Brittle vs. Ductile
A. Teacher demonstration and introduction of ideas.
- In Geology, we talk about the deformation of materials, by which we mean the response of the material to being stretched, bent, squeezed, or otherwise stressed. You can demonstrate material deformation by using a rubber band (stretch and release) and a pencil (bend and release; bend too far -- snap!). When a material is stressed past its strength limit and breaks into separate pieces, as with the pencil, then the deformation is brittle.
- Next, try some silly putty. S-t-r-e-t-c-h it with your hands. You can contort, or deform, the silly putty into extreme shapes without ever breaking it. The deformation is ductile, i.e. the silly putty has not broken into separate pieces.
- Leave your large blob of silly putty prominently on table at front of room.
B. Student activity to investigate these concepts further.
- Silly putty
- Tootsie rolls
The night before, put half the silly putty and all the tootsie rolls in the freezer. (Keep cold in a cooler prior to activity.) The other half of the silly putty should be at room temperature.
- Give each student a blob of room-temperature silly putty. Ask the students to demonstrate ductile deformation of their silly putty. After they have some fun, ask them if they can make the silly putty behave in a brittle fashion. Some of them may get the idea of pulling quickly on the putty.
- Hand out the cold silly putty. Have students try to deform it - it will break if very cold. Encourage students to continue deforming the putty; make observations of putty behavior as it slowly warms up. Once warm, have students pull quickly on the putty. What happens?
- Hand out tootsie rolls. Have students deform tootsie roll in mouth - what happens as the tootsie roll heats up in their mouths?
What controls/influences how materials behave when stressed?
Materials are not inherently brittle or ductile. The material response depends upon conditions of deformation - the temperature and the rate at which stress is applied!
Show rocks which have been ductilely deformed (the most obvious to children are layered rocks in which the layers have been deformed into folded shapes).
Most people think of rocks as brittle materials, i.e. ones which will break if hit hard (demo: break rock with hammer). This is because at Earth’s surface, where it is relatively “cold”, rocks mostly behave brittlely. Students may be familiar with blasting rocks to make highways or tunnels, or blasting in rock quarries. Some students may be familiar with old cemeteries, where old marble benches or markers have ductilely deformed under their own weight.
- Point out your silly putty on front table - what has happened to it? It has flowed under own weight, which rocks can do too.
What is an Earthquake?
A. Teacher demonstration and introduction of ideas
- Ask students what “earthquake” means -- the sudden movement of Earth’s surface. The occurrence of earthquakes depends upon brittle behavior of rocks, and the ability of rocks to store energy. Stretch a rubber band between two hands for students to see. The extended rubber band has stored energy: let go of the rubber band with one hand; it returns to its original size and shape, releasing the stored energy. Now stretch the rubber band again, but keep stretching until it breaks-- ow --(the students love this) - except for the break in the band, it returns to its original size and shape, i.e. the energy was again released.
- Rocks, like the rubber band, can store energy when they are stressed, and if the stress is removed, the rocks release the energy, returning to their original shape. Like the rubber band, if rock is stressed beyond its strength, it will break, suddenly releasing the stored energy. This is the key to earthquakes -- the sudden release of stored energy, breaking rock, or slipping rock past other rock. Rock must therefore behave brittlely in order to cause an earthquake. What happens to the energy when rocks break?
- Demo: Hold up a pencil. Tell students to shut eyes. Break the pencil. Tell students to open eyes, and ask them what happened. They of course will say you broke the pencil. Ask them how they know. They heard it. What does that mean? How did the information get to their ears from the pencil? Discussion should lead to idea of energy waves traveling away from break in all directions, impacting on each of their eardrums. Analogy: the energy from breaking/slipping rock travels out in all directions through the earth, in the form of waves. We can not “see” sound waves, but we can see another type of wave...
B. Teacher demonstration
This demo is best done on an overhead projector, but could also be done with students gathered around a central table.
- Large area container of water (oblong baking dish works well; transparent for overhead projector)
- Pebble or similar small object
- Place container of water on overhead projector.
- Wait until water is still, then drop pebble into water.
- What happens? Waves move out in all directions from the source of the disturbance.
The water waves move out from the source in all directions on the water’s surface. In Earth, the energy waves move out in all directions through the Earth (3-D!) from the break (called the focus of the earthquake) and so the waves will reach the surface all over the Earth, if the energy released is great enough.
- If using oblong dish, ask students to observe where waves “hit” first.
- The waves hit the nearer sides first, obviously taking longer to travel a greater distance. It is like this on Earth. Seismograph stations, where instruments are set up to record the arrival of earthquake waves, which are closer to the earthquake focus will record earthquake waves before stations which are farther away. By knowing the velocity of earthquake waves, geologists can calculate the distance the station is from the earthquake. By determining the distance from three stations, the surface location above the focus, called the epicenter, can be determined.
More advanced discussion
The students may note that the waves reflect from the container edges. You could use this as a lead in to idea of waves reflecting and refracting at layer boundaries in the Earth.
Where Should You Build Your House if you Live in “Earthquake Country”?
A. Teacher introduction
This activity will illustrate the difference between building in solid rock or compact sedimentary material versus building in less compact, water-rich sediments.
B. Student activity (groups of 2 or 3)
Materials (for each group)
- 2 Wide-mouth clear containers (beakers or jars)
- Length of tubing a bit longer than the container is tall, or plastic straw
- 2 Rocks
- Sand (uniform size of sand grains)
- Small funnels to fit tubing/straw
- Half fill one container with gravel; place a rock on top of the gravel.
- Place the tubing or straw into the second container, with one end at the bottom, and the other end out the top; half fill container with sand; place rock on top of the sand. Slowly add water through the tubing/straw, watching as it fills in spaces between the sand grains; keep adding until water is just below the top of the sand.
- Wait a moment, taking care not to touch the containers or the table.
- Choose one student to be the earthquake and the other students to be the scientific observers. The earthquake student either pounds once on the table with a fist, or gives a sudden shake to the table so that the containers shift parallel to the table top*, while the other student(s) observe the two rocks. What happens to the rock in each container? The rock on the pebbles should not do anything, while the rock on the saturated sand should sink into the sand.
* Either move the table suddenly, or if the table is too solid, the container itself can suddenly be shifted parallel to the table surface; I find the lateral shifting usually is more effective than a pound on the table; for older students, trying both can lead to a discussion of different types of earthquake waves (compressional, when pounded, versus shear, for lateral shift; which is more damaging?).
- Thought question: If these rocks represent your house, in which type of material would you want to build your foundation? Why? Houses built on solid rock or well compacted sediments will move less during an earthquake. Loose, water saturated sediments will liquefy when suddenly agitated, as happens when earthquake waves shake them, and houses with foundations in this “liquid” are not stable. Show students a map of San Francisco area, with Candlestick Park and the Marina District labeled. The Loma Prieta earthquake focus was below the Santa Cruz Mtns, some 50 miles away; Candlestick Park was undamaged--it is built on bedrock; the Marina District was seriously damaged, including ruptured gas lines and fires--it was built on bay fill.
Why Does the Earth Quake?
Earth’s surface quakes when rock breaks or slides past other rock along broken surfaces called faults. A very famous break, or fault, in the Earth’s crust is the San Andreas Fault in California. There are other faults in the United States where rock can slip past other rocks, causing earthquakes. The New Madrid Fault in Missouri and neighboring states was responsible for large earthquakes in 1811-12, which would have been felt by people living in Ohio at the time. To understand why earthquakes are normal, regular occurrences on Earth, we can take a look at a powerful theory of Geology called Plate Tectonics.