Waco Gem and Mineral Club, Waco, Tx

10.4 Sedimentary Rocks

 

10.4 Sedimentary Rocks

a) Making sedimentary rocks

Sedimentary rocks start by processes of erosion that create gravel, sand, or mud. These settle to the bottom of a basin (ocean, lake, or river valley) in layers. These layers eventually harden to become what are called clastic sedimentary rocks: conglomerate, sandstone, or shale. Sedimentary rocks also include those that precipitate out of water either through chemical action or evaporation, such as limestone, gypsum, or halite. These are nonclastic sedimentary rocks, or precipitates and evaporites. Via the following activities, kids can make artificial sedimentary rocks, including evaporites, sandstone, conglomerate, and breccia.

A. Creating precipitates and evaporites. Some sedimentary rocks, such as limestone and gypsum, chemically precipitated out of minerals in water or were left behind when water in a lake or sea evaporated. You can demonstrate this process using water solutions created with readily available materials.

Materials:

 Table salt, Epsom salt, or alum Glass jars
 Water Pebbles
 Measuring cups Stick or pencil
 Spoon String (cotton twine), cut into small lengths & dampened
 Cooking pan Food coloring (optional)

Procedure:

1. Heat water to a boil, then turn off the heat.

2. If using table salt, use ½-cup salt with ¾-cup hot water. With Epsom salt, use ½-cup salt with 1-cup water. If using alum, use ¼-cup alum with 1-cup water.

3. Slowly add and stir salt into the hot water until it becomes a saturated solution. A saturated solution contains the maximum amount of mineral that will dissolve in a given amount of water. If all of your salt dissolves, the solution is not yet saturated, and you should add a bit more salt. Stop when no more salt will dissolve.

4. Optional: You can make colorful crystals by adding a couple drops of food coloring.

5. Place a few pebbles in a glass jar and pour your solution over the pebbles. Or, tie a piece of string to a stick or pencil. Dampen the string with your solution and roll it in salt to provide seed crystals. Then pour your solution into a glass jar, and dip the string into the solution. Leave it hanging there from the stick or pencil.

6. Set your jar aside in a spot where it won‘t be disturbed and don‘t bump or bounce it. Check every so often the next few days. As water evaporates, you‘ll see crystals forming on your pebbles or string.

Assign different salts to different kids. Once everyone‘s water has evaporated, bring their jars together to compare the different forms of crystals each produced.

You can also grow crystals using commercially available crystal-growing kits from places like toy and craft stores, museum gift shops, or scientific supply houses. Two reliable supply houses are Ward‘s Natural Science (order their Earth Science and Geology catalogs; phone 1-800-962-2660), or Edmund‘s Scientific (phone 1-800-728-6999).

Note: Kids can use this activity for satisfying requirements toward earning the Rocks & Minerals badge simultaneously (Activity 1.6).

B. Creating sandstone. Sandstone forms when sand is buried and mineral-rich groundwater flows through it. Minerals in groundwater act as cement to glue sand grains together while overlying layers of sediment exert pressure to compact it. Your kids can simulate sandstone formation with an easy activity to make their own artificial sandstone.

Materials:

 Paper cups Pan or Pyrex beasuring cup
 Sand (from beach or hardware store) Spoon, dowel, or popsicle stick
 Epsom salt, sodium silicate solution (also called water glass), or plaster Food coloring (optional)
 Water Paper towels

Procedure:

1. Fill the bottom of a paper cup with a layer of sand about an inch deep.

2. Make a solution of mineral-rich groundwater in a pan or Pyrex measuring cup by dissolving Epsom salt in boiling hot water (keep stirring in salt until no more will dissolve). An alternative to Epsom salt is a sodium silicate solution (water glass) diluted with water. (As an option, have different kids add drops of different food colorings to their solutions to make sandstones of different tints.)

3. Pour the groundwater into the sand and stir it all together with a spoon, dowel, or popsicle stick to make sure all the sand is wet. However, you don‘t want to make soup, so don‘t pour in too much water!

4. Lightly tap the bottom of the cup on a countertop or desktop to settle the sand.

5. Set the cup, uncovered and undisturbed, in a sunny, warm open spot to evaporate the water. If you poured in too much solution, you may find you need to soak up excess water with wadded paper towels after you‘ve allowed the mixture to sit for awhile.

6. After about a week, the mixture should have completely dried. When it has, tear off the paper cub, and you should end up with a rock that looks and feels similar to the sandstone in your sedimentary rock box.

I‘ve had mixed success with using these Epsom salt and water glass solutions. They took a long time to dry, and Epsom salt often produced just a thin crust at the top of the sand. Here‘s an alternative that‘s worked with greater consistency. Fill cups with an inch-thick layer of sand and add a heaping tablespoon of plaster of Paris. Have kids add different food colors to different cups, and then add water and mix the sand and plaster together. This variation also tends to dry more quickly than Epsom salt or water glass solutions.

Kids will notice that the artificial sandstone is softer, crumblier, and not as heavy as the real thing. Ask if they can think of why. (Answer: the real sandstone not only has been cemented together by minerals in groundwater but also has been compacted when it was buried beneath other rocks. The weight of overlying rocks and earth pressures squeezed sand grains together as much as possible, forcing out air pockets and making the real sandstone much denser than our artificial sandstone.)

If you have specimens of real sandstone, you might notice that it comes in different colors, from yellow or brown hues to bright reds, grays, greens, etc. The color of sandstone may have two explanations:

i) Sometimes, sand grains are made of different minerals, and the color of the sandstone is caused by the color of the sand grains themselves. For instance, black sand beaches in Hawaii are derived from the dark basaltic lavas. White sand dunes covering an extensive area of New Mexico were derived from the mineral gypsum.

ii) Other times, the color of sandstone may be due to the color of the minerals deposited around sand grains by the groundwater. For instance, some groundwater holds iron oxide in it, and this will often cause a rusty color, painting the sandstone red.

Many times, the color of a piece of sandstone represents a combination of colors derived from the sand grains themselves along with the color/s of any minerals that were deposited around those sand grains to glue them together. You can demonstrate the coloring effect of minerals in groundwater by having different kids add different colors of food coloring to their ground water solutions. Have some add a couple drops of red, have others add a couple drops of blue, and have others use no food coloring and compare the resulting sandstones when all have dried.

C. Creating conglomerate and breccia. Conglomerate is a clastic sedimentary rock formed by the cementing of rounded cobbles and pebbles that have been worn smooth during transport in streams, rivers or ocean shores. The individual cobbles and pebbles (or clasts) get compacted and cemented together in the same manner as sand grains in sandstone. Breccia is basically the same thing as conglomerate except that its cobbles and pebbles are sharp and angular, indicating that the rock fragments had not been transported very far before being deposited and buried. To make a conglomerate or breccia, you can follow a similar procedure as that used to make sandstone and just add pebbles to your sand mixture:

Materials:

 Paper cups Water
 Sand (from beach or hardware store) Pan or Pyrex measuring cup
 Gravel (smooth & rough pebbles from a beach or river bed, or purchase bags of smooth & rough pebbles  an aquarium supply stores or hardware stores Spoon, dowel, or popsicle stick
 Sodium silicate (also known as water glass) or Plaster of Paris Paper towels

Procedure:

1. Fill the bottom of a paper cup with a layer of sand and gravel about an inch thick. (Give half your kids rounded pebbles and the other half the rougher, angular pebbles.)

2. If using sodium silicate (water glass), make a solution of mineral-rich groundwater in a pan or Pyrex measuring cup by diluting the sodium silicate in water.

3. Pour the groundwater into the sand and gravel mixture and stir it all together with a spoon, dowel, or popsicle stick to make sure all the sand and gravel is wet. However, you don‘t want to make soup, so don‘t pour in too much water!

4. Alternatively, if using plaster of Paris, put a heaping tablespoon of dry plaster into each kid‘s cup of sand and gravel and then add just enough water to be able to stir and mix everything together. (Again, don‘t make soup!)

5. Lightly tap the bottom of the cup on a countertop or desktop to settle the sand, gravel, and water mixture.

6. Set the cup, uncovered and undisturbed, in a sunny, warm open spot to help the drying process. If you poured too much solution, you may find you need to soak up excess water with wadded paper towels after you‘ve allowed the mixture to sit.

7. Once, the mixture has completely dried, tear off the paper cub, and you should end up with a rock that looks and feels similar to the conglomerate or breccia, especially if you break your artificial specimens in half.

Those kids who used the smooth, water-worn pebbles will have created artificial conglomerate. Those who used the rougher pebbles with sharp edges, on the other hand, will have created artificial breccia.

D. Creating a geologic column. The geologic column is the sequence of rocks that document the earth‘s ancient history. For instance, a layer of limestone that‘s capped by a layer of shale that‘s capped by a layer of sandstone might tell of a time when a sea began to retreat. When the sea was deep and clear, it left a deposit of limy, fossil-filled sediments that would eventually become limestone. But as the sea began to retreat and shrink away from its original banks, the floor of the sea would grow muddier from dirt washing in from the land and from swamps and estuaries advancing at the edge of the sea. This mud would eventually become a layer of shale. As the land continued its advance and the sea continued to retreat, a layer of sand from a beach might be deposited over the older layers of limestone and shale and eventually become sandstone.

Geologists study sequences of sediments like this from all around the earth. By studying sedimentary layers, they tease out stories each layer tells about earth history, and they assemble and organize various layers by time into the geologic column, which is like assembling pages in a history book that progresses from ancient history to modern time.

You and your kids can create a small geologic column as follows:

Materials:

 Several capfuls of sand & gravel Petroleum jelly
 Small seashells and thick leaves Waxed paper
 Plaster of Paris Bowl or large plastic cups
 Water Spoon, dowel, or sticks
 Food coloring (red, blue, & green) Apron & paper towels
 Half-gallon cardboard milk carton  

Procedure:

1. Cut the top off a half-gallon rectangular cardboard milk carton.

2. Spread your seashells and leaves across a sheet of waxed paper and lightly coat one side of each seashell and leaf with petroleum jelly.

3. Mix equal amounts of sand and plaster of Paris (about a half to one cup of each) in a bowl or large cup.

4. Add a few drops of red food coloring and water and stir to a thick, smooth consistency.

5. Pour this colored sand/plaster mixture into your milk carton.

6. Take some seashells and/or leaves and gently press them atop your sand/plaster layer with the oiled sides up. (Don‘t bury them completely into the sand/plaster layer; just nudge them in a bit, with the oiled tops showing.)

7. Repeat this process using sand/plaster layers colored by different food colorings (with some layers of no food coloring, just natural sand and plaster), placing oiled seashells or leaves between each layer as you build up a multi-colored layer cake inside the milk carton. For variation, in some of the layers you might mix in some pea-sized gravel along with the sand and plaster. Continue adding different colored layers until the milk carton is filled to the top.

8. Once the milk carton is full, let everything harden for a day or so.

9. When all is dry, peal off the milk carton to reveal your layers of sediment.

10. By tapping between layers with a hammer and chisel, you should be able to split your sedimentary rock into layers to reveal fossils and their impressions in the form of the seashells and leaves you dropped between layers.

E. Observing sedimentation in action. Rather than making artificial examples of sedimentary rocks, send kids outdoors to observe sedimentation in action. For example, they might see:

  • rocks chipping off and piling up at the base of a cliff;
  • a tree with roots growing into and cracking rocks and boulders at an outcrop (or buckling and cracking a cement sidewalk in their neighborhood);
  • a gully cutting into a hillside and carrying away soil, sand, or gravel;
  • sand bars and cobbles piling up in bars along a river bank.

Have kids look around and bring back lists of what they see in the natural environment, including these and any other examples.

F. Observing sedimentation in the lab. Kids can observe sedimentation in the lab with the following activities:

  • Fill a large glass jar or water bottle one-third to one-half full of a mixture of gravel, sand, and dirt. Pour in water to the top of the jar and screw on the cap. Shake vigorously with up-and-down and circular motions. Then set the jar down and allow the mixture to settle. If all goes well, the sediments should have settled in clear layers by weight, with gravel on the bottom, mud on top, and sand in between.
  • Fill a large pan with dirt and tilt it with a brick or wood block under one end. Using a gardener‘s water can, rain water down from the high end to show how erosion occurs on hillsides, carving gullies and transporting sediment downhill with gravity. You might also plant stones in the dirt to show how such obstacles affect the flow of water and erosion.
  • To illustrate the destructive power of water and how—given enough time—water can break down rocks as it expands and contracts between its frozen and liquid states, get a water bottle with a cap. Fill it one-third with cinders (available from the garden supply section of a hardware or garden store) and the rest with water. Cap it and pop it into the freezer. Over the course of several weeks, allow it to freeze and thaw a number of times. Finally, pour the contents into a bowl or pan. What began as cinder rocks should now be a combination of mud, sand, and cinders.

b) Making fossils

Fossils are the remains of past life that got buried within sediments that turned into sedimentary rocks. This includes remains of animals (bones, teeth, shells) or plants (impressions of leaves or stems or petrified wood) or even imprints such as footprints that a dinosaur left on a beach or tubes that worms burrowed through mud.

Kids can make fossil imprints with clay and organic materials they bring in themselves, such as flowers, leaves, ferns, chicken bones, or seashells. Here‘s what they‘ll need:

Materials:

 Self-hardening clay Seashells, leaves, chicken bones, flowers, ferns, or other organic materials
 Paper plates or sheets of waxed paper Vegetable oil or talcum powder
 Rolling pin (optional) Paint (optional)

Procedure:

  • Give each child a sheet of waxed paper or a paper plate and a lump of self-hardening clay.
  • Either with their palms or with a rolling pin, have kids flatten their clay into a thin, even layer about a half-inch thick on the waxed paper or paper plate.
  • Have your kids press their flower, leaf, fern, chicken bone, or seashell gently into the clay and lift it out. (With seashells that have deep ridges or indentations, they first may need to coat the shell lightly with vegetable oil or talcum powder to be able to lift it out of the sticky clay with ease.)
  • Let the clay dry and harden, and each of your kids will have a fossil impression.
  • For a realistic touch with impressions of ferns or other leaves, students can paint the impression with black, brown, or gray paint after the clay has dried. Most plant fossils are carbonized films, and the paint will replicate the film of carbon left on the impression.

Note: For other, somewhat more involved projects to make fossils using clay and plaster, see the back-up page for Activity 3.2. You can use any of these activities to help kids satisfy requirements toward earning both their Earth Processes and Fossils badges simultaneously.

c) Collecting sedimentary rocks

Following are sedimentary rocks kids may be able to collect if they live in the right area of the country, or that they may be able to purchase from rock dealers or to trade through the mail via the AFMS Patricia Egolf Rock Pals program as a club project with kids in other AFMS/FRA clubs who live in areas where sedimentary rocks are common:

  • Breccia is a clastic sedimentary rock composed of cobble- and pebble-sized rock fragments that are sharp and angular, indicating that the rock fragments had not been transported very far before being deposited and buried.
  • Coal originated from compressed vegetation, often derived from swamps, that was buried rapidly in thick masses. High in combustible carbon content, coal-burning facilities are the largest source for generation of electricity.
  • Conglomerate is a clastic sedimentary rock formed by the cementing of rounded cobbles and pebbles that have been worn smooth during transport in streams, rivers or ocean shores.
  • Coquina is similar to conglomerate, but rather than being formed by rounded cobbles and pebbles, it‘s formed by masses of broken seashells, coral fragments, and other biologically-derived materials that are poorly cemented together.
  • Diatomite, a soft chalk-like sedimentary rock, is composed primarily of silica from the fossilized shells of billions and billions of microscopic diatoms, which are algal-like organisms at the base of the ocean‘s food chain. It has many industrial uses as a filter (you‘ll see it in hardware stores with pool supplies), a mild abrasive, and as filler (as in house paints); under high magnification, the individual diatom shells look like snowflakes.
  • Gypsum is a chemical sedimentary rock precipitated from highly saturated salt waters that left behind thick deposits of sulfate hemihydrate. Gypsum is the main ingredient in plaster of Paris and is also used in drywall, so you may well be surrounded by gypsum at this very moment.
  • Limestone is a type of non-clastic, chemical sedimentary rock also called calcium carbonate because of its high content of calcium. It generally forms as a limy ooze precipitated on the ocean floor and includes shells from marine animals.
  • Sandstone is a clastic sedimentary rock formed from the cementing of sand-sized grains, often from minerals in groundwater, along with pressure.
  • Shale is one of the most common sedimentary rocks. It‘s composed of silt, mud, or clay that has been compacted to form a solid rock.
  • Travertine is a form of calcium carbonate (like limestone) deposited through the action of water, such as mineral-rich springs. It‘s often soft and beautifully banded, making it a favored sculpting stone. It‘s also sometimes called onyx and alabaster.

Note: Kids can use this activity toward satisfying requirements for other badges, too: Rocks & Minerals (Activity 1.4) and Collecting (Activity 5.1).