for printing.

Short answers with justifications please. 1. The three rocks ML-1, ML-10, and ML-17 all were collected on the isle of Mull off the

coast of Scotland. We know they are geographically associated. Are they chemically similar? Please explain the different appearance of the three rocks?

2. Rocks HI99-2A and HI99-4A (no thin sections) are from two different volcanoes on Hawaii

(Kohala and Mauna Loa). Based on what we have learned about phase diagrams, give two possible explanations for the differences among these rocks.

3. Rock HI-05(3) (no thin section) from Hawaii contains more than basalt. Describe what you

see and give a plausible explanation for it. 4. Rock ED99-J has very interesting olivine phenocrysts. Look carefully at their interference

colors. In many cases, you can see (look at the edges) systematic variation in the retardation of a single grain. Use information about the birefringence of olivine and about the crystallization of olivine to explain what you see.

5. Rock TR-J-2 should be familiar. It is a sample from the Holyoke Range. Like many

volcanic rocks you may collect, it is somewhat messy because of hydrothermal alteration. Look for epidote, chlorite, and fine-grained mica (sericite). Some of the clinopyoxene in this rock is pigeonite. What is pigeonite? How are its optical properties different from the typical clinopyoxene (augite) in basalt?