I call students to the gathering area. We review how erosion occurs. We discuss how erosion can occur rapidly or over a very long time. I tell students that today we will look at a well-known example of erosion that occurred over thousands of years – The Grand Canyon.
I ask students to tell me in which state the Grand Canyon is located. Students should be able to answer that the Grand Canyon is located in Arizona. I show students a class map of the United States. I ask them to locate Arizona on the map. If students have individual desk maps, they can use those to locate Arizona. My maps are plastic so I have students circle Arizona with a washable marker.
I show students a digital map of Arizona http://www.mapofarizona.net/. I use this map as it allows me to zoom in on the Grand Canyon and show students the Colorado River that flows through the Grand Canyon. We trace the course of the river from east to west through the Canyon.
I divide students into groups of four and provide them with a laptop each. I ask them to go to a digital photograph of the Grand Canyon. Students can also use these photos of the Grand Canyon. Photograph 4 is very useful for the following discussion.
I ask students to brainstorm in their groups how the Grand Canyon was formed. I use a Kagan Placemat strategy where students write their own brainstorms in their quadrant. After two minutes groups convene and write their group’s consensus to the question in the center of the placemat.
Students read information about how sedimentary rocks are formed. They can take notes in their interactive science notebooks as they read.
I call students attention back to me. I explain to them that rivers, lakes, and oceans all have small pieces of sand and rock that float in the water and are carried by the currents. When they settle on the bottom of the river, lake, or ocean, they might stay there and be buried by other pieces of rock or sand. Each of these layers that are left on the bottom has the color and texture of the rock and sand that was left there.
I have students look at the photograph of the Grand Canyon again. I tell them that the layers of rock were deposited there in the way we just talked about. I ask them to try to count the layers. I ask them what different colors they might see in the layers. I ask them why they think the layers are different from each other.
I call students to the gathering area. We discuss that each of these layers built up over time. However, the Colorado River eroded through the layers of rock over time to form the Grand Canyon.
I ask students to think about rivers they have seen. What do the riverbanks look like? Do they reveal layers like the ones in the Grand Canyon? We discuss the reasons why some riverbanks reveal distinct layers, while others just appear muddy or sandy.
Students should realize that not every riverbank reveals layers that were deposited by ancient oceans. Some rivers just cut through the same type of rock from top to bottom, while others cut through soil with high clay content, making the entire riverbank appear muddy. Other riverbanks are so heavily lined with plants that any layers present are impossible to see.