June 19, 2003
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
Impermeable beds trap rain and snow at Mauna Kea's Lake Waiau
Lake Waiau, at an altitude of 3,969 m (13,020 ft) atop Mauna Kea, is the only alpine lake in the Hawaiian island chain. It is thought to have formed at the end of the last glacial retreat, which in Hawai`i was almost complete by 15,000 years ago.
Since that time, sedimentary layers of silt, clay, and ash have accumulated in the tiny, shallow lake-0.73 ha (1.8 acres) in surface area and about 3 m (10 ft) deep. A radiocarbon age obtained 20 years ago from a sample of organic matter at the base of a 5.6-m-thick (18.4-ft-thick) drill core indicates that the lake basin must have been free of an extensive ice blanket by 12,600 years ago.
Lakes are sparse features among the volcanic slopes of the Hawaiian island chain. The few basins that could contain water are commonly underlain by permeable lava flows or cinder and ash that favor the rapid seepage of water. How does Lake Waiau defy the odds?
Lake Waiau lies within its namesake cinder cone, Pu`u Waiau, one of the more conspicuously gullied and eroded of the cones that erupted during the past 65,000 years. In his monograph on Mauna Kea's geology, Edward Wolfe explains that sulfur-bearing steam probably emanated through the cone either during eruptions or soon thereafter, altering the cinders and ash to fine clay minerals that reduce permeability. As a result, water runs off the cone instead of percolating downward, resulting in heightened erosion. The reduced permeability also accounts for the retention of water to form Lake Waiau.
An alternative scenario that may have produced the clay-altered rock was suggested by Stephen Porter, who studied the glacial deposits of the mountain. Porter noted that Pu`u Waiau and nearby Pu`u Poli`ahu are unique among Mauna Kea's summit cones because of their abundant, fine-grained ash beds. Ash of this sort, commonly produced when rising magma interacts with shallow groundwater or surface water, led Porter to suggest that Waiau eruptions occurred beneath glacial ice. Both interpretations recognize that Pu`u Waiau's impermeable beds slow the seepage of water from its small crater.
A third explanation invokes permafrost as the impervious layer beneath Lake Waiau. This proposal was offered by Alfred Woodcock, who reported the existence of permafrost at nearby Pu`u Wekiu (Summit Cone). The Wekiu permafrost is of limited breadth, perhaps only 25 m (80 ft), as estimated from Woodcock's study of surface exposures. Shallow holes drilled 30 years ago passed through the permafrost at depths of 10 m (33 ft).
No permafrost has been reported at Pu`u Waiau. It seems unlikely that permafrost could survive beneath a Mauna Kea lake, because the average air temperature is too warm much of the year, and the lake would melt the underlying ice. Nonetheless, Woodcock's proposal cannot be dismissed outright.
According to precipitation records and geochemical information, the source of Lake Waiau water is the rain and snow that fall yearly upon Pu`u Waiau. However, the fancy of some writers is captured by the novel idea that a conjectural Waiau permafrost block might feed the lake. A web-based search using the string "Mauna Kea permafrost" uncovers several tour-oriented articles that describe Lake Waiau as the world's only tropical lake fed by melting permafrost. An alpine lake in the tropics?-yes. But fed by melting permafrost?-no.
Eruptive activity at the frost-free Pu`u `O`o vent of Kilauea Volcano continued unabated during the past week. Three incandescent patches outline the surface flows on Pulama pali. Numerous surface breakouts occur in the coastal flats from the base of Paliuli to the Highcastle sea cliff. The Highcastle delta has multiple ocean entries and is slowly expanding seaward.
The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying sudden collapses of the new land. The steam clouds are highly acidic and laced with glass particles. The National Park Service has put warning signs in critical places. Do not venture beyond these signs and onto the lava deltas and benches.
Two tiny earthquakes were felt during the past week. A resident of Volcano felt the earth shake at 7:48 a.m. on June 13. The magnitude-1.7 earthquake was located 6 km (3.6 mi) northwest of Ka`ena Point at a depth of 7 km (4.2 mi). At 10:02 p.m. on Sunday, June 15, an earthquake was felt in Hilo. The magnitude-2.2 temblor was located 7 km (4.2 mi) southwest of Volcano at a depth of 3.1 km (1.9 mi).
Updated: June 23, 2003 (pnf)