CRATER LAKE NATIONAL PARK
FIELD TRIP STOP – INSIDE A DORMANT PLEISTOCENE CALDERA
LOCATION: Located in southwest Oregon about 40 miles northeast of Medford, OR.
GEOLOGIC FEATURES: Caldera; Crater; Stratovolcano; Cinder Cone Volcano.
DESCRIPTION: Crater Lake (actually a Caldera nearly 6 miles across) is the result of the eruption of Mt. Mazama nearly 7000 years ago. Mt. Mazama was a stratovolcano composed of andesitic lava and pyroclastics, similar to some of the other volcanoes of the Cascade Range (e,g, Mt. Rainer, Mt. St. Helens, Mt. Hood, Mt. Adams). Each of these volcanoes is the result of the Juan DeFuca Plate subducting under the North American Plate at a Convergent Plate Boundary.
Pyroclastic Flows accompanied the eruption and ash from the volcanic eruption reached northwest Yellowstone, central Nevada, and southern British Columbia. Ash is estimated to be about 150x that of Mt. St. Helens.
After the eruption of Mt. Mazama, the volcano collapsed upon itself into a voided magma chamber forming a caldera having lost about 4000 ft in height. Crater walls rise 1000 to 2000 ft above the lake level. Three Cinder Cone Volcanoes subsequently grew from the base of the caldera. Only one of them, Wizard Island, breaks the surface. It rises nearly 2700 ft. from the crater bottom with the upper 755 ft. extending above the lake level. The lake itself is 1949 ft in depth (with an average depth of 1148 ft) and as such it is the deepest lake in the U.S.
STUDENT QUESTIONS:
(1) How does a Crater differ from a Caldera? Compare their methods of formation.
(2) Crater Lake is not considered to be extinct. Explain the fundamental reason why this is so.
(3) Crater Lake has no streams in or out. Why might the lake level be consistent year-round?
(4) CHALLENGE: Why might the magma responsible for the formation of Mt. Mazama (precursor to Crater Lake) be Andesitic (richer in silica than Basalt)?
SELECTED REFERENCES:
-Harris, A. and E. Tuttle. 1983 (3rd ed.). Geology of the National Parks. Kendall/Hunt Publishing Company. Dubuque, IO. 554 pp.
-Harris, D.V. and E.P. Kiver. 1985 (4th ed.). The Geologic Story of the National Parks and Monuments, John Wiley and Sons, New York, 464 pp.
PHOTOS: (All photos by Eric Marintsch)
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Figure 1 - Crater Lake is about 6 miles in diameter. Walls along the edge of the crater vary between 1000 and 2000 ft. The crater (caldera) represents the collapse of Mt Mazama into the magma chamber responsible for the eruption. (Explorer Eric Marintsch observes the lake from the crater edge.)
Figure 2 - The deep blue waters of Crater Lake extend to a maximum depth of 1949 ft. making it the deepest lake in the U.S. Wizard Island is a Cinder Cone volcano within the caldera. It stands 755 ft. above lake level. Some lava flows can be seen to project from the base of the volcano.
Figure 3 - Looking towards the south, Wizard Island is seen to the right of the lake,