Download Sedimentary Rocks

Illinois State Museum Geology Online – http://geologyonline.museum.state.il.us
Sedimentary Rocks
Grade Level: 9 – 12
Purpose: The purpose of this lesson is to introduce sedimentary rocks. Students will
learn what sedimentary rocks are and how they form. It will teach them how to identify
some common examples.
Suggested Goals: Students will shake a flask of sand and water to see how particles
settle. They will draw a picture showing where the various rocks form and they will
identify some common examples.
Targeted Objectives: All of Illinois is covered by sedimentary rocks. In most places,
they reach a thickness that could be expressed in miles rather than feet. Ancient seas
dropped untold numbers of particles for hundreds of millions of years creating the layers
that bury the original igneous rocks in 2,000 to 17,000 feet of sedimentary layers. These
valuable layers are used to construct our homes, build our highways, and some were
even used to heat our homes.
The Illinois Basin dominates the geology of southern Illinois. There the layers dip down
until they are over three miles thick. The geology of Illinois cannot be told without
discussing sedimentary rocks because they are what Illinois is made of.
Students will learn to tell the difference between the major sedimentary rock
varieties.
Students will learn in what environments different sedimentary rocks form.
Students will learn what sedimentary rocks are.
Background: Most sedimentary rocks are formed when weathering crumbles the
parent rock to such a small size that they can be carried by wind or water. Those
particles suspended in water collide with one another countless times gradually
becoming smaller and more rounded. When water is moving quickly due to flooding or
due to a rapid change in elevation, larger particles can be carried by the streams and
rivers but when the water slows down the particles begin to settle out. The particles
dropped from water and wind are called sediment. As layers become thicker, the bottom
particles get squeezed closer and closer together under the crushing weight.
Many of the organisms that live in the oceans have shells and skeletons that are made
of calcium carbonate. Their decay releases a natural cement into the water. The cement
glues the particles of rock back together forming sedimentary rock. People who live in
homes with hard water see the amazing affect of this calcium carbonate glue in their
showerheads as the holes gradually become closed until the spray is reduced to a
trickle.
The variety of sedimentary rock often depends on the size of the particles that make up
the rock. Conglomerate, sandstone, and shale form from different sized particles.
Illinois State Museum Geology Online – http://geologyonline.museum.state.il.us
Conglomerate usually forms in places with fast moving water. Finding conglomerate
made of large particles often suggests that when they were formed there were
mountains nearby. Conglomerate means a variety and that is a fitting name because
there are usually large and small particle intermingled within the rock.
Sandstone forms where the flow is slower. Sand grains bounce down streams without
much difficulty so even slow moving streams can carry vast amounts of sand.
Shale is made from silt (dirt) in the water. The particles are so small that they can stay
suspended in water for quite a while even after the river or stream has stopped flowing.
The silt gradually settles to the bottom.
Limestone and Dolomite do not form from particles washed from the land. They
usually form on the ocean bottom far out to sea where no particles from the land ever
reach.
Limestone is made from an infinity of microscopic skeletons that rain down on the sea
floor. In some cases the shells of larger organisms such as brachiopods are found, too.
Limestone is almost totally calcium carbonate.
Dolomite is similar to limestone except that it forms in the ocean in places that have
magnesium dissolved in the water. It often forms from limestone as the calcium in
calcite is partially replaced by magnesium.
Materials and Preparation:
Several 500 ml flasks (or larger) filled with 250 ml of sandbox (river) sand and then filled
to 400 ml with water. Tightly seal each with a cork. A long jump pit is a good
place to find the sand.
Containers to catch any water that might spill from the flasks
Typing paper
Colored pencils
Collections of limestone, sandstone, shale, and conglomerate. If possible, have enough
collections so that there can be groups of two or three.
Magnifiers
Procedure: Pass out the flasks of sand created as described above and give each
group a container for spills; in case there is an accident. Have the students shake the
flasks quickly back and forth and then turn them while the water is still churning so that
the cork end is down. Stop shaking. Have the students observe the sand and answer
the following questions.
Discussion:
1. Where are the larger particles and why? Near the bottom because they could not
stay suspended in the water very long when the shaking stopped.
Illinois State Museum Geology Online – http://geologyonline.museum.state.il.us
2. Are there just large pieces at the bottom? No the large pieces are surrounded by
hundreds of grains of sand.
3. As you look higher in the flask what is happening to the size of the sand grains?
They are getting smaller.
4. Did all of the particles settle out right away? No the water remained cloudy and a thin
layer of silt is being deposited on top.
5. When a river enters the ocean, the water slows down suddenly just like when you
stopped shaking the flask. Which particles would you expect to drop closest to the
mouth of the river? The largest ones.
6. If you were to sample the sand near a rivers mouth and then take another sample
father out to sea, what would you expect to find? Farther out the sand particles would
be smaller.
7. What would settle out even farther that the sand? Silt (mud)
Preparation And Procedure: Draw the following diagram on the board and have your
students copy it onto a piece of typing paper. Have them label and color it for a
homework assignment. As you draw the diagram point out why each particle forms
where it does.
Gravels are close to shore because they are too large to stay suspended. Sometimes
gravel does not occur if the stream flow is too slow.
Sand is the next particle to settle out because it is the next largest but the sand grains
become smaller and smaller the farther they are from shore.
Mud is next to settle out because the tiny particles of silt can stay suspended for a very
long time, like they did in the flask, after the water has stopped moving.
The material farthest out forms from shells because no material from the land can make
it far out to sea. This is where limestone forms. The only things that settle on the bottom
far out to sea are those from the sea itself such as shells and dissolved minerals.
Illinois State Museum Geology Online – http://geologyonline.museum.state.il.us
Discussion:
1. How would sedimentation be different in an environment where a very slow moving
river enters the ocean? The first particles to settle out would be sand or mud. Where
there was a fast current? The first particles would be larger.
2. None of the beaches in Florida have gravels. What is the major reason? Florida is
very flat. The highest point is only 345 feet and most of the state is much lower. Florida
streams are very slow moving so sand particles are the largest particles that they can
carry.
3. What property of sedimentary rocks is used to identify most of them? The size of the
particles
4. Which sedimentary rock is NOT identified by its particle size? Limestone, its
identification is based on the chemical that it is made of which is calcium carbonate.
Illinois State Museum Geology Online – http://geologyonline.museum.state.il.us
Activity
Identification of Sedimentary Rocks
Materials
Collections of sedimentary rocks
Magnifiers
A Drawing from the previous activity for each group
Weak hydrochloric acid or vinegar in acid bottles
CAUTION…If you are diluting strong hydrochloric acid be sure to add the acid to water
and NOT water to acid. Have your students wear safety goggles if they are using
hydrochloric acid.
Preparation and Procedure: Prepare several collections of rocks so that each
contains a specimen of sandstone, limestone, shale, and conglomerate. Make enough
so that you will have enough sets for groups of two or three students.
Hint…Small baskets, boxes, or baggies work well and can be kept from year to year.
Purchase dilute hydrochloric acid and place it in acid bottles with droppers. If you are
mixing the acid from concentrate, be sure to add the acid slowly to water and not
water to the acid. Wear gloves, long sleeves, and safety glasses while doing the
mixing.
If you are using vinegar, make sure before the lab that the limestone specimens you are
using will bubble when a drop of vinegar is placed on them. Some do not!
Have your students use the descriptions on their lab sheet to identify each specimen.
Have the students use one of their groups drawings for this activity. Ask your students
to place each variety of rock on the section of their group’s drawing where it would
occur. A piece of conglomerate will be placed near the shoreline where it appears on
the picture and so forth.
Illinois State Museum Geology Online – http://geologyonline.museum.state.il.us
Student Activity Sheet
Identification of Sedimentary Rocks
Now that you have learned about the different kinds of sedimentary rocks and know
how each forms, you are going to try to identify each. As you learned, most sedimentary
rocks are identified by the size of the particles that make them.
Use a copy of the picture that one of your group drew in the previous activity. It shows
where each variety of sedimentary rock is likely to form. When you determine what a
rock in your collection is, place it on the section of the picture where that rock would
form. Conglomerate would form near the shore so your piece of conglomerate would be
placed over the drawing where the conglomerate was drawn.
How to Tell the Rocks Apart?
Conglomerate…Pieces of gravel cemented together (see ISM Geology Online picture of
“conglomerate”)
Sandstone…Feels gritty, often pieces of sand can be rubbed off. With a magnifier
grains of sand may be visible. (see ISM Geology Online picture of “sandstone”)
Shale…Feels smooth, does NOT bubble when you put a drop of acid or vinegar on it.
Limestone…Feels smooth and bubbles when you place a drop of acid or vinegar on it
since it is made of calcium carbonate. Fossils may be visible. If none are visible try
using a magnifier. It is possible that none will be visible, even with the magnifier if the
limestone is made of microscopic fossils. (see ISM Geology Online picture of
“limestone”)
Illinois State Museum Geology Online – http://geologyonline.museum.state.il.us
Assessment:
1. Correct the pictures showing the environment where each rock variety would form.
2. Give a lab test in which each student identifies each of the four specimens of
sedimentary rocks.
3. Have your students answer the following essay questions,
A. How are sedimentary rocks different from one another
B. Tell what conditions would produce each variety of sedimentary rock and
where each variety of sedimentary rocks would form?
C. If forces were to create mountains in Florida, how would that affect the
beaches there?” They would become rock beaches instead of sand.
4. Have your students reproduce the rock drawing from memory.
Extensions:
Place shale, sandstone in a bag and have students try to identify them without looking.
Collect specimens from a stream or other source.
Have students create sedimentary rock collections.
Make a list of things made from sedimentary rocks; such things as plaster, plasterboard,
grinding stones, coal, cement, driveways, etc.
Create a poster similar to the drawing that the students did in class. Permanently glue
specimens to it showing which rock would form in each place.
Print Resources
Geology Underfoot in Illinois by Raymond Wiggers, Mountain Press Publishing
Company, Missoula, Montana 1997 phone 800-234-5308
The Audubon Society Field Guide to Rocks and Minerals available in most bookstores
The Pocket Guide to Rocks and Minerals by Michael O’Donoghue. London: Parkgate
Books. 1991, 1999. Available in most bookstores
Peterson Field Guide Rocks and Minerals by Frederick H. Pough. Boston: Houghton
Mifflin Company. 1996. Available in most bookstores
Video Resources
RockMan Video http://www.rockman-tm.com/ Infinity Multimedia, LLC Eugene, Oregon
(541) 683-6650 a great video that shows uses for rock and gravel
Illinois State Museum Geology Online – http://geologyonline.museum.state.il.us
Web Resources
The Illinois Geologic Survey, http://www.isgs.uiuc.edu/isgshome/isgshome.html
Mineral Identification Project http://www.ironorchid.com/minerals/
ISM Geologyonline geogallery http://geologyonline.museum.state.il.us/geogallery
Lesson Specifics:
This activity will take about two class periods.
It is desirable to have groups of two or three for the lab portions
Lab tables should be used so that there is no spillage of the acid
Illinois State Board of Education Goals and Standards:
11.A.4c: Collect, organize and analyze data accurately and precisely.
12.E.4b: Describe how rock sequences and fossil remains are used to interpret the
age and changes in the Earth.
12.E.5:
Analyze the processes involved in naturally occurring short-term and longterm Earth events (e.g., floods, ice ages, temperature, sea-level
fluctuations).