Lab 8. Igneous rocks Today s exercise is an introduction to rock identification and the crustal structure of the seafloor. All rocks are composed of two or more minerals, and can be classified based on their mineralogy. For these rock samples, common minerals include: Olivine characteristically yellowish-green, translucent crystals with a vitreous luster. Pyroxene in some hand samples these are large dark crystals that refract light at ~90 because of the pyroxene groups distinctive cleavage. Plagioclase light colored, some have rectangular fracture edges that look like broken glass. Pyrite hydrothermal mineral, typically a very light yellow and either cubic or roughly dodecahedral. Chalcopyrite hydrothermal mineral, darker yellow than pyrite and rarely forms good crystals. We will be looking at igneous rocks, which are formed as magma cools. Intrusive rocks are formed beneath the surface in the magma chamber, while extrusive rocks form on the surface of a continent or the seafloor. Some of the samples are from collections undergoing active research and have significant scientific value. For this reason the samples will only be available during class and by arrangement with the TA. Be very careful with the samples - a couple are very fragile. There are 9 stations - you can look at the rocks in any order. Station 1. At the other stations you will see some rocks that are fine-grained and some that are coarse grained - Why is this? At this station you will see rocks with large crystals in a finegrained matrix. Theorize how this rock formed. 1
Station 2. At this station you will see a fine grained rock that includes vesicles (gas bubbles) that are exposed on the cut surfaces. How do vesicles form? These rocks are fragile. Be careful. Station 3. At this station you will see a rock with a glassy crusts. How would this form? 2
Station 4. At this station you will see a rock with veins (planar fractures) filled with quartz (Silica dioxide). How does a vein form? Is there anything else unusual about this rock? Stations 5-8. You will examine four different rocks (pick one from each group when there is more than one sample). Sketch the rock, identify crystal size, mineralogy, and defining characteristics (like vesicles, glassy crusts, and veins). Assuming that the rock is part of the oman ophiolite, where abouts did it come from? The rock at station 6 is extremely fragile Be very careful. A worksheet is on the following pages 3
Station 9. Here are two examples of hydrothermal chimneys. These form on the MOR as water circulates deep within the crust and then returns to the surface in very hot focused upflow zones called black smokers. Sketch the smaller sample, identify the dominant mineral present, and clearly label on your sketch the interior and exterior of the chimney. Sketch the larger sample from a basal view, clearly marking any mineral zonation present. Although this sample is heavily altered, what do you think the original mineralogy was? (Hint, Fe weathers to red, Cu weathers to green/blue). 4
Visible crystals? If so sketch texture and estimate crystal size. Station 5 Station 6 Can you identify any minerals? Which ones? How? Is this rock extrusive or intrusive? Why? Any other defining characteristics? Assuming this is part of an Omanophiolite type suite. Which section is it likely to come from? 5
Visible crystals? If so sketch texture and estimate crystal size. Station 7 Station 8 Can you identify any minerals? Which ones? How? Is this rock extrusive or intrusive? Why? Any other defining characteristics? Assuming this is part of an Omanophiolite type suite. Which section is it likely to come from? 6
Depth Below Seafloor 0 km 0.5 km 2 km 6 km 7
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