Activity 4: Faults

WHOSE FAULT IS IT?

Summary:

Earth's crust is broken into many pieces called plates. Within each plate, smaller breaks are found along which movement occurs. These breaks are called faults. Students will use blocks of wood to distinguish among fault types and describe the movement along each type.

Instructional Method: Experiment

Goal: Introduce students to three different fault types and the movements along each type.

Objectives: Students will be able to:

• Sketch the three fault types.
• Describe fault movements using wood blocks.

Time:

Activity time: 30 minutes
Discussion:15 minutes

Materials Needed:

• Angled wood blocks with rough surfaces

Vocabulary:

earthquake
fault
fault plane
normal fault
strike-slip fault
thrust fault
transform fault

Background:

A fracture in rock along which movement occurs is called a fault. Geologists consider faults to be a planar break in rock, along which there has been movement of one side relative to the other. Pieces of crust on either side of a fault are known as fault blocks. Each block moves as an individual unit during earthquakes. It is helpful to imagine breaks in this manner because it allows you to use common objects to simulate fault block interactions.

Faults can be very short or can be miles in length, continuous or intermittent. There are three common fault types, classified according to fault block motion. They are:

 

Images of the three common fault types.

USGS

• Normal faults are distinguished by fault blocks that slide in such a way that one block is down-dropped (lowered) relative to the other block. This happens when the crust extends, or stretches. The Basin and Range in the western United States is a geographic province dominated by normal faults. It encompasses all of Nevada and portions of surrounding states. The Basin and Range is a region with rows of mountains and valleys trending north and south formed by normal faults.
• Thrust faults are distinguished by a package of rock (fault block) that pushes up and over another rock package resulting in crustal thickening. This fault movement happens when plates collide or push together.
• Transform faults (also called strike-slip faults) are distinguished by side to side sliding of fault blocks. Think of a transform fault as two cars, moving opposite directions, passing on the road. A well known example of a transform fault is the San Andreas fault in California. They occur where ever the crust is being stretched or pulled in opposite adjacent directions.

Faults can often be seen at the surface. Many man made items are affected by fault movement: a fence that has been separated, underground pipes that are sheared, or a canal that has been set off course by a fault.

In many places, faults are detected by rock associations. To look for a fault in nature you search for younger rock beside older rock (normal or reverse faults) or older rock on top of younger rock (thrust faults). In many places along a fault plane broken rock (fault breccia) or even powdered rock (fault gauge) are found. Sometimes if the plane of the fault is visible, there are little lines called slickensides on the rock surface showing the direction of fault movement, created by the scraping of the rocks against one another. Large fault planes are often exposed at the surface and can be seen cutting across hills and mountains.

When movement occurs between two fault blocks we feel an earthquake. For more information about earthquakes and movement caused by earthquakes, link to Surfing Rock Waves. The following activity uses wood blocks to experiment with fault movement.

Instructional Procedures:

 

1. Divide class into groups of two to three students.
2. Provide each group with blocks cut into varying angles. (You may want to paint 1 inch lines on the block representing rock layers.)
3. Have students slide the blocks together and apart simulating each type of fault movement: normal, thrust and transform.
4. As one student is performing the interactions have another student place a finger on a block to feel the friction caused by each interaction.
5. Have each student create all three fault type movements individually while the other students observe.
6. For each fault type have the students sketch the rock movements. Label the up or down movement of each block using arrows and label the fault plane. An example of the desired drawings can be seen above in the USGS drawings.

Discussion:

Are there any faults in your area? If so, do you know which type is near you? Which type of fault would result in the most damage? Why? How do you know?

Variation:

Play-doh can be used in place of wood blocks. Use three different colors. Stack the colors forming layers. Once the layers are formed and stacked, cut a diagonal or vertical line in the middle of the dough to represent the fault surface. Now use the two blocks of Play-doh to perform all the fault movements.

Included National Parks and other sites:

Aniakchak National Monument
Bryce Canyon National Park
Denali National Park
Glacier National Park
Great Basin National Park
North Cascades National Park
Point Reyes National Seashore

Photos:

Normal fault near Bryce Canyon
San Andreas Fault
Thrust Fault

Utah Science Core:

5th Grade Standard 2 Objective 1,2,3

 

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