51 Volume 29 – 1998

Volume 29, 1998

All material courtesy of the National Park Service.These publications can also be found at http://npshistory.com/
Nature Notes is produced by the National Park Service. © 1998


By Stephen R. Mark, Editor

This is the 50th issue of Nature Notes from Crater Lake, a milestone that took 70 years to reach. Publication began in 1928, but ceased twice, with the longest hiatus being for three decades after 1961. A symposium held in 1992 brought about the current revival of this serial and it has appeared every year since then.

The first issues of Nature Notes were mimeographed by park staff and appeared between one and three times each summer for the first ten years of publication. The present format and restriction to an annual volume started in 1950. National Park Service naturalists at that time wanted to economize on effort, but not at the cost of sacrificing the qualities which made Nature Notes a popular and inexpensive sales item. The background of contributors has since broadened to include employees working in other fields besides interpretation, in addition to long time friends of the park.

Volunteers came to the aid of this year’s volume with articles that underline the importance of observation. They are simply the latest in a long line of individuals who believe in the importance of an educational program at Crater Lake National Park. Two other volunteers, Jamie Halperin and Randall Payne, deserve special recognition, as they have put back issues of Nature Notes on the park’s web site (http://www.nps.gov/crla) last winter.

The Crater Lake Natural History Association encourages the reprinting of articles that appear in Nature Notes so long as credit is given to the authors and the association. CLNHA sponsors this publication as part of its commitment to assist the educational and resource management programs of the National Park Service. Please join them in this effort by becoming a member of CLNHA, and receive a 15 percent discount on all sales items. A list of these items can be obtained from the Business Manager, Crater Lake Natural History Association, P. O. Box 157, Crater Lake OR 97604.

This issue of Nature Notes is dedicated to the memory of Richard M. “Dick” Brown, who passed away on May 31, 1998. His career spanned the years from 1952 to 1970 at Crater Lake, where he had stints as assistant park naturalist, chief park naturalist, and then research biologist. Trained as a plant taxonomist, he combined academic rigor with a gift for interpreting to the public, thereby inspiring two generations of NPS employees who were privileged enough to know him.

Drawing by L. Howard Crawford, Nature Notes from Crater Lake, 7:3, September 1934.

Understanding the Bimodal Eruptions of Mount Mazama

By Brandon L. Browne

Crater Lake partially fills a caldera within what was once Mount Mazama, one of the greatest volcanoes in the Cascade Range. Around 7,700 years ago it awoke with great fury and power. Roughly 13 cubic miles of magma erupted from the volcano, covering 500,000 square miles to the north and northeast. This eruption is considered by many volcanologists to be the most violent and devastating that the world has seen in the past 10,000 years.

This climactic eruption occurred in two phases. The first was a “single vent phase” in which ash and pumice were erupted from a single vent forming an eruptive cloud estimated to be 25 miles high. This disturbance emptied the uppermost levels of the chamber beneath Mazama, leaving it weak and unstable. As the eruption continued, it drained the chamber underlying the mountain so that Mazama began to collapse inward. This brought on the “ring vent phase,” whereby the remaining magma was pushed out of the mountain along the multiple areas where the upper volcano was cracking and falling. This occurred along circular ring fractures which is now where most of Rim Drive is located. This sequence of events brought about formation of a caldera, much of which we see today.

Perhaps the most unusual aspect of the climactic eruption is that it exhibited two distinct chemistries. The purpose of this article is to describe some field examples of those chemistries, because their appearance from the same volcano during one eruption is rare. Those examples should also help to illuminate how long differentiated magma compositions have characterized what we see around Crater Lake.

Volcanoes are commonly classified by their composition and their structure. Mount Mazama is an andesitic stratovolcano. Andesite is the most common lava type found at the volcano, and you can get a great look at andesite along West Rim Drive around the Watchman. The mountain’s structure was that of a sharp faced or angular peak, making it a stratovolcano. (Mounts Hood, Rainier, and Shasta are all stratovolcanoes as well). Mount Mazama is believed to have been a cluster of several overlapping volcanoes that began erupting at least 420,000 years ago. With each new eruption, more and more bulk to the mountain would be added. The Crater Lake volcanic system consisted of three main components: 1) several peaks like Phantom cone, Hillman peak, and Mount Scott that piled on one another to form a conglomerate main cone, which the lake partially fills at present; 2) about twenty smaller volcanoes called cinder cones located within the caldera and on its perimeter (of which Wizard Island is one of the more recognized examples); and 3) volcanic domes, the only ones of which are now beneath the water line. All three of these surface structure groups are believed to have been fed by a large magma chamber. Figure 1 illustrates the locality of that chamber in relation to the volcano.

Figure 1. This is an illustration that modifies a figure drawn by C.R. Bacon of the U.S. Geological Survey. It shows how the chamber may have been layered into these two main zones during the time of the climatic eruption. Notice that the lighter, more silicic magma floats above the heavier, mafic magma. The magma chamber is less than four miles from the surface, and it is in the general shape of a lens.


Each type of volcano is usually made up of a specific type of lava. This is due to different lavas having characteristics which result in distinct types of volcanoes. A few characteristics like viscosity, temperature, and water content vary widely when you look at different types of lavas. These variations that are observed in lavas play an important role in the way that the molten rock will eventually behave when they reach the surface. For example, if you were to drop cookie dough on one side of a plate, and pancake batter on the other side, they would behave differently. This is mainly because the thick and pasty cookie dough has much higher viscosity than does the runny pancake batter. When the dough and batter “cool” and harden, you would see two different shapes (one flat and one tall).

The cookie dough would be analogous to “silicic” lavas , while the pancake batter would be called “mafic” lavas. Silicic lavas usually build stratovolcanoes and domes, and mafic lavas make up the large and broad shield volcanoes and cinder cones. Figure 2a shows how lavas are broken-up into specific classes based mainly on compositional changes in the silica and oxygen.

Figure 2a. There are six main lava and ash types that are found in the Cascade Range. These types are classified as to the amount of silica they contain. Rhyolites have a high amount of silica, while basalts are at the low end of the scale. Lava and ash types (such as numbers 2, 3, 4, and 5) are intermediate.

Figure 2a. There are six main lava and ash types that are found throughout the Cascade Range. These different types are mainly classified in terms of the amount of silica that is in them. Rhyolites have a high amount of silica, and the basalts have a lower amount. Lava and ash types like numbers 2, 3, 4, and 5, are intermediate.
Figure 2b. Let’s say that after a volcanic eruption, you go out into the field to gather up a sample of each type of lava and ash that was deposited. In the majority of eruptions, several types of lava and ash are found. If we were to plot each type found from a common eruption with a dot, the distribution would look like the figure above. Notice how all of the samples that you collected are randomly dispersed along a wide range of different types. There are no clumped groups.
Figure 2c. Now, if you were to do the same thing that you did in 2b, the types of ash and lava that you would find after the Crater Lake climactic eruption would only be from the basaltic-andesite (2.) and the rhyodacite (5.) groups. Let’s plot them on the same kind of graph that we did before. The dots would only be in those two regions. When the products of an eruption are limited to two distinct, clumped groups, the system that fed this rare eruption is said to be bi-modal. This is one of the aspects of the climactic eruption that makes Crater Lake so unique. 

** I should note that the 4 lava flows associated with the climactic eruption occurred up to 4,000 years prior to the collapse. Some geologists who studied Crater Lake do not even consider them to be part of the climactic eruption, but rather lava flows that simply preceded it. So, the dots in figure 2c are the ash deposits from the climactic eruption.

It is important to realize that the majority of volcanoes in the Cascade Range, as well as those around the world, are composed of closely related lavas and other eruptive material. In other words, it is uncommon for volcanoes to display material of one composition as well as large amounts of a completely different type. There is commonly some diversity in the lavas and ash in most eruptions, but their compositions are usually very similar to one another as shown in figure 2b.

Figure 2b. After most eruptions in the Cascades, field samples would show that a range of different lava and ash types (shown in Figure 2a) were deposited. The samples collected would be randomly dispersed among the various types, with their distribution similar to the above figure, so that no clumping of samples were evident in any group.

At Crater Lake, the volcanic system exhibits a pattern where there are only two very distinct groups of eruptive material (figure 2c). This is called a bimodal system. In the climactic eruption of the Crater Lake volcanic system, materials of two distinct compositions were erupted almost right after the other. In the first phase, an enormous (up to 30 miles high) eruptive cloud was produced. This sent large amounts of ash and pumice into the atmosphere and was eventually deposited in an area of nearly 500,000 square miles.