Lesson Plan

Liquid Rock

crack revealing molten lava

Red hot lava is visible beneath the cooled surface of a flow in Hawaii

USGS

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Grade Level:
Fourth Grade-Seventh Grade
Subject:
Earth Science, Geology, Physical Science
Duration:
1 - 2 hours
Group Size:
Up to 36
Setting:
classroom
National/State Standards:
NGSS.SEP.2, NGSS.SEP.6

Overview

Students learn about the properties of lava by experimenting with liquids having varying gas contents and viscosities. (CLASSROOM ACTIVITY)

Objective(s)

  • Students will be able to describe liquids in terms of their viscosity.
  • Students will be able to explain how heat affects a liquid's viscosity.
  • Students will understand how dissolved gas and pressure influence the behavior of an eruption.

Background

When we think about the properties of liquid, water usually comes to mind. But there are many different liquids, each with unique freezing points, boiling points, and viscosities. A liquid's behavior can also be greatly modified by the presence of dissolved gases (e.g., soda vs. water).

Craters of the Moon was once a liquid sea of lava (although not all at once) until it "froze" and turned to a solid. Early eruptions were violent, like a shaken can of soda, while later eruptions were sedate, like water being poured into a glass. Some lava raced across the land like heated olive oil, while other flows crept along like tons of tepid toothpaste.

Two basic types of lava made Craters: rhyolite and basalt.The main difference between the two is the amount of silica (SiO2) they contain. Rhyolitic lava's high silica content causes it to be quite viscous.That viscosity prevents gas within it from readily escaping as the magma rises to Earth's surface through a break in its crust. When lava can no longer contain the increasing gas pressure within, cataclysmic rhyolitic eruptions occur.

Basaltic eruptions are gentler because gas bubbles to the surface before it generates explosive pressure. Boiling water does not cast large volumes of liquid into the air in violent fits and starts like, for example, a boiling vat of mud. The high viscosity of the mud causes it to behave like rhyolitic lava. Basaltic lavas are runnier, like maple syrup, while rhyolitic lavas are more like molasses.

Early volcanic activity in the Snake River Plain (up to 17million years ago) consisted of calamitous rhyolitic eruptions that created enormous calderas up to several 100 miles square miles in area! At Craters this evidence has since been entirely hidden by the more recent, gentler, low-silica basaltic eruptions. All the rock you see at Craters is basalt.

Very hot basalt, whose top layered cooled like the "scum" at the surface of a mug of hot chocolate, became the smooth, ropy lava we call pahoehoe. Cooler lava that crept downhill and was slowly turned, twisted, and ground up into irregular chunks became aa.

See "Additional Resources" below for links to introductory materials about the geology of Craters of the Moon.

From the Teacher's Guide to Craters of the Moon.

Materials

  • Two empty film canisters (black with gray lid)
  • Alka-seltzer tablets (about 30)
  • Bunsen burner, hot plate, or camp stove (optional)
  • A pot for water (optional)
  • Balloons, smallest available (one per 2-3 kids)
  • Means to fill balloons part-way with water
  • A bottle of carbonated mineral water
  • Thermometer
  • Watch with second hand
  • Several liquids with different viscosities (e.g., water, cooking oil, honey, syrup, corn syrup, molasses, water, or mayonnaise)
  • A smooth surface such as a lunch tray or a dry erase board

Procedure

Additional Resources

Geology of Craters of the Moon:
For Teachers
For Students
Glossary
Analogs

Vocabulary

viscosity
gas pressure