Erosion:the Changing of the Earth's Surface

Time: Two or three fifty-minute class periods

Objectives: Students will be able to identify the different types of erosion and their causes.

Purpose: The purpose of the following lessons and activities is to familiarize students with causes and effects of erosion and for them to realize that this is an ongoing process. Even in the times of Washington these processes occurred. Students should realize that erosion could have affected the surveying outcomes such as boundaries (ex. rivers, creeks, gullies).

Materials: Student workbook and pictures of erosion you supply-two per student for the entire class. Computer for students to access pictures of erosion on the park's Education page at the website (www.nps.gov/gewa).

Procedure: Erosion is the wearing away of earth or rock. Students should have a preliminary understanding of the earth's composition, which is various forms of rock. This may take a quick five minute review if there seems to be any confusion. All land portions of the earth look different. (All underwater bottom profiles are different, too, in case someone asks.) Ask the students, "Why do various places on the earth look different?" This question should be directed toward the students, and they should be able to respond that erosion (by acting on different kinds of rocks) is one cause of why different places on the earth look different. (Start out general, then become more specific.) Agents of erosion are the different things that cause erosion. Wind, water, gravity, and biological (wildlife, plants, and humans) are among the agents of erosion. Water erosion includes tides washing shorelines, rain such as flashfloods, freezing and thawing in rocks, and glaciers (in the Ice Age and presently).

The teacher should use the pictures on the Education page at the web site to describe each picture as an example of erosion. In doing this, it should be mentioned that there are both mechanical and chemical means of weathering-erosion. The mechanical type of weathering is exemplified through rocks of the mountains changing into particles of sand upon reaching the oceanfront. The students should ask "How does this occur?" If a student cannot explain it, then you must. A rock starts out large in the mountains, travels downhill via water with bumping, crushing, grinding, and rounding occurring until it looks as sand looks when it reaches the beach.

Chemical weathering will be more difficult for the students to comprehend. One way is organisms decomposing in rocks, releasing acids and breaking down rocks into soil. Another way is in damp regions moisture dissolves some minerals that may contain acids and change the appearance of surrounding rocks. The best examples of chemical weathering are monuments in cities and historic sites. Monuments erode from acid rain and deteriorate from pollution (biological) and natural elements. Try to differentiate among the change in appearance only, and change in composition. The students should have discussed these types of changes. It is a difficult concept to some and may take ten minutes to review. Ask the students if chemical weathering would occur in a damp environment or in a dry one. (Answer: Damp One). Mechanical erosion can occur in dry and damp environments.

Activities: The students' workbooks do not have the pictures of erosion that are in the teacher guide. Have the students access the Erosion Pictures . These pictures can be printed to pass out to the students to show various examples of erosion. If you have ensured that the students understand this topic, group the students into 4-5 members and supply them with two pictures. Have the students decide if erosion has occurred in each picture and what type of erosion. Have each group present the information for two pictures to the class. Have the students agree or disagree. If the students disagree, have them give reasons why. If this lesson runs out of time for one period, have the students continue the presentations on the following day.

Homework: Have the students freeze a full plastic bottle (with the top on it) of water one night. Have the students take it out of the freezer (to thaw) the next night for homework and write a report on what happened to the bottle. The following day the students are to bring the empty bottles to class with their reports. In class share how some of the bottles look different. Why do the bottles look different? After the students observe the differences, hypothesize how freezing and thawing activity affects rocks. Students should point out that if the plastic bottles look different, then rocks should also look different after this type of event. You can recycle the bottles after this activity.