• Assess the general knowledge of the class through the large-group discussion of sedimentary rocks.
• Observe student worksheet responses to assess student understanding of the relationship between rock physical properties and identification.

• Collect and review individual performance on the closing question: How over time can a beach become a clastic sedimentary rock? in order to assess student comprehension of sedimentary rock forming processes.

“Today we’ll be working with sedimentary rocks, examining specimens of the rocks and investigating how they form. Just from their name, where do sedimentary rocks come from? Right, sediments. The word sediment comes from the Latin word sedimentum, which means a settling or sinking down. What are sediments and where do they come from?” Facilitate a class discussion to activate and assess prior knowledge. In general, sediments are loose materials—like pieces of rock, mineral grains, and the remains of once-living plants and animals—deposited on the Earth’s surface by wind, water, and ice. Sediments can also form through chemical reactions and precipitate from oceans, lakes, and rivers. “Most sediments come from pre-existing rocks that are weathered (broken down into smaller pieces or dissolved) and eroded (moved by wind, water, or ice). Other sediments may come from the remains of once-living plants and animals.”

Divide students into small groups and distribute the Sedimentary Rocks Worksheet (S-6-1-2_Sedimentary Rocks Worksheet and KEY.doc) and the sedimentation tubes. Instruct students to leave the tubes undisturbed and note their observations for questions 1 and 2 in Part I of their worksheets. Monitor students, providing feedback. Distribute the magnifying lenses and plastic bags containing the samples of the solid materials in the sedimentation tubes. Instruct students to complete questions 3 through 5 in Part I of the worksheet. Monitor students, providing feedback. Facilitate a short class discussion of group findings.

Share with students pictures of sedimentary rocks (S-6-1-2_Sedimentary Rock Pictures.doc). “Clastic sedimentary rocks are made of broken pieces of other rocks, pieces that look like mud, sand, and gravel. The word clastic comes from the Greek word klastos, which means broken in pieces.” Instruct students to complete questions 1 through 4 in Part II of the worksheet. Monitor students, providing feedback. Distribute the boxes of sedimentary rock specimens. Allow students time to examine the specimens. “Two of these rock specimens are samples of clastic sedimentary rocks. Remember, clastic sedimentary rocks are frequently made of rock particles like mud, sand, or gravel. Which two rock specimens do you think are clastic sedimentary rock samples? Why?” Monitor students, providing feedback. Prompt students to compare the rock specimens to the materials in the sediment tube. Instruct students to answer questions 5 and 6 in Part II of the worksheet. Facilitate a short class discussion of student ideas.

The sandstone and shale are the clastic rock samples. The sandstone will be easier for students to pick out because of the grain size. The shale is composed of much smaller sized particles, like mud, so students will not be able to see the individual grains. Relate the shale to the muddy sediment in the sediment tube. A photomicrograph (picture taken through a microscope) similar to the ones found at http://www.uta.edu/paleomap/homepage/Schieberweb/Picture%20Pages/shphoto3.htm may help students distinguish the grains.

“So, clastic sedimentary rocks start as sediments similar to the soil, sand, and gravel we have in the sediment tube. These sediments came from the wearing down (weathering) of pre-existing rocks. How do these loose sediments become sedimentary rocks like the sandstone and shale specimens we examined?” Facilitate a class discussion, noting the steps on the board. Lead students to understand that the sediments are eroded (moved) and deposited (laid down) by running water, wind, or ice. The sediment particles in these sediments become squeezed together (compacted) by the weight of the overlying sediments that are being deposited on top of them. These buried, compacted sediments are then cemented together by natural processes when minerals crystallize around the particles and bind them together, forming a sedimentary rock. Instruct students to complete question 7 in Part II of the worksheet. Monitor students and provide feedback.“Sedimentary rocks that come from organic sediments are called organic sedimentary rocks. Biology is the science of living things, so organic sediments come from living things. Examine the limestone rock specimen and complete questions 1 and 2 in Part III of the worksheet.” Monitor students and provide feedback.

“The most common type of limestone is formed from the remains of marine (ocean) organisms like clams, snails, corals, and foraminifera (tiny one-celled organisms that float in the upper part of the oceans; see pictures at http://www.ucmp.berkeley.edu/foram/foramshapes.jpg . The shells and skeletons of these organisms precipitate (solidify) out of calcite found in ocean water. Corals precipitate skeletons; clams, snails, and foraminifera precipitate shells. When these organisms die, they leave their shells and skeletons as organic sediments after their soft parts rot away.” Note: Commonly, the shell and skeletal material is broken down and recrystallized by natural processes until it does not resemble the original shells and skeletons, even when viewed with a microscope.

“I’m going to model how calcite can be precipitated. I’m going to blow through the straw, bubbling carbon dioxide through a solution of limewater. I made the limewater by dissolving lime, the white, powdery chemical in this plastic bag (hold up the bag for students to view; do not pass it around for students to handle). In this model, the lime represents limestone rocks that are dissolved by rivers and streams. Limestone rock is made of the mineral calcite. This dissolved limestone (calcite), is dumped into ocean waters by the rivers and streams. The carbon dioxide from my breath represents the carbon dioxide from the air, which reacts with the lime (or ‘calcite’ in this model) dissolved in the water to create the white solid flakes (precipitate) of calcite.” Explain that the limewater irritates skin and eyes, so you are wearing safety goggles, gloves, and apron. Caution students to stay back while you are blowing through the straw so the limewater does not splash into their eyes or get on their skin. “Watch for the flakes of calcite to form as I blow through the straw.”

“So, marine organisms like clams, snails, corals, and foraminifera precipitate calcite out of the ocean water to make their shells and skeletons. And then what happens to make limestone rock?” Facilitate a class discussion of how this type of limestone forms, guiding students to learn that after the organisms die, their shells and skeletons sink to the ocean floor and become buried by other sediments. These organic sediments turn into rock (lithify) through the same processes as discussed earlier for clastic sedimentary rocks (compaction and cementation). “Answer question 3 in Part III of the worksheet.” Monitor students, providing feedback.

“The last rock specimen we’ll examine today is bituminous coal.” Instruct students to examine the coal specimen and complete question 4 in Part III of the worksheet. “Bituminous coal is also a organic sedimentary rock. What is coal made of?” (large, thick accumulations of plant material) Instruct students to examine the coal specimen and complete question 5 in Part III of the worksheet. “These thick accumulations of plant material are the organic sediments that become the organic sedimentary rock, coal. Why do you think the coal is black in color?” (The carbon from the plant material makes it black.) “Knowing that bituminous coal is a sedimentary rock, how do you think coal forms?”

Facilitate a short class discussion. Instruct students to complete question 6 in Part III of the worksheet. Monitor student completion of questions 4 through 6. Provide feedback and guide students in understanding the process. Lush, thick plant growth of a wetland (swamps and peat bogs) dies. Plant material (like leaves, twigs, and branches) falls into the soggy soil. This plant material is rapidly buried and immersed in water so that it is protected from the bacteria that would make it rot. (The bacteria need oxygen and the rapid burial and immersion cut off the oxygen supply to the bacteria.) The plant material continues to build up, creating peat (a porous brown mass of organic material in which you can still see plant parts). As more plant material keeps falling down, it buries the peat deeper and deeper. The weight of this overlying material squeezes the plant material in the peat closer and closer together (compaction). With continued burial (compaction) and increase of heat, the peat becomes lignite (a very soft, brownish-black coal-like material) and then bituminous coal (soft coal, like the specimen from the sedimentary rock box) with further burial.

Instruct groups to return the activity materials, clean up their work area, and wash their hands. “Let’s go back to our desks, now, and rethink what we know about the sedimentary rocks we investigated today. We’ll start with sandstone, shale, and limestone.” Display the unsequenced steps necessary for a pre-existing rock to become a clastic sedimentary rock (in this case, sandstone or shale) or an organic sedimentary rock (in this case, limestone).

Sedimentary Rock Notes

• Formed when sediments are compacted and cemented together at or near the Earth’s surface.

• Most characteristic feature is stratification, or layering of sediments depending on the sediment’s size, color, and kind. Layers are referred to as strata

Types of Sedimentary Rocks

1. Clastic

• Composed of other fragments of rocks and minerals

• Classified by the size of the fragments they are made of

• Arranged from largest to smallest

• ex.: Conglomerate (larger size pebbles), sandstone, siltstone, shale

2. Chemical

• Formed from solutions of minerals and water

• Allows water to dissolve minerals away from rock as water passes through it; minerals then crystallize out of the solution

• ex.: Rock salt, chemical limestone, iron ore (hematite)

3. Organic

• Formed from the remains of dead organisms

• ex.: Coal, fossiliferous limestone

Facilitate a class discussion to construct the correct sequence of the rock-making processes. Distribute the Sedimentary Rock Graphic Organizer (S-6-1-2_Sedimentary Rocks Graphic Organizer and KEY.doc). Leave the unsequenced rock processes posted. Monitor students and give feedback as they construct the sequence in the correct order on their graphic organizers.

“Let’s take another look at the processes in the graphic organizer. How are the rock-forming processes similar for the two types of sedimentary rocks (clastic and organic)?” Facilitate a class discussion leading students to understand that the process begins with a pre-existing rock that gets weathered. Then this weathered material is eroded (moved) to where it can be deposited and buried. Next the material is lithified (turned into rock) by compaction and cementation.

Weathering Erosion Deposition Lithification

(compaction/cementation)

“How are these processes different?” Guide students in understanding that the clastic sediments come from the physical breaking of the pre-existing rock into smaller pieces. The organic sediments (in this case for limestone) come from the dissolving of the pre-existing rock and then precipitation of the dissolved material (in this case calcite) as shells and skeletons by marine organisms.

“How is the formation of coal similar to the rock-forming processes of clastic sedimentary rocks and of other organic rocks (in this case, limestone)?” The pieces of plant material come from a pre-existing source (in this case, plants growing in a wetland area). The dead plant material is deposited and buried when it falls down accumulates in layers on the wet ground. This material is lithified (turned into rock) by compaction (from the weight of overlying plant material) and cementation (changes in the plant material due to burial and heat).

“Explain how over time a beach could become a clastic sedimentary rock. Hint: Use your Sedimentary Rock Graphic Organizer to determine which part of the sedimentary rock forming sequence the beach represents (erosion, weathering, deposition, lithification). 1) State which part of the sequence the beach represents. Explain your reasoning. 2) Explain the Earth processes that formed the beach.
3) Explain the Earth processes that might change the beach into a sedimentary rock in the future.”

Instruct students to answer the question posted in complete sentences. Post a sketch of a river running downhill to a beach area along an ocean for students to refer to while answering the question.

Extension:

• Students who might need opportunities for additional learning can create a modified graphic organizer to supplement the sedimentary rock organizer used in the lesson. Students can make a linear graphic organizer out of a common process (i.e., baking brownies) to use as a comparison to the process of sedimentation from start to finish. Students can add their own annotations to the sedimentation graphic organizer, defining key terms and steps in the process in their own words.

• Students who may be going beyond the standards can research foraminifera and coral reef environments, relating their findings in essay form to the formation of limestone rock layers.