Time and Time Again:
A Stratigraphic Band Activity
Goal: To provide students with opportunity to observe the record of 13,000 years of Mount St. Helens history at a cliff on the lahar which reveals stratigraphic bands, and to impress upon them the immensity of geologic time.
Objectives: Students will demonstrate
- Understanding of the cyclic and varied nature of volcanic activity
- Understanding of the ways in which these varied forms of activity help to determine the surrounding topography
- Ability to differentiate and describe various layers of deposition
- Appreciation of the challenges faced by geologists in the tracing of natural history.
Key Concepts: Volcanic activity constructs topography. Volcanic eruptions have been varied in form, and will continue to occur in a variety of ways. The stratigraphic layers at Mount St. Helens help to reveal its history, and aid in predicting its future.
Summary: At the stratigraphic bands site, the teacher will lower from the top of a low cliff a series of signs, fastened at intervals to a rope, which are maintained in that location and mark the strata revealed there. Students observe, describe characteristics and record evidence of six of Mount St. Helens' eruptive periods. They identify specific traits of lahars, pyroclastic flows, and airfall found in stratigraphic depositions from these periods.
Content Areas: Earth Science, History
See also Classroom Supplements folders.
- Stratigraphic Bands Activity Worksheet
- Stratigraphic Bands Activity Key
- Summary of Geological History
- clipboards or notebooks
Evaluation: Completed, corrected worksheet may be used to study for quiz or project on eruptive periods.
- Lower and arrange signs at the stratigraphic bands site, and explain to students that the signs identify layers corresponding to the six eruptive periods on their worksheets. Point out that the distance between knots on the rope is one foot. Note that the bands revealed here account for only the last 13,000 years of the 40,000 years of volcanic activity presently identified, and that inter-pretations of the evidence are, by nature, speculative.
- Explain how to complete the Stratigraphic Bands Activity worksheet, to include thickness, description of eruptive material, and artifacts of plant and animal life contained in the layers.
- Discuss the challenges of determining what each layer represents, the cyclic nature of Mount St. Helens eruptive activity, and the use of examination of past depositions to predict future eruptions.
- Ask questions to stimulate thinking, such as what root artifacts in the layers can tell us about former environments; what the layers reveal about how lakes in the area were formed, etc.
- Give the group ample time to complete and to discuss the questions at the bottom of the worksheet. It is imperative that the teacher move from group to group during this time to address concerns and answer questions. Have the group suggest mnemonic devices -- image associations, limericks, phrases, acronyms, etc. -- as aids to remembering the names of the periods in order.
- Correlate this activity with A Place in Time, and dates of volcanic eruptions with concurrent events in world history.
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