# Yellowstone Lesson Plan: How Yellowstone Geysers Erupt

Clepsydra Geyser erupting in Lower Geyser Basin
Jim Peaco

### Overview

In “Why Yellowstone Geysers Erupt,” students will learn the key ingredients necessary to form geysers and how they function naturally. In addition to a geyser model demonstration (all grades), older students can build their own geyser models, while forming and testing hypotheses about the eruption behavior using different variables.

### Objective(s)

Students will:
• Be able to define a geyser
• Identify the ingredients necessary to form a geyser
• Create a model of a geyser
• Hypothesize which geyser model will most accurately represent a real geyser
• Evaluate different geyser models for their performance simulation of a real geyser
• Accept or reject hypotheses after viewing the eruption demonstrations

### Background

A geyser is a hot spring that erupts periodically and forcibly ejects water. Three ingredients are necessary for a geyser to exist: a source of heat, an abundant supply of water, and a special underground plumbing system.

Much of the water in Yellowstone’s geysers begins as rain or snow. The moisture seeps into the ground and then rises back up as it flows through the plumbing system of the feature. The round trip may take hundreds, or even thousands, of years. Faults, fissures, and cracks in the Earth provide paths in which the water passes. The narrow places, or constrictions, of a geyser’s plumbing system distinguish it from that of a hot spring.

Eventually, the water encounters rocks that have been heated by underlying areas of magma. This rocky stove warms the water to the point where it becomes so hot that it can dissolve the silica from the surrounding bedrock. When the water approaches the surface, it begins to cool. The silica drops out of solution and coats the geyser’s plumbing system. This coating makes the system strong enough to withstand tremendous pressure.

A geyser eruption is triggered when the superheated water fills the geyser’s plumbing system and the geyser begins to act like a pressure cooker. The boiling point of a liquid is dependent upon the pressure. The boiling point of pure water is 212°F (100° C) at sea level. In Yellowstone, where the elevation is about 7,500 feet (2,250 meters), pressure is lower, and the boiling point is 199° F (93°C). Within a geyser’s plumbing system, much of the water can reach temperatures greater than 400°F (205 °C) and still remain in a liquid state due to the great pressure exerted by overlying surface water and rock.

As more hot water continues to enter the geyser’s plumbing at depth, the water temperature climbs high enough to overcome the pressure. Some of the water converts to steam. As the steam bubbles become larger and more plentiful, they can no longer rise freely through the constrictions in the plumbing system. Temperatures build and the boiling becomes more turbulent. Eventually the violent bubbling forces some of the underlying water through the constriction. This release creates an instant reduction in pressure. Much of the water in the system flashes instantly into steam and forcibly ejects the remaining water.

The character and function of a geyser are determined by its plumbing system and every geyser is unique. In 1992 and 1993, a probe equipped with a video camera and temperature and pressure sensors was lowered into Old Faithful Geyser’s vent. At a depth of approximately 45 feet, the probe encountered a constriction that was barely four inches wide. Beyond this narrowing was an opening about the size of a large automobile.

### Materials

For Demonstration:

• Antacid seltzer tablets, cut into halves
• Plastic film canisters with lids
• Needle, safety pin, or other small sharp object to puncture hole in lid
• Warm water

For Creating Geyser Models and Testing Hypotheses:

• Vinegar
• Baking soda
• Soda bottles
• Clay
• Straws
• Plastic cups
• Plastic bowls
• Funnels
• Paper
• Pencil

### Procedure

The instructor will:
1. Review the process and ingredients that make a geyser.
2. Instruct students to write a definition for a geyser and to list the three natural ingredients necessary to form a geyser on the Geyser Worksheet and Observation Guide.
3. Make a small hole in the lid of a plastic film canister with a ball point pen, needle, nail, or other small, sharp object.
4. Select a student to assist with the demonstration in Step 5.
5. Fill the film canister ¾ full with warm water. Instruct the student to add a piece of an antacid seltzer tablet (approximately ½ tablet). The student should replace the punctured lid quickly and place a finger over the hole in the lid. Instruct the student to shake the canister a few times and then remove his/her finger from the hole. Instruct class to observe the ensuing eruption.
6. Ask students to describe what happened and why.
7. Divide the students into groups of 3-4 persons.
8. Inform students they will be given approximately 15 minutes to assemble a geyser's plumbing system and will have a variety of household items from which to choose (See the items listed under "Materials" for suggestions. Instructors may either collect the materials themselves or suggest items that each student should bring from home.)
9. After the allotted time, instruct each group to explain why they chose to assemble their geyser in the manner they did.
10. Ask students to hypothesize in writing which of the geyser models will most accurately represent a real geyser.
11. Remind students that heat creates pressure in the Earth. In the classroom, pressure is being created by the chemical reaction of adding baking soda to vinegar.

### Park Connections

Yellowstone protects the largest concentration of natural geysers in the world. Though the plumbing systems of real park geysers are hidden underground, it is possible to create a model of a geyser's plumbing that will demonstrate how a narrow constriction contributes to geyser eruptions.

### Extensions

Lead group discussion or ask students to predict what might happen to the behavior of geysers in Yellowstone if…

• large earthquakes change the plumbing systems of geysers
• there is extended drought for centuries
• the Yellowstone volcano erupts cataclysmically
• geothermal energy drilling takes place in or just outside park boundaries
• people toss materials (garbage, rocks, other debris) into a geyser or otherwise clog or "cap" the opening of a geyser

Discuss/debate whether or not geysers in Yellowstone should be preserved in their natural state or tapped into for energy production.