You can start the activity after you have completed the reading assignment.
In this lab you will be applying the principles that you have been studying and determining properties for a selection of the igneous rock you will study from photos on the Internet.
In addition to texture and the intrusive/extrusive determination for each sample, you will also be asked to determine the mineral content of the sample to the best of your ability. For example, you can expect to find abundant olivine, and maybe a little pyroxene and Ca-rich plagioclase in an ultramafic rock called peridotite or komatiite. Pyroxene, plagioclase, and possibly some olivine or amphibole may be present in a mafic rock such as gabbro or basalt. You can also expect to see quartz, muscovite, potassium feldspar, and maybe a little biotite and Na-rich plagioclase in a felsic (or silicic) rock such as granite or rhyolite.
The classification of an igneous rock depends partly on the minerals that may be present in the rock, and since the minerals have certain colors due to their chemical makeup, then the rocks must have certain colors. For example, a rock composed of mostly olivine will be green in color due to olivine’s green color; such a rock would be called ultramafic. A rock that has a large amount of ferromagnesian minerals in it will be a dark-colored rock because the ferromagnesian minerals (other than olivine) tend to be dark colored; an igneous rock that is dark in color is called a mafic rock (“ma-” comes from magnesium, and “ c” from ferric iron). An igneous rock with a large amount of nonferromagnesian minerals will be light in color, such as the silicic or felsic rocks (“fel” from feldspar, and “sic” from silica-rich quartz). So, based on color alone, we’ve been able to start classifying the igneous rocks.
Instructions
A. Before you begin this activity, you may also wish to review the
Igneous Rock Identification
page. Some optional resources you can use are:
B. Select each photo below to view the igneous rock samples larger. You can also view the images in Flickr by clicking the link, and zoom in on each of these samples in the picture to get a better view of their texture:
Is the rock mafic, felsic, intermediate, or ultramafic (based on mineral content you identified above)?
Is the rock extrusive or intrusive (based on its texture you identified)?
For each sample, compare your observations to the sample(s) listed in the Rock and Mineral Guide. Are your findings consistent with the guide?
For one of the rock samples of your choice, write a half-page summary describing the physical characteristics you can determine from the photo or from any personal sample you have collected. If using a personal sample, please include a photograph. For all the other samples above, include a table summarizing the characteristics you observed (1 – 6). You will be graded on this activity as described in the rubric below.
Grading Rubric
10 points
: Report accurately summarized the igneous rock characteristics, spelling and grammar are correct and complete sentences are used, including a photograph. Characteristics of the other samples are included in a table and are accurate.
8 points
: Report mostly captured the igneous rock characteristics, spelling and grammar are mostly correct and complete sentences are used, including a photograph. Characteristics of the other samples are included, but one or two details were missing.
5 points
: Report missed two or more characteristics, included spelling and grammar errors, and/or did not include a photograph. Characteristics of the other samples are included, but over half of the minerals and/or characteristics are missing.
2 points
: Report was inaccurate, included significant spelling and grammar errors, and/ or did not include a photograph. Characteristics of the other samples are not included. 0 points: Did not complete the assignment.