- Grade Level:
- High School: Ninth Grade through Twelfth Grade
Geologists generally accept that events of the future will resemble those of the past. For this reason, they investigate the origin of rocky layers (deposits) set down by previous volcanic events. They recognize that understanding a volcano's general behavior patterns now can lead to a society that is better prepared because they can anticipate likely types of future volcanic activity. Geologists have developed ingenious methods to identify the area, origin, and sequence of events that formed the rocky layers set into place by lava flows, pyroclastic flows, tephra fall, lahars and glacier, stream, and erosional processes. They conduct complex chemical analyses to determine the composition of rocks, their source, and the process that formed them. They perform sophisticated laboratory methods to identify the age of rock layers. Yet, one of the most useful methods requires little more than good observational skills, a small trowel or rock hammer, notebook and map, and a basic understanding of the Law of Superposition. This fundamental principle, which states that younger layers will be on top of layers that are older, is one of the guiding principles of geological investigation.
This activity provides a glimpse into the methods used by geologists as they study the rocky layers at Mount Rainier. Students manipulate "Earth Blocks" into the likely sequences observed at Mount Rainier, and practice their logic skills as they employ the Law of Superposition.
Look for students' understanding of the Law of Superposition. As the activity progresses, students should recognize that this principle enables scientists to determine the chronological order in which geological events happened. Students should understand why this law is helpful to volcanologists who seek to know the geological story of an area. Their understanding of past events helps them to determine the most likely geological events for the future, and ultimately the hazards. This process in due course saves lives and property and improves the well being of nearby communities. Use questions on the student pages to assess students' ability to apply the principle of Superposition to real-world situations. After completing this lesson, students should be able to design their own data and observation tables. Students who select an incorrect response may be able to provide a justification for their answer that demonstrates understanding of the principle.
This activity provides a glimpse into the methods used by geologists as they study the rocky layers at Mount Rainier to discover the volcano's history.
- Clothes hamper geology. Introduce the concept of layers and the “Law of Superposition” by instructing students to envision a clothes hamper in their bedrooms. They come home from school on Monday and take off a red shirt and throw it into the hamper. On Tuesday, they toss in a green shirt. On Wednesday, they put in a yellow shirt, on Thursday, a purple shirt, and on Friday, a blue shirt. Each shirt represents a layer deposited by the student. Ask students which layer is oldest and which is the youngest? From this, ask students to derive the “Law of Superposition.” Alternatively, students pile up jackets and sweaters worn that school day.
- Snow bank stratigraphy. If you live in an area where snow banks develop during the winter, take students to one of them and ask them to interpret the relative ages of the layers while hypothesizing about the events that created the layers.
- The earth’s ticking clock—radiometric dating. Use this extension activity to explain how radiometric dating works. Provide each student with a graham cracker and ask students to take exactly one minute to eat half of the graham cracker. After the one minute of eating the first half, instruct them to eat half of the remaining graham cracker, and split the remaining amount in half. Ask students to repeat this until the amount of graham cracker is so small that they can no longer divide it in half. The amount of time it takes for half of the graham cracker to be eaten is known as the half-life. Carbon-14 has a half-life of 5,730 years. Make sure students keep track of how long it takes them to reach the point when the graham cracker can no longer be split apart. At this point, the amount of cracker left represents the amount of Carbon-14remaining when the sample has completely degraded to one molecule. The molecule cannot be split and so there is essentially no Carbon-14 left in the sample. Instruct the students to plot the relative size of the cracker (one half, one quarter, one eighth, etc.) versus time on graph paper. If the students were to find a sediment layer with a graham cracker in it that had degraded to one-eighth its original size, how old would the sediment layer be? Make an analogy to Carbon-14 dating.
- Design earth blocks with character. Make a set of “Earth Blocks” that include common physical characteristics found in the different tephra layers at Mount Rainier. You can find this information in published geologic papers on the website of the USGS Volcano Hazards Program. See Internet Resources Page
Cas, R.A.F., and Wright, J.V., 1987, Volcanic Successions Modern and Ancient—ageological approach to processes, products and successions: London, Allen andUnwin, 528 p.
Fisher, R.V., and Schmincke, H.U., 1984, Pyroclastic rocks: New York, Springer–Verlag,472 p.
Scott, K.M, Vallance, J.W., and Pringle, P.T., 1995, Sedimentology, behavior, andhazards of debris flows at Mount Rainier, Washington: U.S. Geological SurveyProfessional Paper 1547, 56 p., 1 pl.
Vallance, J.W., and Scott, K.M., 1997, The Osceola Mudflow from Mount Rainier:Sedimentology and hazard implications of a huge clay-rich debris flow: GSABulletin, February, 1997, v. 109: no. 2: p. 143-163, 6 tables.
Zehfuss, P.H., Atwater, B.F., Vallance, J.W., Brenniman, H., Brown, T.A., 2003,Holocene lahars and their by–products along the historical path of the White River between Mount Rainier and Seattle: in Swanson, T.W., ed, Western Cordillera andadjacent areas: Boulder, Colorado, Geological Society of America Field Guide 4,p. 209-223.