Explore 3: Rock Color, Shape, and Texture
Target Concepts
• Rocks come in different shapes, colors and textures.
• Color and texture help determine the type of rock.
• The same rock type can come in many different shapes and sizes.
Addressed Standards
• MA-4
• MA-6
• ME-3
Assessment Task
• Classify rocks by their color and texture.
• Tell what attributes can and cannot be used to determine a rock’s identity.
Vocabulary
texture
Background
Geologists use several different techniques, which when combined, can tell
them the type of rock they are analyzing. In this process they are looking at
color, texture, crystals, hardness, and layering. They observe the rock first,
looking for specific color, layering, texture, and crystallization. For instance, a
rock without crystals, with smooth layers, and a smooth gray color is usually
slate, while a rock with all the same attributes except wavy layers would be
shale. After looking at the rock, several tests are conducted to further
determine its identification. These include a test for magnetic properties,
hardness, powdered color, and carbonates.
Magnetic Properties
The test for magnetic properties is conducted using a strong magnet. If the
rock is attracted to the magnet then it contains metal such as magnetite. If you
want to try this with your students you can try “Mining for Iron” from the Primary
Adaptation module, Elbow Deep in Mudpies and Minerals. You can also try to
make a magnetometer by gluing a piece of thread to one end of a very tiny bar
magnet. Suspend the magnet by the thread and slowly bring rocks near the
magnet one at a time. If no deflection occurs, rotate the rock. Tiny bits of
magnetite can often be detected in granite rocks even if the magnetite cannot
be seen when observing it.
Hardness
Geologists use hardness tests because each mineral has its own unique
hardness which based on the Mohs’ Scale of Hardness, shown on page ____.
It is based on a mineral’s ability to resist scratching by a set of standard
minerals ranked one through ten. A mineral will scratch all other minerals of
less hardness than itself, and scratch minerals of equal hardness with
difficulty. A convenient hardness test for students consists of a fingernail,
penny, piece of glass, and a common non-galvanized steel nail.
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The streak test looks at the color of a mineral when it is in powdered form. It i s
not commonly used by geologists to identify rocks unless it is homogeneous.
When using the streak test with a non-homogenous rock it is hard to tell exactly
what part of the rock is streaking. If you would like to try this test with your
students you can use pennies, nickels or dimes. Streak plates are
commercially available or you can use the rough side of a ceramic tile.
Color
The acid test is used for certain rocks and minerals which contain carbonates
(a specific combination of carbon and oxygen). These rocks react with acid and
release carbon dioxide gas. The escaping gas forms bubbles causing a “fizz”.
The fizzing can sometimes be heard better than the bubbles can be seen.
Geologists generally use diluted hydrochloric acid, but for the classroom
students can use diluted vinegar or lemon juice with fairly good results. When
using these mild acids it is often necessary to scratch the surface of the rock a
bit first. Students must wear safety glasses when doing this test as they tend
to hold the rock close to their face to see the bubbles. If you try this test, be
sure to rinse and dry the specimens when you are finished.
Carbonates
In this module only color, texture, and hardness have been addressed. The
important concept here is not whether or not a student can pick out a schist
from a granite, but rather that rocks have attributes that help distinguish one
kind from another.
The Rock Key has been included on page 24 to show the dichotomous key for
rock identification as an optional addition.
Materials per Every 2
• 15-30 rocks or pebbles (should be of differing sizes and shapes) – do NOT
use polished rocks.
• 15 Paper plates (5 larger plates and 10 smaller plates)
• 2 rocks of the same type, but differing sizes. Beach rocks are good for this.
Procedure
1. Show the class a large rock and a small rock, both of the same type.
(Maybe both are granite.) Ask if these two rocks are the same kind of rock?
Why or why not. Make a “same” and a “different” column. Have students
look closely at the 2 rocks to fill out the columns. Discuss their ideas. They
should come to the conclusion that they are both the same color and
texture, but they’re different in size and shape. The use of a Venn Diagram
may help with this concept.
2. Explain that rocks come in many shapes and sizes, but it is their color and
texture, as well as their hardness (which will be addressed in the next
activity) that determines the kind of rock.
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3. Brainstorm words for rock colors and list them on the board. Read the list
together several times. You may have colors like dirty pink, dark gray, silvery
gray, etc.
4. Divide the class into groups of 2. Give each group a pile of rocks and paper
plates.
5. Ask each team to sort their rocks into like piles based on color only. Rocks
can be placed on the paper plates to help keep track of differing piles.
6. Explain the word texture. You may want to have examples to show such a s
sand, cotton balls, and a piece of soap. Brainstorm a list of texture words
that apply to rocks such as bumpy, scratchy, smooth, cool, lumpy, holey, oily,
etc. (Stop here is you are teaching kindergarten).
7. Next have them take one color pile and divide it into like piles of texture. Let
the students sort them any way they wish. After this has been done, sort the
other color piles in the same way.
8. Some students may find they can divide their rocks into further categories.
Have students continue dividing their rocks into piles until they feel they can
go no further. They may want to add a sticky note to the plate listing the
attribute of that set of rocks.
Generalize
Ask the students:
1. How many different piles did you make?
2. Did you think it was easier to sort by color or by texture? Why?
3. What was the most common color? Let’s make a graph to find this out.
4. What was the most common texture? Let’s make a graph to find this out.
5. Did you have rocks of different sizes within each pile? Do you think they are
the same kind of rock even though they are a different size or shape? Why?
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Assess
Looking at their final piles, students name each rock category with a fictitious
name which helps describe the rock. For instance the gray lumpy pile may be
called “dull lumps”.
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Rock Color, Shape, and Texture Assessment Rubric
Science Content
•
•
Communication of
Science Content
•
In Progress
Towards the
Standard
Sorts the rocks
into like piles of
color and
texture.
Confuses
shape and size
while sorting,
mistakenly
demonstrating
that shape and
size do
determine the
type of rock.
When
describing
his/her fictitious
rock, color and
texture are not
used to name it
and/or, shape
and size are
used to name it
(ie: hugo
lumpis).
Meets the
Standard
•
•
•
Exceeds the
Standard
Sorts the rocks
into piles of like
color and
texture.
Shows, through
his/her sorting,
that size and/or
shape does not
determine the
type of rock.
In addition to Meets
the Standard:
• Sorts the rocks
into various
shapes and
sizes.
• Sorts the rocks
into like piles of
color and
texture until all
of the rocks
within a
category are the
same, differing
only in shape
and size.
When
In addition to Meets
describing
the Standard:
his/her fictitious • Uses accurate
rock, color and
descriptions
texture are the
which use
only attributes
neither shape
used to name it.
nor size and are
exceptionally
creative.
National Science Benchmark
Chunks of rocks come in many sizes and shapes, from boulders to grains of
sand and even smaller.
From Benchmarks for Literacy from the American Association for the Advancement of Science
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