The Top Mauna Kea

Story by Pat Duefrene
Aloha magazine
August 1, 1984, v7 n4

An ice cap glacier existed on the summit of Mauna Kea during the last of the Ice Ages, in the Pleistocene epoch of geologic time. It was the only mountain peak in the central (tropical) Pacific known to have had a glacier during this time, roughly between 8,000 and 2,500,000 years ago.

Geologists have speculated that as many as four previous glaciers occurred, although the evidence of that is minimal. The conditions under which a future glacier could form involve surprisingly little change from the summit's present climate. The average rainfall would need to increase by only two inches per year and the average temperature would need to drop only a few degrees. Glacial ice would soon form as the result of continued year round accumulations of snow. This condition may have existed in recent times with only the duration being too short. Many early travelers reported that Mauna Kea was known to have a year round cap of snow that "looked down as winter upon summer."

Scientists know, from the abundance of evidence, that Mauna Kea's ice cap of the Pleistocene epoch covered a twenty-eight-square-mile area. The amoeba-like shape of the glacial body remained above 11,000 feet, but some of its lobes dipped to that elevation. One of the lobes, located above Pohakuloa, extended as low as 10,500 feet. Meltwater from these extremities carved gulches into the mountainside and left ridges of glacial moraine where rubble, picked up "en route," was released from its suspension in the solid ice. The gulch and moraine ridge (on either side of the gulch) above Pohakuloa are prominent features that can easily be seen from the Saddle Road. Similar terminal moraine deposits encircle the summit and indicate the extent of the ice cap.

When the glacier melted, its former presence was well marked by erosion. Broad, low-lying areas were blanketed with a layer of residual rubble. The source of the rubble was other nearby areas that were scraped bare of cinder and ash. ‘A’a flows, with their loose, clinkery crusts were likewise stripped of all that was not firmly anchored. As the icy mass crept in its multidirectional pattern, it sculpted ledges of rock. It left other boulders and rock surfaces striated and polished.

The larger cinder cones were partly exempted from the glacier's aggression, though the lower slopes, brushed by the passing ice, are steepened from the erosion. Measuring the height of that erosion probably helped geologists determine the thickness of the ice. It averaged less than 200 feet thick, but in some areas was increased to about 350 feet. That was relatively thin compared to the great continental ice sheets of the same period. Some areas of those glaciers accumulated ice in excess of 5,000 feet deep.

Weathering of the summit terrain continues, primarily as the result of alternate freezing and thawing. The rocks are gradually split apart as moisture penetrates holes and cracks, then expands when frozen. This continuing process fractures the rock, reducing it to pieces. Frost action is also responsible for arranging the pockets of rock fragments into mosaic-like patterns.

Features, like the stony mosaics, are noticeable only by those with a discriminating eye.

Fossilized organisms have been discovered, frozen in time within a year round permafrost layer, itself only recently found. The extent of the permafrost is not yet known, but parts of it lie beneath the cinders of the cones. An underlying layer of permafrost, acting as a seal, may account for why the water of Lake Waiau does not drain through the porous lava. Other speculation of why the water seems permanently pooled is that it is perched above a dense layer of tuff.