Volcano Watch — Mauna Kea a good place to observe glacier action

The snow that fell this past week at the summits of Mauna Kea and Mauna Loa volcanoes reminds us that it can get cold enough to snow, even here in the tropics. The name "Mauna Kea" means "white mountain," so apparently snow was common at the summit during ancient Hawaiian times. The variations through time in global climate discussed last week in this column, with reference to changes in sea level, also change the average temperature here in Hawaii. During these colder times, snow accumulated at the summits of Mauna Kea, and, presumably, Mauna Loa, and formed perpetual mountain glaciers.

The evidence for this assertion is that glacial deposits are found near the summit of Mauna Kea Volcano. These glaciers advanced and retreated several times and left behind glacial drifts of rock that had been ground up by the moving ice. The glaciers covered about 28 square miles of the summit of Mauna Kea and had a maximum thickness of perhaps 350 feet. These deposits, which extend down to about the 10,500-foot elevation, are preserved on Mauna Kea because it hasn't erupted very often since that last advance of the glaciers. On Mauna Loa, which was also clearly tall enough to accumulate snow and form glaciers, all evidence has been buried by voluminous younger lavas.

The glacial deposits consist of gravels and rock "flour" produced by the grinding action of ice; there are also some deposits with rounded cobbles. The youngest of these deposits, named the Makanaka Glacial Member of the Laupahoehoe Volcanics, is thought to mark the greatest advance of ice and to slightly precede the 15,000-year age of the youngest drowned coral reef surrounding the island. There are two earlier glacial drifts; the Waihu and the Pohakuloa that may correspond to earlier advances of ice during the last 150,000 years. These deposits are best exposed along the gulches that cut the south-facing upper slopes of Mauna Kea, including Waikahalulu, Kemole, and Pohakuloa gulches. The boundary marking these deposits can also be clearly seen from Saddle Road as a near-horizontal color change on the upper slopes of Mauna Kea.

These glacial periods in Hawai`i had imporant ramifications for the Hawaiian people, who arrived much later. The largest adze quarry on Hawai`i is located on the upper slopes of Mauna Kea. The rock that was quarried here was from a lava flow that erupted beneath a glacier. When an eruption occurs under ice, the heat melts a hole in the ice, but the meltwater is trapped in a large pothole in the ice. The lava actually erupts into water, and the outside of the flow is quenched to form volcanic glass. The Hawaiians found this location, which became their main tool source because the glassy, dense lava was sharp and made an excellent cutting edge.

The meltwater formed by these subglacial eruptions eventually flows beneath the ice and escapes as a catastrophic flood. In Iceland, where these events have occurred during historic times, such floods are called jokulhlaups. The gulches on the south face of Mauna Kea are thought to have originally formed by such glacial outwash floods, which were probably triggered by subglacial eruptions. The gulches have since been enlarged by erosion.

There are lasting effects from the last glacial period on Mauna Kea Volcano. As many of you know, the only natural lake at high elevation on Hawai`i is located near the summit of Mauna Kea at Waiau. The reason why lakes do not occur elsewhere in the islands is that the rocks and soils are too porous to trap and perch rainwater at the surface. We have all watched enormous puddles formed during heavy rains disappear into the ground in a matter of a few hours. What prevents the water in Lake Waiau from percolating through the ground is that the water is perched on a zone of permafrost left over from the last glacial stage.

Hawai`i may seem like a strange place to study the glacial history of the Earth, but the coral reefs offshore and the glacial deposits near the summit of Mauna Kea record global climate change at different parts of the cycle. The reefs die when the polar ice caps begin to melt at the end of glacial maxima, whereas the glacial deposits on Mauna Kea record the greatest advance of the ice and, therefore, the peak of the cold part of the climatic cycle.