Volume XXX - 1999

Crater Lake is certainly a sight to behold. It is especially spectacular for those who see it for the first time. Pictures cannot really capture the lake's magnitude and its image seems ageless. The base of the great volcano, which stood here once, properly frames the cobalt blue of the lake. These rugged cliffs are mostly bare, undoubtedly stripped of vegetation by continual rockfall. What of Wizard Island? It somehow seems fitting to have a winged dinosaur fly in circles around the cone. These creatures were long gone, however, when the fires of Mount Mazama first began to burn less than a half million years ago.

The apparent serenity of this lake is somewhat misleading. We must face the reality that this image is fleeting, or in the geologic time scale, ephemeral. Why should we expect Crater Lake to always be the same when it has changed so much in the past? A deep lake with a small volcanic island near one shore are its dominant features, though how each formed has been somewhat of a mystery until recently. By analyzing lake sediments obtained from the bottom, scientists can now tell us much more about what has happened here since Mount Mazama last erupted 7,700 years ago.

This crater (or caldera) holding Crater Lake today was created by a great eruption. That event ended with the collapse of the mountain summit, and was much more catastrophic than the Mount St. Helens incident in 1980. It may have been the biggest occurrence of its kind in North America during the last several million years. During the eruption, Mount Mazama released great quantities of ash and pumice. At least 12 cubic miles of mountain top disappeared as it slumped back into a drained magma chamber located beneath the volcano, now below the lake bottom.

During the summers of 1988 and 1989, a piloted submersible was used for the first time to explore the bottom of Crater Lake. Videos identified active thermal springs on the caldera floor, indicating the presence of heat below. Hot water rises up, in various locations, through layers of lake sediment that have accumulated since Crater Lake formed. Five sediment core samples were taken from selected sites on the lake bottom during these dives. The sample taken east of Wizard Island, on a series of lava domes called the Central Platform, lacks the rock debris covering other sites and therefore better displays the materials that have settled here from above. These include volcanic ash, soot from forest fires, pollen grains, and diatoms layered in a fine mud about five feet in depth. By looking closely at this core sample, geologists can now work out what kind of events have transpired in and around the caldera over the last several thousand years.

Diagram courtesy of U.S. Geological Survey.

At the core's base is mud, whose age indicates that it was deposited very soon after the caldera was formed. Dr. Charles Bacon, a volcanologist with the U.S. Geological Survey, speculates that only 300 years may have been needed to fill the lake basin to its present level. By the time water flowed over the top of the Central Platform, something that took a minimum of 150 years, the lake was already 1,000 feet deep over in the eastern basin, Obviously, winter snow and ground water easily found a repository in the large cavity left by the devastating eruption of Mount Mazama.

Crater Lake was approaching its present depth when Wizard Island appeared. We know this by examining its lava rocks 250 feet below the present water level. This was the water line for the lake when the event that produced Wizard Island occurred. The lava above this point looks different than the same lava below it. The lava, which was forced to cool in water, is left with a somewhat glassy appearance. The same rock, which cooled above the level attained by water at that time, has an oxidized surface and appears rusty red or brown. Since Crater Lake was lower when Wizard Island formed, it was not yet an island, At least several more decades were needed to allow the lake to enter Skell Channel and inundate the lava flows that connected Wizard Island to the caldera walls. Some of this lava made its way into the sediment over the Central Platform. Since the sediment appears near the core sample's base, it provides more evidence that Wizard Island appeared not long after Mount Mazama collapsed.

The core sample extracted from the Central Platform shows that a second volcanic event occurred. Located approximately at the midpoint in the lake sediment is an ash deposit from activity related to creation of the Rhyodacite Dome, a feature situated immediately east of Wizard Island. Organic material within the ash layer indicates this eruption occurred 5,100 years ago. This is possibly the most recent volcanism related to Crater Lake. The absence of other activity since that time, however, does not indicate the volcano's extinction.

Sediments on the lake bottom also contain pollen grains and diatoms. Scientists can broadly ascertain vegetation history patterns and related climate changes over the past few thousand years by studying them. Pollen, carried by the wind, entered the caldera soon after it formed, Even though the local pine and fir forests were decimated during the climactic eruption, other conifers located further away persisted. As water began to accumulate, pollen grains became a permanent part of the lake sediment. The appearance of pollen from Abies (true fir) in the core samples may be evidence for the recovery and reestablishment of the local forest, Fir pollen is relatively heavy and does not travel far from its source, It first appeared in lake sediments after a few hundred years, but just as this species became well-established, it and certain pine species began to decline. The pollen record shows that incense-cedar, western juniper, and Douglas-fir (all of which prefer a warmer and drier climate than the true fir) replaced them. These conditions prevailed for a thousand years and affected the entire Pacific Northwest. Pronounced change took place at lower elevations, where Lower Klamath Lake and Tule Lake all but disappeared. What effect the dry climate had on Crater Lake is not known. Currently, 67 inches of annual precipitation are needed to maintain the lake level. Evaporation and seepage would lower the lake level without a steady input from rain and snow. Perhaps Wizard Island was, once again, connected to the caldera walls during this period.

Pollen on lake in Steel Bay. NPS photo

Diatoms in the core samples provide researchers with information about lake water at various times. These microscopic silica skeletons are the remains of a type of algae and there are many different forms. As with other plants, the presence of one species or several related species can tell scientists something about the local environment, Eighteen different species of diatoms were examined from the core samples. Nine of them drift in the water column, whereas the other nine prefer the bottom of the lake.

The first species to establish itself was Stephanodiscus, a type of diatom that prefers fresh water, Others requiring a more mineral-rich and basic supply of water slowly replaced them and in abundance. This change may indicate that the lake water was slowly enriched with chemicals from hydrothermal vents, not unlike those seen with the submersible. Diatom concentration reached a maximum about 4,000 years ago following the dome building event that occurred east of Wizard Island, Since then, concentrations have slowly decreased for most species, though why is not well understood, Lake transparency has improved with this decrease since diatoms are effective scattering agents. In large concentrations they prevent light rays from travelling very far. With this in mind, Dr. Hans Nelson, a limnologist with the U.S. Geological Survey, is convinced that the lake's exceptional clarity is a relatively recent phenomenon.

The peaceful setting of Crater Lake at the present time stands in sharp contrast to the violence which produced this picture. In the absence of continued volcanic activity, the water has slowly been purified by an abundance of rain and snow. We are often reminded by geologists that this volcano is only dormant. If the past is any indication of the future, the tranquility we enjoy today is only temporary. A new mountain may stand here someday and it is possible that our descendants may be limited to only imagining the lake we now see.

References

C.H. Nelson, et al., "The volcanic, sedimentologic, and paleolimnologic history of the Crater Lake caldera floor, Oregon: Evidence for small caldera evolution," Geological Society of America Bulletin 106 (May 1994), pp. 684-704.

C.R. Bacon, "Geological Observations and Sampling," pp. C1-C3, in Robert W. Collier, et al (eds.), Studies of Hydrothermal Processes in Crater Lake, OR, OSU College of Oceanography Report #90-7, Corvallis, May 31,1991.

Tom McDonough teaches science at Chemeketa Community College in Salem, Oregon and began working seasonally at Crater Lake National Park thirty years ago.