PLANAR PLATEAUS - CREATING FLAT-TOP MOUNTAINS
Have you ever seen a flat-topped mountain? How are they created? In this activity, students see how broad uplifted areas breakup, forming plateaus such as those on the Colorado Plateau.
Instructional Method: Experiment
Goal: To present an example of a landform that resulted from plate tectonics.
Objectives: Students will be able to:
- Identify a plateau in nature or from photos
- Describe how plateaus are formed
- Draw a picture of a plateau
- Name the region of the United States containing many plateaus
Preparation: 15 min.
Activity: 20 min.
Discussion: 15 min.
- Moist sand
- Student's hand
- Newspaper for cleanup
- Wood blocks
- Large bucket
- 5 lb. ice block
Plateaus are broad flat-topped mountains with steep sides. They form due to a number of tectonic forces. This activity will discuss four different ways plateaus form.
As upwelling magma pushes its way toward the surface but does not break through, it can push up large flat areas. This magma plume can be created from a melting subducted plate. Scientists believe that this is one process that helped to formed the Colorado Plateau. (Plate Tectonics concepts can be further examined by connecting to the Plate Tectonics Unit page.)
Colliding crustal plates can form mountains and plateaus. The compressed crust buckles like the hood of a car in a collision. Mountains form nearby the collision, while planar uplifted areas form farther away from the collision source. An example of this is found on the Mongolian Plateau in Asia. As India collided with Asia, the Himalayan Mountains were buckled and uplifted like the hood of a crashed car but the area behind the collision zone is undeformed, creating the Mongolian Plateau.
Alleviating overburden or lightening of the crust by erosion allows large planar surfaces to uplift. If a large amount of heavy sediment is removed from a region, that area of the crust becomes lighter and rises as a result. This same process occurs to melting glaciers. As the glacier mass diminishes it becomes lighter and sticks higher into the air. For plateaus to uplift, the entire area must erode continuously.
As a plateau region elevates, stresses cause breaks in the crust known as faults. A fault often separates smaller plateaus from one another on the Colorado Plateau. As a result of faulting, some of the uplifted regions are pushed higher than the surrounding area.
Erosion helps to define one plateau from another. Rivers follow the path of least resistance, where rocks are weakest. Rock near faults tend to be weak and broken. Flowing water easily cuts into broken rock along a fault and over time forms a broad river valley. Separation by the new valley allows us to easily differentiate between plateaus.
A large concentration of plateaus is found within the Colorado Plateau. It is an uplifted region broken into many smaller plateaus. It comprises portions of four states: Utah, Arizona, New Mexico and Colorado. All of the Colorado River drainage basin is located on the Plateau, and thus its name.
Smaller structures resembling plateaus are known as mesas and buttes. Mesas are like plateaus, only smaller. Buttes are even smaller than mesas. On the side bar there is a list of National Parks found on the Colorado Plateau. Each National Park protects a unique feature created by the Plateau uplift. The following activity allows students to create plateaus in a few different ways.
1. Sand plateaus (represents upwelling forces)
- Spread newspaper on table.
- Place a student's hand flat on a table.
- Pour sand over student's hand, forming a layer 2 inches thick. Make sure sand is 5 inches around the student's hand.
- Compact the sand so that it is flat.
- Have the student lift his/her hand upwards a little. (This is represents magma upwelling in the mantle). Repeat the process multiple times with more than one hand in the sand.
- Have students sketch what happened to the sand when the student's hand moved upward.
2. Wood block plateaus (represent compressional and extensional forces)
- Now experiment with wood blocks.
- Cut wood into angled pieces of 45 degrees or more.
- Drill a hole through each piece so the holes line up.
- Slide a rope through the blocks and tie a knot at each end. Make sure to leave 3 to 4 inches between the knot and the end of the block.
- Push them together and pull them apart. (This represents crustal compression and extension).
- Have students note what happens when the blocks are pushed together and pulled apart. What type of block structures are formed?
3. Ice uplift (represents removal of overburden)
- Fill large five gallon bucket with water 5 inches from top.
- Place ice block into water.
- Measure how high the ice block sticks out of the water.
- Take block out of water and cut a v into the side of the ice block. Do not let it break in half.
- Replace the ice block into the water and re-measure how high it sticks out of the water.
Explain to students that their action in uplifting the sand cake is taking the place of magma rising to the surface of Earth. The irregular uplifting caused by the students resulted in small breaks in the sand. In nature these would be called faults. Ask students how erosion may separate the broken sand cake into plateaus. How can the Colorado Plateau be more than one plateau? Why is the entire region uplifted? What can happen to land that is uplifted? Do plateaus always have flat tops? Why? Ask students to draw sketches of the blocks and label plateaus with sides and top.
Instead of using your hand to show how the plateau formed, you can use the sand to allow students to make a plateau. Have students draw a plateau on paper and then sculpt a plateau out of sand. Have them imagine that their hands are erosional forces carving the sand away. On their picture write what forces their hands represent, i.e. rivers, ice, gravity, etc.
Included National Parks and other sites:
Arches National Park
Bryce Canyon National Park
Canyonlands National Park
Capitol Reef National Park
Cedar Breaks National Monument
Grand Canyon National Park
Hovenweep National Monument
Mesa Verde National Park
Zion National Park
Utah Science Core:
4th Grade Standard 4 Objective 1,2
5th Grade Standard 2 Objective 1,2,3