Medium-Activity Volcanic Lakes
With a temperature structure similar to the previous category, medium-activity lakes are less affected by venting at the bottom. Fumaroles release into the lake salts and acids, but buoyant plumes are unable to reach the surface. Total dissolved solids range between 1 and 4 percent and pH values vary between 1 and 3. A good example of a medium activity lake is one named “TAP” by western scientists. It is another lake on Keli Mutu, on the island of Flores, This lake’s temperature is 20°C (69°F) and it has 1.7 percent total dissolved solids in it. The water is very acidic with a pH of 1.8. Most of the year the lake has a dark green color due to the presence of barium, copper, and arsenic precipitates. When oxygen rich rainwater enters the lake, the lake color changes to blood red because ferric oxide precipitates are produced.
Low-Activity Volcanic Lakes
These lakes tend to be larger bodies of water with low heat flow into their basin. Heat may enter a low-activity lake through vents or sediments. Some warm salty water may circulate up and into the lake’s top layers that are thermally stratified. Low-activity lakes are capable of accumulating large amounts of carbon dioxide, a substance released during overturning events. A good example of such lakes is Lake Nyos in Cameroon, where a large volume of carbon dioxide (~1 km3) was released in 1986, killing 1700 people living down slope. The source of the gas was (and still is) hot magma beneath the lake. The gas slowly accumulated on the lake bottom and was released once the stable layers of lake water overturned. It is worth asking whether an event like this could occur at Crater Lake. Carbon dioxide enters the bottom of Crater Lake, but in the form of carbonic acid (H2CO3). The acid ionizes once in lake water and remains in an ionic form of hydrogen (H+) and bicarbonate (HCO3–). The slow process of turnover at Crater Lake appears rapid enough to prevent a build up of dissolved carbon dioxide at the bottom.
A new low-activity crater lake can be found at a national park in the Alaska Peninsula. At the summit of the collapsed Mount Katmai is a lake, 250 meters deep, one whose level is still rising. It is surrounded by a 9 km wide caldera with steep walls which measure 500-1000 meters high. This stratovolcano collapsed in 1912 when its magma chamber was drained by the eruption of nearby Novarupta. The lake is a blue-green color, but yellow-green plumes are still visible in the water.
No-Activity Volcanic Lakes
Many stable volcanic lakes display little or no activity. Crater Lake, the lakes in Newberry Crater, and Medicine Lake are of this type. The lake water is relativity pure and the color from a distance is perfectly blue. The chemical content of Crater Lake is 80 mg/liter or .008 percent by volume. These figures contrast sharply with all active systems having chemistry greater than 1 percent by volume. There are active hydrothermal springs on the bottom of Crater Lake, but the flow rate is minimal and the released minerals are greatly diluted by more than four trillion gallons of water. This warm hydrothermal spring water is, however, an important factor in the slow mixing process occurring within the lake.
Newberry Crater is located 30 km southeast of Bend, Oregon. This is one of the largest volcanoes in the Cascades and has been active for about 500,000 years. At the summit of this shield volcano is a caldera that measures 3 km by 7 km, and it appears to be the most recent of a series of overlapping depressions that have formed over time. The last caldera forming eruption occurred about 200,000 years ago and since then debris from other eruptions has filled the basin of the caldera. There may have been only one lake here in the past and its depth may have been close to the depth of Crater Lake. Two lakes now occupy the caldera separated by a narrow strip of pyroclastic material (airfall debris), with Paulina Lake being deeper of the two lakes. It is 76 meters deep and has a surface area of 1531 acres, about one tenth the size of Crater Lake. Hot springs and vents feed this lake on its northeast side. East Lake is roughly two-thirds the size of Paulina Lake. Its surface rests 15 meters higher than Paulina Lake, but its depth is about half. There are no surface inlets for either lake, but Paulina Creek drains Paulina Lake.
Medicine Lake is located atop the largest shield volcano in the Cascade Range, the Medicine Lake volcano. It is located 50 km northeast of Mount Shasta and began forming less than a million years ago. Resting in the summit is a caldera, measuring 7 km by 12 km, which may have formed when a series of smaller craters circling the summit collapsed. The lake has a depth of 46 meters and is oblong in shape. Although it is nowhere near as deep as Crater Lake (at 1882 meters), the surface of Medicine Lake is higher above sea level—at 2036 meters.
Each of these low-activity lakes is relatively stable since the potential of a volcanic eruption in the near future is minimal. This does not mean, however, that any of these volcanic systems are extinct. They still produce enough heat for some engineers to consider each of the volcanoes as a potentially safe source of geothermal energy.
A Final Word
Volcanic lakes appear in a variety of forms around the world. Those located in the Pacific Northwest, specifically Crater Lake, are examples of inactive systems where the water is clear and blue amid a placid setting. These lakes are volcanically stable and tend to be older bodies of water. The more recent volcanic lakes are temporary features since they sit atop active magma bodies. During eruptive events, their water content may be ejected or will simply boil away at high temperatures. Nevertheless, no lake, whether volcanic or not, will last forever. Seismic activity, volcanic blasts, or the forces of erosion will eventually alter the appearance of every volcanic lake. Even Crater Lake, given enough time, will be replaced by other volcanic systems.
References
C. R. Bacon, et al., Volcanic and Earthquake Hazards in the Crater Lake Region, Ore., Vancouver, WA: U.S. Geological Survey Open-File Report, 97-487 (1997).
G. B. Pasternack, Volcanic lake systematics: physical constraints, Bulletin of Volcanology 58 (1997), pp. 528-538.
G. L. Rowe, et al., Fluid-volcano interaction in an active stratovolcano. the volcanic lake system of Poas Volcano, Costa Rica, Journal of Volcanology and Geothermal Resources 49 (1992), pp. 23-51.
Tom McDonough teaches at Chemeketa Community College in Salem, Oregon, while also pursuing his scientific interests each summer at Crater Lake.
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