Observing Igneous Rocks
Integrated Science
Name ______________________ Per. _____
Introduction
Igneous rocks are classified according to their color, which indicates chemical composition, and their
crystal size, which indicates where they were formed. Intrusive rocks form beneath the Earths crust when
magma cools slowly. Extrusive igneous rocks form on the surface when lava cools very rapidly. Differences in
cooling rate determine the texture of igneous rocks. When magma cools slowly, it forms large crystals and
coarse-grained rocks. When magma cools rapidly, such as in water, it forms extremely small crystals and glassy
rocks.
The color of igneous rocks indicates their mineral composition. Dark rocks usually contain large amounts
of iron and magnesium and are called mafic. Light rocks contain large amounts of silicon and aluminum and
are called felsic. Andesitic rocks are a mixture of light and dark colors, and are also intermediate in terms of
iron, magnesium, aluminum and silicon levels.
In this laboratory, we will observe color and crystal size in igneous rock types. We will use these
observations to infer chemistry and depth of formation.
Consider the following focus questions as you complete this activity:
• What are the observable differences amongst common igneous rocks?
• What are the differences in chemical composition and depth of formation amongst common igneous
rocks?
Procedure
1. Using the Introduction and any supplemental resources pertaining to igneous rocks, complete the unshaded
sections of Data Table 1.
• First, consider the mineral composition of each igneous rock type: felsic, mafic, and andesitic. Record the
characteristic Colors of each rock type in the appropriate row in Data Table 1.
• Next, consider the chemical composition of each rock type, felsic, mafic, andesitic. Record the
characteristic Chemistry in the appropriate row in Data Table 1.
• Then, consider the depth of formation for each igneous rock type (extrusive and intrusive). Based on this,
determine and record the relative Crystal Sizes of each rock type in the approprate column in Data Table 1.
2. Using the background information and any supplemental resources pertaining to igneous rocks, complete
the shaded sections of Data Table 1. To do this, write the Name of each rock sample in the appropriate box
in Data Table 1. The igneous rock names we will be observing are: andesite, basalt, diorite, gabbro, granite,
obsidian, pumice, rhyolite, and scoria. Note: the color and crystal size observations should match for each
rock type. Although there are 9 types of rocks listed, we will only be observing 7 types. You need to
identify the two missing rock types.
3. Gather the actual rock samples. Observe each rock’s characteristics and place them into the appropriate box
in Data Table 1. After completing Data Table 1, have your teacher initial your work.
4. With your naked eye and hand lenses or stereo microscopes, make thorough observations of each rock.
Record all crystal sizes, colors, and a sketch (labeled or colored, if possible) for each sample in Data Table 2.
After completing Data Table 2, have your teacher initial your initial work.
Data Table 1: Classification of Common Igneous Rocks (Color, Chemistry and Crystal Size Trends)
ROCK TYPE:
Andesitic
Felsic
Mafic
Colors:
Colors:
Colors:
Chemistry:
Chemistry:
Chemistry:
Name:
Name:
Name:
Relative crystal
size:
Name:
Name:
Name:
Relative crystal
size:
Name:
Name:
Name:
DEPTH OF
FORMATION:
Relative crystal
size:
Extrusive
Intrusive
(Shallow
to
moderate
depth)
Intrusive
(Moderate
to great
depth)
Data Table 2: Observations of Common Igneous Rocks (Listed in alphabetical order)
Andesite
Color(s) and sketch:
Basalt
Color(s) and sketch:
Diorite
Color(s) and sketch:
Crystal Size(s):
Crystal Size(s):
Crystal Size(s):
Gabbro
Color(s) and sketch:
Granite
Color(s) and sketch:
Obsidian
Color(s) and sketch:
Crystal Size(s):
Crystal Size(s):
Crystal Size(s):
Pumice
Color(s) and sketch:
Rhyolite
Color(s) and sketch:
Scoria
Color(s) and sketch:
Crystal Size(s):
Crystal Size(s):
Crystal Size(s):