19 Assignment: Igneous Rock Lab

Igneous Rock Lab

An illustration of a volcano and the magma chamber below. Text says, Extrusive ignous rocks cool quickly and as a result these rocks are fine grained or has a lack of crystal growth. Intrusive igneous rocks are formed from magma that cools slowly and as a result these rocks are course grained.
Figure 1.


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.


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:

Select an image to view larger


Glassy textured black rock
Figure 2. Obsidian is an easily recognizable igneous rock. It is a glassy-textured, extrusive igneous rock. Obsidian is a natural glass – it lacks crystals, and therefore lacks minerals. Obsidian is typically black in color, but most obsidians have a felsic chemistry. Felsic igneous rocks are light-colored, so a felsic obsidian seems a paradox. Mafic obsidians are scarce, but they have the same appearance. Obsidian is an uncommon rock, but can be examined at several famous localities in America, such as Obsidian Cliff in the Yellowstone Caldera (northwestern Wyoming, USA) and Big Obsidian Flow in the Newberry Volcano Caldera (central Oregon, USA). Obsidian is moderately hard, has conchoidal fracture (smooth & curved fracture surface), and has exceedingly sharp edges. Freshly-broken obsidian has the sharpest edges of any material known, natural or man-made (as seen under scanning electron microscope).Obsidian forms two ways: 1) very rapid cooling of lava, which prevents the formation of crystals; 2) cooling of high-viscosity lava, which prevents easy movement of atoms to form crystals. An example of obsidian that formed the first way is along the margins of basaltic lava flows at Kilaeua Volcano (Hawaii Hotspot, central Pacific Ocean). This is an example of obsidian that formed the second way . – James St. John


speckled rock with colors of pink, black, and white
Figure 3. Granite from the Precambrian of Wyoming, USA. (8.9 cm across at its widest) This is a sample of Precambrian basement rock from the Rattlesnake Mountain Uplift (a.k.a. Rattlesnake Mountain Anticline) in northwestern Wyoming. The uplift is part of a extensive set of mountain ranges from New Mexico to western Canada that were formed during the Laramide Orogeny (late Mesozoic to mid-Cenozoic). Locality: float from roadcut on the northern side of Rt. 16 / 20, just west of third tunnel west of Cody, next to Buffalo Bill Reservoir, central Park County, northwestern Wyoming, USA – James St. John


black matte rock with holes and some areas of white and black
Figure 4. Basalt from the upper Holocene of Hawaii (~6.5 cm across at its widest). This attractive piece of lava is from the 13 January to 19 February 1960 Kapoho Eruption at the easternmost point of Hawaii. The former town of Kapoho was located in the Kapoho Graben (Puna Rift Zone) segment of Mt. Kilauea’s East Rift Zone. Kapoho was famously burned, buried, and destroyed by aa lava flows erupting continuously for a little over a month in early 1960, despite the hasty construction of barriers. The lava flows overtopped the barriers and even “burrowed under” the barriers. The rock is a vesicular porphyritic olivine basalt (more technically, it can be called vesicular tholeiitic picrite basalt). It has abundant subrounded to stretched vesicles and common large greenish forsterite olivine phenocrysts. The basalt matrix is lustrous and cryptocrystalline, with very small crystals of plagioclase feldspar and pyroxene. The sample has an obvious black-and-white speckled xenolith of gabbro from the lower oceanic crust. – James St. John
a white and gray rock with a frothy appearance
Figure 5. Pumice is a distinctively frothy-textured, felsic, extrusive igneous rock. Frothy textured igneous rocks have large numbers of tiny holes formed by the presence of many gas bubbles in the original volcanic lava. If cooling & solidification was completed before all the gas bubbles escaped to the lava surface, the result is a rock with lots of empty space (porosity).Pumice is actually a volcanic glass having a frothy texture. The solid portions of pumice are not crystalline. So, pumice has no minerals – it has the elemental chemistry of granite & rhyolite, but it doesn’t have any minerals.Pumice is typically whitish, light grayish, or very light brown in color. Most pumice samples are so lightweight that they float on water. After the August 1883 eruption of Krakatoa in Indonesia, sailors observed pumice floating in the Indian Ocean for many months.Pumice is typically encountered in the form of small chunks littering the land surface around eruptive centers. They typically fall to Earth and form air fall pumice deposits. James St. John
a triangle shaped white/gray rock
Figure 6. Rhyolite is a felsic, extrusive igneous rock. It has the same chemistry & mineralogy as granite, but is very finely crystalline (aphanitic texture; crystals <1 mm in size) due to cooling of high-viscosity lava. Rhyolites are often light gray to pinkish to somewhat reddish in color. Rhyolite lavas are highly viscous (thick & slow-moving) when erupted and often appear as if they have flowed like oozing toothpaste. Sometimes the term felsite is applied to rhyolites. There does not seem to be a consistent definition of felsite, cf. rhyolite. However, felsite appears to be the broader term, applicable to all light-colored volcanic rocks. – James St. John

C. Determine the following characteristics to the best of your ability from the photos of the samples above:

  1. Color
  2. Texture (aphanitic, phaneritic, glassy, vesicular, frothy, porphyritic, pyroclastic?)
  3. Mineral content
  4. Is the rock mafic, felsic, intermediate, or ultramafic (based on mineral content you identified above)?
  5. Is the rock extrusive or intrusive (based on its texture you identified)?
  6. For each sample, compare your observations to the sample(s) listed in the Rock and Mineral Guide. Are your findings consistent with the guide?
  7. 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.

0 points: Did not complete the assignment.


Icon for the Creative Commons Attribution 4.0 International License

Physical Geology Copyright © by Lumen Learning is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

Share This Book