(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)
As days get shorter and nights cooler, we look to the summits of majestic Mauna Kea after rainstorms to see whether there might be a cap of pure white snow.
Not very long ago, in geological terms, the white mantle was ever-present on Mauna Kea. The last of the glacial episodes that Mauna Kea experienced covered the summit in ice and snow from about 40,000 to 13,000 years ago.
The glaciers that flowed from the summit left distinctive features. The most prominent of these are extensive deposits of glacial till–accumulations of jagged boulders, stones, cinder and ash. There is also evidence of eruptions interacting with the glaciers. The lava flows that chilled very quickly against glacial ice produced the especially fine-grained, dense rock that was so prized by ancient Hawaiians for making adzes. The famous Mauna Kea adze quarry complex, covering over 12 square kilometers (about 4.6 square miles) is near the summit, just below Lake Waiau.
Lake Waiau, in the Mauna Kea Ice Age Natural Area Reserve, is the only alpine lake in the Hawaiian island chain, lying within Pu`u Waiau at about 3,969 m (13,020 ft) elevation. Besides the significant archeological and geological features, Lake Waiau has cultural and spiritual significance to many Hawaiians.
The lake has been shrinking at a rapid rate during the last year. While fluctuations in lake level have been observed in the past, the recent rate of decline seems to be unusually rapid and severe. Theories about the shallowing are tied into theories about why the lake is there at all.
Studies of the chemical signatures of the lake and geomorphology of the area show that the source of the water is precipitation falling within Pu`u Waiau, mainly from winter storms. The reason for the ponding of the rainwater and snowmelt is less clear, however.
Several mechanisms have been proposed to explain the impermeable layer that traps the lake waters. One suggestion is that sulfur-bearing steam emanating from Pu`u Waiau either during or just after eruptions altered the cinder and ash to fine clay minerals that reduced permeability.
Another suggestion is that the impermeable layer that traps the lake water is made up of the fine-grained ash beds that are found at Pu`u Waiau and that this ash is a result of Waiau eruptions occurring under glacial ice.
One scenario suggests that permafrost makes up the impervious layer beneath the lake, based on the existence of permafrost at nearby Pu`u Wekiu. However, temperatures taken in the bottom sediment of the lake down to 70 m (230 ft) in the winter of 1966 did not fall below freezing—the lowest temperature recorded was about 4 degrees C (39 degrees F).
What implications do these mechanisms have for the shrinking of Lake Waiau? If the impermeable layer beneath the lake were due to permafrost, then the shallowing of the lake could be a result of the permafrost slowly starting to melt, starting at the edges of the lake where, perhaps, insulating sediment layers are less thick. In this scenario, the lake would surely disappear as the permafrost continues to melt with increasing temperatures on Mauna Kea. The temperature increase is about three times faster than the global rate, and is observed at high elevations throughout Hawai`i.
If the lake is retained by clay-rich ash beds, then the most likely reason that the lake is shrinking is that the evaporation rate is higher than the precipitation rate. The higher average temperatures, along with recent drought conditions, make this the most plausible scenario.
Through a collaboration between the Department of Land and Natural Resources, the Office of Mauna Kea Management and the University of Hawaii at Hilo, research is underway to monitor lake level changes and to reconstruct past lake levels and climatic conditions. Please contact the Office of Mauna Kea Management if you have historical photographs of the lake that you are willing to share.
Although the rising temperatures do not bode well for the future of Lake Waiau, a winter season rich in storms will do much to replenish the lake, as well as provide us with magnificent views of the snow-capped volcano.