Modeling Rock Deformation: Fun with Faults!
Materials Needed:
- Several blocks of different colored modeling clay or play-doh (enough to make a layered block about 10x10x5 cm)
- Plastic knife or clay cutting tool
- Ruler
- Notebook and pen/pencil
- Optional: A smartphone with a level app and compass app OR a physical Brunton compass/clinometer (for demonstration)
- Optional: A sturdy book or board
Lesson Activities:
1. Introduction: Stress, Strain, and Deformation (15 mins)
Let's start with the basics! Rocks deep within the Earth are under immense pressure and temperature. Forces acting on these rocks are called stress. How rocks respond to this stress is called strain, which results in deformation (a change in shape, size, or orientation).
Discuss: What happens when you squeeze play-doh? (It deforms). What happens if you try to bend something brittle, like a dry twig? (It breaks). Rocks behave similarly!
- Elastic Deformation: Temporary change, rock returns to original shape when stress is removed (like a rubber band).
- Ductile (Plastic) Deformation: Permanent change in shape without fracturing (like bending metal or clay). Often occurs deeper in the Earth where it's hotter and pressure is higher. Results in folds.
- Brittle Deformation: Fracturing or breaking of rock. Occurs closer to the surface where rocks are colder and pressure is lower. Results in faults!
Today, we're focusing on brittle deformation and the resulting structures: faults.
2. Modeling Faults (30-40 mins)
Time to get hands-on! Create a block using different colored layers of clay/play-doh to represent sedimentary rock layers. Make the block roughly rectangular.
Fault Definition: A fault is a fracture or zone of fractures between two blocks of rock. Faults allow the blocks to move relative to each other.
Activity: Model the Main Fault Types:
- Normal Fault: Caused by tensional stress (pulling apart).
- Gently pull the two halves of your clay block apart slightly.
- Make a diagonal cut through the block (this represents the fault plane).
- Let the block that's 'above' the cut (hanging wall) slide DOWN relative to the block 'below' the cut (footwall).
- Observe how the layers are offset. This often happens in areas where the Earth's crust is stretching, like the East African Rift Valley. - Reverse Fault (and Thrust Fault): Caused by compressional stress (pushing together).
- Gently push the two halves of your block together slightly.
- Make a diagonal cut through the block (fault plane).
- Push the blocks together so the hanging wall slides UP relative to the footwall.
- Observe the offset layers and the shortening/thickening of the block. This happens where tectonic plates collide, forming mountain ranges like the Himalayas. (A Thrust Fault is just a low-angle reverse fault). - Strike-Slip Fault: Caused by shear stress (blocks sliding horizontally past each other).
- Make a vertical cut through the block.
- Slide one half of the block horizontally past the other half, parallel to the cut.
- Observe how the layers are offset horizontally. The San Andreas Fault in California is a famous example.
Discuss the type of stress associated with each fault type as you model it.
3. Introduction to Strike and Dip (15 mins)
How do geologists describe the orientation of these rock layers or faults in the real world? They use strike and dip.
- Imagine a tilted rock layer (or use a tilted book as an example).
- Dip: This is the angle that the rock layer tilts downwards from the horizontal. It's measured in degrees (0° for horizontal, 90° for vertical). It also includes the direction of tilt (e.g., 30° towards Southeast). Use the level app (clinometer function) to demonstrate measuring this angle on the tilted book.
- Strike: This is the compass direction of a horizontal line on the surface of the tilted layer. It's perpendicular to the dip direction. Imagine water filling up to the level of the tilted book - the line where the water meets the book surface is the strike line. Use the compass app to show the direction of this imaginary line.
Understanding strike and dip is crucial for mapping geological structures and understanding the subsurface.
4. Wrap-up and Application (10 mins)
Review the three main fault types and the stresses that cause them. Briefly discuss how recognizing these structures is important in fields like civil engineering (building stability), resource exploration (oil, gas, minerals often trap near faults), and hazard assessment (earthquakes occur along faults).
Assessment/Check for Understanding:
- Can you sketch the three main fault types and label the relative movement?
- What type of stress causes each fault type?
- In your own words, what do 'strike' and 'dip' tell us about a rock layer?
This practical understanding of faults and orientation is fundamental for interpreting geological maps and cross-sections, a key skill in structural geology!