Virtual Igneous Rock
Identification Lab
Name: _____________________
Date: _______
Period: ___
Igneous Rocks
Igneous rocks are rocks which solidify from molten material ( magma). Cooling of the magma can occur beneath
the surface (plutonic) or on the surface (volcanic). Igneous rocks can be identified by the determination of the
composition and texture of the rock. Once these two characteristics have been identified, the Igneous Rock
Identification chart is used to identify the rock name.
IGNEOUS ROCK IDENTIFICATION CHART
COMPOSITION
TEXTURE
Felsic
Intermediate
Mafic
Pegmatitic
Granite Pegmatite Diorite Pegmatite Gabbro Pegmatite
Phaneritic
Granite
Diorite
Gabbro
Aphanitic
Rhyolite
Andesite
Basalt
Porphyritic
Rhyolite
Andesite
Basalt
Glassy
Vesicular
Pyroclastic
Obsidian
Pumice
Ultramafic
Dunite
Basaltic Glass
Scoria
Volcanic Tuff
Composition
Composition of igneous rocks is properly identified by determination of the rock's chemical composition. This,
however, requires chemical equipment and apparatus that is unavailable in this lab. Fortunately determination of the
exact chemical composition is not necessary. Color is often an indicator of the composition of a rock or mineral and
can be effectively used to identify the composition of most igneous rocks. Light colors, including white, light gray,
tan and pink, indicate a felsic composition. Felsic compositions are rich in silica (SiO2). Dark colors, such as black and
dark brown, indicate a mafic or ultramafic composition. Mafic compositions are poor in silica, but rich in iron (Fe)
and magnesium (Mg). Intermediate compositions have an intermediate color, often gray or consisting of equal parts
of dark and light mineral . Beware that even though an igneous rock may have a felsic composition (light color), the
rock can contain dark colored minerals. Mafic rocks may contain light colored minerals as well. As mentioned above,
the composition of most igneous rocks can be identified using this system, formally known as the Color Index.
However, there are exceptions. The two most notable are obsidian and dunite. Obsidian is volcanic glass which
erupts as a lava flow. Most obsidian is felsic in composition, yet typically it will have a very dark color (dark brown
to black). Dunite has an ultramafic composition yet is apple green to yellowish green in color. Dunite is composed
almost entirely of the mineral olivine which usually contains both iron and magnesium.
Texture
The texture of an igneous rock does not refer to the roughness or smoothness of the surface. Textures are based
primarily on crystal size. Pegmatitic texture is composed of very large crystals (larger than 2-3 cm). Phaneritic texture
is composed of crystals which are large enough to see but smaller than pegmatitic texture, and the entire rock is
composed of crystals. Aphanitic texture is a fine grained texture but the crystals are too small to see. Porphyritic
texture is composed of crystals of two different sizes. Typically the large crystals ( phenocrysts) are visible while the
smaller crystal are not (referred to as groundmass). Glassy texture is the most readily recognized. The rock is
composed entirely of glass. Few, if any, crystals will be visible. Vesicular texture is formed when lava solidifies before
gases are able to escape. The result is a "bubbly" appearance. Lastly, pyroclastic texture is composed of volcanic
fragments. These fragments or clasts can be very fine (ash) or coarse (lapilli) or very coarse (bombs and blocks).
Terms to know prior to completing this lab: intrusive, extrusive, plutonic, volcanic, texture, composition, mafic, felsic,
intermediate, aphanitic, phaneritic, vesicular, porphyritic, pegmatitic, glassy . These are the terms you needed to know
for the pre-lab quiz.
This page is based on information from http://facweb.bhc.edu/academics/science/harwoodr/Geol101/Labs/Igneous/index.htm
PART 1: Fill in the following table from these 6 common igneous rocks: (gabbro, diorite, rhyolite, andesite, granite, basalt)
Mafic
Intermediate
Felsic
APHANITIC
PHANERITIC
PART 2: Go to the following website and practice identifying rocks prior to completing the remainder of this assignment.Richard
Hardwood at Black Hawk College)
PART 3: Fill in the chart below. Sample numbers are linked to pictures of each rock (click on each one to view the sample).
PROCEDURE:
1. Determine the overall color of the sample: some examples include pink, light grey, dark grey, black, white, etc.
2. What is the composition of the sample? Mafic, intermediate, or felsic are your options
3. What is the texture of the sample? Aphanitic, phaneritic, vesicular, glassy, porphyritic, and pegmatitic are your options
4. What are the major minerals present in the rock? Refer to Rock Classification Chart handout or table within the lab or
textbook.
a. Mafic: plagioclase, pyroxene, olivine
b. Intermediate: plagioclase, amphibole, biotite
c. Felsic: potassium feldspar, quartz, biotite, muscovite
5. What is the rock name – refer to the list on the first page of this handout. There is only one of each!
Sample
#
1
Color
Composition
Texture
Major Minerals
2
3
4
5
6
7
8
9
10
11
12
This page is based on information from http://facweb.bhc.edu/academics/science/harwoodr/Geol101/Labs/Igneous/index.htm
Rock Name
PART 4: Post-Lab Assignment
Objective: See the relationships between the terms and the rocks. There will be more than one rock listed for each.
Procedure: Classify the following rocks into the categories listed below:
Granite, basalt, obsidian, rhyolite, pumice, scoria, andesite, diorite, gabbro
1. List all of the rocks that are felsic.
2. List all of the rocks that are intermediate.
3. List all of the rocks that are mafic.
4. List all of the rocks that are intrusive.
5. List all of the rocks that are extrusive.
6. List all of the rocks that are volcanic.
7. List all of the rocks that are plutonic.
8. List all of the rocks that can be aphanitic.
9. List all of the rocks that can be phaneritic.
10. List all of the rocks that can be vesicular.
11. List all of the rocks that can be porphyritic.
12. List all of the rocks that can be pegmatitic.
This page is based on information from http://facweb.bhc.edu/academics/science/harwoodr/Geol101/Labs/Igneous/index.htm