Unit C: The Solid Earth

How fast does a stalactite grow?

1. Get Set to Explore

Vocabulary

  • cave: A hollow underground space, made of rock.
  • limestone: A kind of sedimentary rock made up of calcium and carbon.
  • stalactite: Rock formation found hanging from the ceiling of a cave.
  • stalagmite: Cone-shaped rock formation found on the floor of a cave; stalagmites are usually formed from water dripping off stalactites.

Building Background

  • Show students a picture of the inside of a famous cave, and ask students to identify what kind of place the picture shows. Ask the class if anyone has ever been inside a cave. If so, let volunteers describe their experiences and any rock formations they saw.
  • Review the vocabulary words and their definitions with the class. Make sure students know how to pronounce stalactite and stalagmite. You may wish to introduce the following mnemonic device to help students remember which rock formation is which: stalactites grow from the ceiling of the cave; stalagmites grow from the ground (or floor) of the cave.
  • Introduce the Discover! question: How fast does a stalactite grow? Let students make guesses and write their guesses on the board. If necessary, give students plausible units to use, such as inches/year, centimeters/year, inches/century, centimeters/century, inches/millennium, or centimeters/millennium. If necessary, go over the units of time with the class. Let students vote on the rate they think is most likely, and keep this up on the board for students to refer to later.

2. Guide the Exploration

  • Let students launch the Discover! Simulation. They should listen closely to the introductory information and to the question. You may wish to distribute blank charts students can use to record their predictions of how fast each type of rock formation grows and key information about the formations.
  • Students should spend time examining the different formations and features of the cave. They can learn about the formations by moving the cursor over each one. In addition, clicking the stalactite will start a narrated animation that shows how a stalactite grows and forms a column with a stalagmite.
  • When most students have watched the animation, briefly go over how a cave forms. Ask volunteers to describe the process, and record the steps in the correct order, as described in the animation. Make sure that students realize that caves form because dripping water dissolves rock that is under the ground. This will allow them to better understand how the different rock formations in the cave form.

3. Review/Assess

  • Go over Step 3's Wrap-Up text, discussing each step the text presents.
  • Ask the Discover! question again and have students refer to the Wrap-Up text to find one answer. Have them compare this answer to their previous predictions.
  • Pose the Extension questions. Have students refer to the charts they made while doing the simulation. After students have formulated their answers, share these ideas:
    • All the cave structures are formed by limestone from the roof or walls of the cave dissolving in rainwater and being redeposited through dripping or flowing.
    • The structures differ in shape, size, age, and the amount of water or the flow of water involved in their formation.

If time permits, present students with the following questions and activities:

  • Critical Thinking Classify What common Earth-changing process is cave formation an example of? What common Earth-changing process is stalactite formation an example of? Answer: Cave formation is an example of weathering; stalactite formation is an example of deposition.
  • Inquiry Skill Use Numbers Compare the rate you predicted stalactites would grow with the rate that the simulation gives for stalactite growth. Make sure that you use the same units to make your comparison. Answer: The simulation gives a rate of 2 meters/4000 years. This reduces to a rate of 1 meter/2000 years or .5 meters/millennium. Help students change the units of their predictions to make an accurate comparison.

4. Reaching All Learners

English Language Learners

Print a copy of the picture of the cave from the simulation. The copy should include all of the cave features. Distribute copies to English Language Learners. Work with students to label the different features of the cave. Make sure that students can pronounce all the words and that they know what each refers to. Pair students with native English speakers to review and describe how the different features form.