How Sedimentary Rocks are Formed

To answer a central question: How are the stripes of sedimentary rocks formed?

Middle-school students should be able to recognize the rock cycle as erosion, transport, and deposit. When prompted, they may share activities they have done, including using rock tumblers to create smooth rocks simulating erosion; picking up rocks along the beach as an example of transport; and making a fossil from clay to help them with the idea of rock layers as deposit.

This lesson briefly reviews the rock cycle in general, then focuses on sedimentary rocks. Research tells us: "It is especially important that students come to understand how sedimentary rock is formed periodically, embedding plant and animal remains and leaving a record of the sequence in which the plants and animals appeared and disappeared." (Benchmarks for Science Literacy, p.73.)

In this lesson, students investigate the stripes in sedimentary rocks using a structure at Petra, in Jordan, as an example, and then do a hands-on activity using mixed nuts to illustrate the layering of sedimentary rocks. Throughout the lesson, students are asked to reflect on the central question: How are the stripes of sedimentary rocks formed?

Materials:

• Can(s) of mixed nuts with at least 3-4 types of nuts in the can. Brazil nuts must be the largest type of nut in the mixture. Note: Check for nut allergies before beginning the lesson.
• 1 wide-mouthed glass jar (approximately a liter or quart)
• Uncooked rice to fill the glass jar about three fourths of the way to the top
• Small rubber ball
• Paper towels or napkins
• How Sedimentary Rocks are Formed student E-Sheet

As directed on the student E-Sheet, have students view the rock cycle diagram found on the Nebraska Rocks and the Rock Cycle page of the Nebraska Earth Science Education Network. Ask a volunteer to explain the diagram. If computers are not available for students, you could draw the diagram on the board leaving out the key words (igneous, sedimentary, and metamorphic) and ask a volunteer to fill in the words and explain the diagram.

Once the review of the rock cycle is complete, tell students that they will focus on just one type of rock in this lesson—sedimentary rock.

Have students view the images of the sandstone structures at Petra by visiting the Science NetLinks online resource, Images at Petra. If access to online images is limited, color copies of the photographs can be distributed. If possible, show the location of Petra using a world map.

While looking at the photos, ask students to answer the following questions:

• What do you think this is? (Students should recognize that it is something, possibly a building, carved out of stone.)
• What type of sedimentary rock is this? (Sandstone.)
• Why do you think there are so many colors in the stone? (Students should understand that over time, many different plants and animals died and were buried here along with different kinds of rock sediments.)
• Can you describe the pattern on this building? (Striped.)
• Does this support the idea that layers of sediments exposed to heat and pressure form solid rock? (Yes.)

If the resources are available, have students make observations of sedimentary rock using a rock sample kit. Have them draw and record the name, color, size, shape, and texture of each rock sample. Ask them to identify the sample that most closely resembles the structure at Petra (a sandstone sample should be identified).

Note: A possible resource for rock kits that include sedimentary rock samples is the 24 Hours7Days website.

Finally, tell students that the rest of this lesson will focus on how stripes are formed in sedimentary rocks. Tell them that they will discuss how different kinds of sediments, sand, and other small particles layer and eventually become "striped" rocks.

Write the central question for this lesson in a visible place: How are the stripes of sedimentary rocks formed?

Have students read Stratification And Segregation: Why Does Some Sandstone Have Stripes? on the SEED: Schlumberger Excellence in Educational Development website. They can access it online through the student E-Sheet or you can print it for them ahead of time. At this time, students should not follow the links within the article.

Have students record notes on the article in the following way:

A sheet of lined paper should be divided in half vertically. The left side of the page should be labeled, "Information about Sedimentary Rocks." Students should record a bulleted list of notes from the article. On the right side of the page, students should draw a picture/diagram to illustrate just one important concept from the left column (chosen by you or the student).

The following are sample responses for the left side of the paper (Information about Sedimentary Rocks):

• wind blows over thousands of years
• sand deposits on ocean floors, lakes, rivers
• sand mixes with quartz or calcite
• pressure over time causes them to become rock
• not all sand is the same
• the structure at Petra is at least two colors of sand
• maybe smaller grains of sand layered under larger grains of sand
• Hernan Makse theory that a mixture of different kinds of sand separated into layers over millions of years
• similar to process that occurs in avalanches

Have volunteers share their notes and drawings. In order to assess student understanding and work so far, be sure to check all of the students' responses to this activity.

Note: This method of interacting with the material has been very successful in the writer of this lesson's middle-school classes. The process of reading, taking notes, and drawing pictures of the information has worked to stimulate thinking.

Then ask the central question of the lesson, checking to be sure that student understanding is on target: How are the stripes of sedimentary rocks formed?

At this time, students should suggest that layers are formed by the wind, or shuffling of different kinds of sand over time, mixed with minerals like quartz and calcite, exposed to heat and pressure. They may refer to the formula in the article read earlier: accumulated sand + cementing minerals + pressure + time = sandstone.

Now have students once again access the article, Stratification And Segregation: Why Does Some Sandstone Have Stripes?

Review Hernan Makse's theory with the students, and have them click on the Hernan Makse link toward the bottom of the article to read about the background of this scientist.

Next, to demonstrate the Hernan Makse theory that suggests particles separate, settle, and form layers based on their size, try the following hands-on activity: The Brazil Nut Effect-Experiment. This activity is simple, fun, and demonstrates the principle to a middle-school student. Begin the activity by using uncooked rice, a small rubber ball, and a glass jar. Follow the steps outlined on the site, and ask students the accompanying questions. In addition, when the ball comes to the surface, ask the students why they think that happend. (Some students will suggest the ball is lighter than the rice and "floated" to the top, while others may think it came to the top because it is larger than the rice.)

Tell students that they will investigate this idea further using a can of mixed nuts. Show them a can of mixed nuts as well as a sample of each type of nut in the mixture. Have them rank the different kinds of nuts according to size, from largest to smallest. Depending on the available resources, you can divide the class into small groups (giving each group a can of mixed nuts), or continue with this activity as a whole class.

Following are instructions for small groups; these can be easily modified for whole group instruction:

Ask each group to open the can carefully and record the number of each kind of nut on the surface. Then ask the following questions:

• If we shake the can from side to side, what type of nuts do you think will settle on the top of the can? Why? (Accept all answers.)
• What type of nut is the largest? (Brazil nut.)

Have students remove only the Brazil nuts from the top layer and place them on a paper towel or napkin. Then have them replace the lid and gently shake the can side to side for one minute. When the time is up, have them open the can and record the number of Brazil nuts on the surface, and then remove them. Have them repeat this process three more times and record their results.

Now ask the following questions:

• What type of nut traveled to the top every time? (Brazil nuts.)
• Why? (Because they are the largest.)
• What kind of nut ended up on the bottom layer? (The smallest; most likely peanuts.)
• If you had a mixture of sand and pebbles in a jar and shook it, which material would most likely end up forming the bottom layer? (Sand.)
• Why? (The grains of sand are smaller.)

As directed on the E-Sheet, have students look at Images at Petra once again and ask the central question: How are the stripes of sedimentary rocks formed?

Discuss this as a class, and/or have the students write a short reflection on how this lesson helped them better understand how sedimentary rocks are formed. In either case, students should be specific and cite information learned in this lesson.

In general, students should now clearly understand that smaller pieces of sediments form the bottom layer of sedimentary rock, larger sediments form the next, and so on. Over time, these layers are mixed with cementing substances, exposed to heat and pressure, and form rock.

The Schlumberger SEED website used in this lesson contains several other activities and articles for students to explore.

For example, to explore why sedimentary rocks have stripes, have students read the article, Stratification and Segregation - Avalanches. After reading the article, ask students to explain why larger rocks end up at the foot of the slope and smaller rocks and sand are found higher on the slope. Their answers should refer to size and shape as contributing factors.

In addition, you could have students read the article Spontaneous Stratification and Segregation. In this article, students will view a model of the Hele-Shaw cell, which Hernan Makse used to explore how certain mixtures of two types of grains spontaneously stratify and segregate.

Students could view the slide show on Sedimentary Rocks at the Volcano World website. Using rock samples in your classroom and a digital camera, students could create their own slide show and share it with family and classmates.

Students could start a rock collection of their own. They could identify their samples by size, shape, texture, and other important features using classification information found at Rocks and Minerals, provided by the Franklin Institute.

Created :07/23/2001