CLASS SET
Classifying Rocks
Objectives
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Become more familiar with and practice criteria for distinguishing among sedimentary,
metamorphic, igneous plutonic, and igneous volcanic rocks.
Become more familiar with several common rock types within each of the classifications
above.
Standards Addressed
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Earth Science
o 3c. Students know how to explain the properties of rocks based on the physical
and chemical conditions in which they formed, including plate tectonic processes.
Investigation and Experimentation
Materials
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rock kits
hand lenses
Procedure
Identifying rocks can be a challenging exercise even for experienced geologists. Rocks in their
natural setting might be found in various states of decomposition due to weathering. Depending
on the degree of weathering, it may be challenging even to tell if a particular rock is igneous,
sedimentary, or metamorphic! In any case, when learning to classify rocks there is no substitute
for looking at rocks themselves. In this activity you'll be introduced to several criteria that can
help you identify rocks according to their mechanism of formation (igneous, sedimentary, or
metamorphic), and identify several common rock types within those categories.
In this activity you'll follow the jigsaw small-group collaborative approach. In your permanent
group, one of you will choose to focus on sedimentary rocks; one on metamorphic rocks; one on
igneous volcanic rocks; and one on igneous plutonic rocks.
The people specializing in sedimentary rocks from the various permanent groups will then form
several temporary groups, each with three or four sedimentary rock specialists, and similarly for
specialists in the other three rock types. Each of these specialist groups will be provided with a
rock kit with several unidentified rocks. Your task is:
1. Using criteria described below try to pick out the individual rock(s) in the category in
which your temporary group is specializing (sedimentary, metamorphic, etc.). Consult
with your instructor about your criteria and your application of them. They can perhaps
suggest other criteria, too.
CLASS SET
2. Once you've picked them out, try to identify which particular type of rock each is, using
the tables in the accompanying handout. As a hint, these are the types of rocks that you've
been given:

Granite or diorite, gneiss, schist, basalt, slate, conglomerate or breccia, shale,
sandstone or siltstone, pumice, obsidian
3. Create a Rock Classification Note-Taking Chart on your own paper (see the sample
below).
Rock Classification Note-Taking Chart
Sample
Rock
Properties
Rock
Classification
Rock
Type
How did this rock
form?
(textures, minerals, fossils,
colors, etc.)
(igneous, sedimentary,
metamorphic)
(granite, basalt,
sandstone, gneiss,
etc.)
(cooling of magma or lava;
deposition/compaction/cementation;
precipitation from solution; modification of
existing rock under increased
temperature/pressure or chemical modification
by contact with hot fluids)
What three
changes the
rock could
undergo?
(Refer to the rock cycle.)
A
B
C
Etc.
4. Record your reasoning on your Rock Classification Note-Taking Chart. Return to your
permanent group and explain to the other group members what your group concluded and
why. (You might want to take them back to see the particular rock samples that you
classified.) Each of you is responsible for understanding as much as possible about what
the others learned. (Filling out your Rock Classification Note-Taking Chart with
information about all of the rocks identified by your permanent group, for example,
might help.)
5. Use the diagram of the Rock Cycle below to help you complete your Rock Classification
Note-Taking Chart.
CLASS SET
Some Criteria for Identifying Rocks
As described above, in this activity you will first classify a set of unidentified rocks as igneous
plutonic, igneous volcanic, sedimentary, or metamorphic. Then you will use a set of
classification tables to try to identify the particular rock types.
Rocks are classified using a variety of criteria. Here are a few:
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texture (Can you see individual mineral grains? If so, the texture is medium coarse to
coarse. If the mineral grains are too small to be resolved by the naked eye, the texture is
fine.)
grain arrangement (Is the rock grainy? If so, are the grains interlocking? Interlocking
grains are formed by crystallization, so the rock is probably either igneous or
metamorphic. If interlocking, are the grains aligned and hence show foliation—that is, a
kind of layering caused by differential pressure applied to the rock? Foliation indicates
metamorphism. Are the grains, in contrast, separated by irregular spaces filled with
cementing material? This arrangement indicates sedimentary rock.)
color (Color often tells us something about the chemical composition of a rock, one of
the important determinants of rock type.)
presence of fossils (Fossils in a rock are usually a dead giveaway that the rock is
sedimentary. You don't find fossils in igneous rock [why not?], and they rarely survive
the processes that create metamorphic rock from sedimentary rock.)
A. Identifying Igneous Rocks
Molten magma welling up from within the earth produces igneous rock of two types—
intrusive (or plutonic) and extrusive (or volcanic). Intrusive rocks are formed from
magma that solidified below the earth’s surface and extrusive rocks are formed from
magma erupted as lava at the earth’s surface.
Step 1: Observe their texture. Texture is related to the cooling rate of magma in a
rock’s formation. Magma that solidifies below the earth’s surface cools slowly, forming
large, visible interlocking crystals that can be identified with the unaided eye, forming a
coarse-grained texture. In contrast, magma that reaches the surface (lava) tends to cool
rapidly, forming very fine-grained rocks.
Step 2: Observe the color, and hence the chemical composition of an igneous rock
sample. Igneous rocks are either rich in quartz (a mineral made of silicon dioxide) or
poor in quartz. Quartz-rich (and hence silicon-rich) igneous rocks tend to be light in color
whereas quartz-poor (silicon-poor) igneous rocks tend to be dark in color. Igneous rocks
can therefore be divided into a light-colored group—quartz, feldspars, and muscovite;
and a dark-colored group—the ferromagnesian minerals—biotite, pyroxene, hornblende,
and olivine. (Ferromagnesian refers to minerals that contain iron [ferro] and magnesium
[magnesian]).
CLASS SET
CLASSIFICATION OF IGNEOUS ROCKS
Composition
Light-Colored
10-20% quartz
K-feldspar >
plagioclase
≈10%
ferromagnesian
minerals
Texture
Pegmatitic
(very coarsegrained)
Porphyritic
(mixed crystal
sizes)
Phaneritic
(coarse-grained)
Aphanitic
(fine-grained)
Intermediate-Color
No quartz
plagioclase >
K-feldspar
25–40%
ferromagnesian
minerals
Dark-Colored
No quartz
plagioclase ≈ 50%
50% ferromagnesian
minerals
Granite
Granodiorite
Diorite
Gabbro
Rhyolite
Andesite
Basalt
Very Dark Color
100%
ferromagnesian
minerals
Pegmatite
Porphyry
Peridotite
Dunite
Glassy
Obsidian
Porous
(glassy, frothy)
Pumice
Pyroclastic
(fragmental)
Scoria
Volcanic tuff (fragments < 4mm)
Volcanic breccia (fragments > 4mm)
B. Identifying Sedimentary Rocks
Rock material that has been weathered, subjected to erosion, possibly transported
elsewhere, and eventually consolidated into new rock (that is, lithified) is sedimentary
rock. Sedimentary rocks are classified into two types—clastic and nonclastic. Clastic
sedimentary rocks are formed from the compaction and cementation of fragmented rocks
(sediment). Nonclastic sedimentary rocks are formed from the precipitation of minerals in
a solution. ("minerals in a solution" means that minerals are dissolved in water). This
chemical process can occur directly, as a result of inorganic processes, or indirectly, as a
result of biochemical reaction involving living organisms.
Observe the texture of the rock. If you see particle grains then the rock is probably
clastic. Clastic sedimentary rocks are classified according to particle size:
o
o
Large-sized particles range from boulders (largest) to cobbles to pebbles (less
large).
 If the large-sized particles are rounded, the rock is a conglomerate.
 If they are angular and uneven, the rock is a breccia.
Medium sized particles produce the various types of sandstone.
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o
Fine-size particles include silts and clays (smallest). Silt sized particles produce
siltstone and clay sized particles produce shale.
If the rock does not fit into the clastic classification, the rock is probably nonclastic (e.g.,
limestone, gypsum, etc.).
CLASSIFICATION OF CLASTIC SEDIMENTARY ROCKS
Particle size
Rock Name
Characteristics
Conglomerate
Rounded grain fragments
Breccia
Angular grain fragments
Quartz
Quartz grains
(often well rounded, well sorted)
Arkose
Quartz and feldspar grains
(often reddish color)
Graywacke
Quartz grains, Small rock fragments,
and clay minerals (often grayish color)
Fine (1/256–1/16 mm)
Siltstone
Silt sized particles,
surface is slightly gritty
Fine (< 1/256 mm)
Shale
Clay sized particles,
surface has smooth feel, no grit
Coarse (> 2mm)
Medium (1/16–2mm)
Sandstone
C. Identifying Metamorphic Rocks
Rocks that have been changed into a different form (i.e., metamorphosed) by high
temperature or pressure, are metamorphic rocks. Metamorphic rocks are most easily
classified and identified by their texture and, when a particular mineral is very obvious,
by their mineralogy. Metamorphic rocks can be divided into two groups: foliated and
nonfoliated.
Foliated metamorphic rocks have a "directional" texture and a layered appearance. The
most common foliated metamorphic rocks are slate, phyllite, schist, and gneiss.
o
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Slate is very fine-grained and is composed of tiny mica flakes. The most
noteworthy characteristic of slate is its excellent rock "cleavage"—a tendency to
break into plate-like pieces along flat planes.
Phyllite is composed of very fine crystals of either the mineral muscovite or the
mineral chlorite that are larger than those in slate, but not large enough to be
clearly identified. Phyllite is distinguished from slate by its glossy sheen.
Schists have a very distinctive texture with a parallel arrangement of the minerals,
which structured in sheets. (The minerals are mica, chlorite, and/or biotite). The
minerals in a schist are often large enough to be easily identified with the naked
CLASS SET
o
eye. Because of this, schists are often named according to the major minerals in
the rock (biotite schist, staurolite-garnet schist, etc.).
Gneiss contains minerals in the form mostly of grains, rather than minerals
organized in plates. The most common minerals found in gneiss are quartz and
feldspar. The foliation in this case is due to the segregation of light and dark
minerals rather than alignment of platy minerals. Gneiss has a composition similar
to granite and is often derived from (metamorphosed from) granite.
Nonfoliated rocks are composed of single minerals and lack any directional texture. Their
texture can be described as coarsely crystalline. Common nonfoliated metamorphic rocks
are marble and quartzite.
CLASSIFICATION OF METAMORPHIC ROCKS
Foliated Metamorphic Rock
Crystal Size
Rock Name
Characteristics
Very fine, crystals not visible
Slate
Excellent rock cleavage
Fine grains, crystals not visible
Phyllite
Well developed foliation; glossy sheen
Muscovite schist
Chlorite schist
Crystals visible with unaided eye
Biotite schist
Micaceous minerals
Often contains large crystals
Schist
Coarse texture
Garnet schist
Staurolite schist
Mineral content reflects
increasing metamorphism
from top to bottom
Kyanite schist
Sillimanite schist
Coarse
Gneiss
Banding of light and dark minerals
Analysis
1. What characteristics are used to classify different rock types?
2. Name the primary characteristics common to each rock type.
3. How are sedimentary rocks formed?
4. How are metamorphic rocks formed?
5. How are igneous rocks formed?
6. What are the differences between intrusive and extrusive igneous rocks?