General Features and glacial geology of Mauna Kea, Hawaii
by Herbert Gregory and Chester Wentworth
bulletin of the Geological Society of America
Vol. 48, 1937
Mauna Kea…its profile is a series of broken, irregularly placed steps which lead to a summit plateau with an area of about 10 square miles.
Mauna Loa is an active volcano and any traces of glacial action which may have taken place at its summit during Pleistocene time have...been effaced by subsequent lava flows. Mauna Kea, on the contrary, has long been dormant and shows evidence of little post-Wisconsin volcanic activity. Hence it is the only place in Hawaii and probably in the entire central Pacific, where a record of Pleistocene glaciation can be found.
In certain respects the most remarkable drainage feature of the Mauna Kea summit area is Lake Waiau — a perennial body of water in the bowl of the comparatively old Waiau ash cone…an area of 1 ½ acres, a depth of 8 to 15 feet. The freshness of its outlet channel suggests that each spring the surplus water from melting snows finds its way to Pohakuloa gulch. Around the southern half of its shore the beach is moist with seepage water and a spring hole dug on the southeast side was maintained full to a level of about a foot higher than the lake.
The lake has a yellowish green color derived from organic matter in the water. The water from Waiau Lake is a veritable infusion. Bacteria are extremely numerous and probably the chief factor in causing the turbidity of the water. The sample of muck contains several blue-green algae, desmids, diatoms, at least two species of nematodes, hosts of bacteria and many kinds of protozoa.
Lake Waiau is one of the few perennial water bodies in Hawaii. Its position in an area of porous rocks at an altitude of 13,000 feet is worthy of special comment.
At Lake Waiau the highest temperature measured during 11 days in August, 1935, was 57.1 degrees, the lowest, 18.9. All observers report that the water in Lake Waiau freezes at night during all seasons of the year. During the summer at least the ice melts during the day.
Lake Waiau lies in the bowl of Pu‘u Waiau — a cone built chiefly of fine-grained and much-weathered cinders and ash. As the average depth of the lake when full of water is about 15 feet and the muck at its bottom as much as 8 feet, the floor of its basin lies 23 feet below the lowest part of its rim. In superficial view, Waiau has the appearance of an ordinary crater lake, but striae directed toward the basin from the northeast, morainal deposits high up on its southern slope, and scour marks on its outlet bar, show that it was occupied by glacial ice. It seems probable that ice to a depth of 100 feet or more was forced into the basin and after a temporary halt was forced out to join the larger ice tongues moving down Pohakuloa Gulch.
Scouring by the ice doubtless deepened the original basin, and it may be that some ice remained after the glaciers disappeared. The possibility is suggested that downward seepage of lake water is impeded not only by fine-grained ash and organic material but also by ground ice that probably forms each year.
The surface [of the upper slopes] is like that at the tops of isolated peaks in the Rocky, Sierra Nevada and Cascade mountains which have been demonstrated to result from long-continued ice-scouring during Pleistocene time...but, as compared with many other regions, the evidences of glaciation at Mauna Kea are somewhat obscure, and...the distinctive proofs may not be recognized.
Mapping of the features produced by ice scour and ice deposition serve to outline a glaciated area of 27.98 square miles, within which the cinder cones which stood as nunataks occupy 1.97 square miles. The lower edge of the ice lay at altitudes between 10,550 and 12,500 feet.
The ice moved radially outward from the saddle east of Pu‘u Poliahu in three main channels. By far the larger part passed westward down the mountain slope between Pu‘u Poliahu and Pu‘u Waiau. It is interesting to note that these three lines of glacial movement correspond in position to the three main series of lava flows.
Field observations show that ice-scouring, though long continued, was comparatively feeble. There is no evidence of deep erosion. The preglacial ledges are smoothed but not destroyed, and rock basins and striated boulders characteristic of intense ice action are conspicuously rare.
….the volume of glacier ice...a thickness of as much as 100 feet. At their source the ice streams were doubtless thicker. The estimated average is 150 feet. The maximum thickness was as much as 350 feet.
On both sides of Pohakuloa Gulch, a conspicuous cover of glacial debris, including large blocks, extends down the valley for nearly half a mile from an altitude of about 11,250 feet to a lower limit of about 10,500 feet. Adjacent to the upper parts of these tongues of debris, and extending laterally each way along the contour of the mountain, is a marginal moraine especially prominent on its steep, downhill slope. In places the deposit is 150 feet or more in thickness. The relationship of the marginal moraine to the dome of Mauna Kea is clearly seen when looking upstream from the flats near the Pohakuloa Camp, especially at sunrise or sunset when the glacial pile is brought into sharp relief in the oblique lighting.
The glaciers on Mauna Kea are the local expression of the world-wide lowering of temperature during the ice age which resulted in the accumulation of ice a few hundred to several thousand feet thick over an area of more than 6,000,000 square miles of mountain and lowland. In many regions, perpetual snow and unmistakable features of glaciation make it possible to mark the position of the lower limit of Pleistocene snow and ice. Thus in the latitude of Hawai‘i, the snow line in the Andes, now at 15,000 to 19,000 feet stood 3,000 to 4,000 feet lower. On Mauna Kea, where a perpetual snow cap is lacking, the altitude of the ancient snow line can only be estimated. It seems probable, however, that the shift on mountains in mid-ocean would be less than on continental areas.
The world-wide decrease of land temperatures during the Pleistocene period was accompanied by lowered temperature of sea water which, in time, brought a changed environment for the fauna and flora. Doubtless also, the environment was further modified by a re-alignment of winds and currents incident to shift in zones of temperature and increase in amount of snow. During glaciation, plants that may have grown about the summit of Mauna Kea were destroyed, and no buried remains have been found.
For marine organisms, whose distribution and rate of growth is influenced by even slight changes in temperature and composition of sea water, the glacial climate was of profound significance. In particular, the shift from warm to cool climate and back again to warm climate is recorded by the plants and animals that constitute the coral reefs.
In Hawaii the minimum temperature of sea water is 23 degrees C, only 3 degrees above the lowest limit at which corals grow. During the Pleistocene various lines of evidence show it to have been 3 to 10 degrees lower. In waters at these temperatures, corals could not thrive, and, lacking the defensive cover of organic life, reefs that may have been present in pre-glacial time would have been destroyed by the battering surf. It therefore seems highly probable that while the summit of Mauna Kea was capped with ice the waters about its base held no reef-building organisms.
While Pleistocene climates prevailed, the earth was not only chilled; the relation of sea to land was also affected. During the successive glaciations that increased the amount of polar ice and expanded the continental and mountain glaciers, much water must have been taken from the oceans, resulting in a lowered sea level. Daly estimates that the water locked up as ice during the Pleistocene is sufficient to have lowered the surface of the oceans approximately 90 meters. In other words, since Mauna Kea was capped with ice, the sea level of Hawai‘i has risen about 300 feet. It is further estimated that if all the existing ice masses were melted the surface of the ocean would stand about 165 feet still higher.
The last expansion of glacial ice, and probably the earliest expansion, post-dated most of the volcanism that gave Mauna Kea its present form. Several of the cones have been scoured by ice which passed their bases, and the freedom of the summit area from cinders and ash, except at the immediate slopes of cones, indicates that no explosive eruptions from a major vent have taken place in post-glacial time. However, a few features indicate that some volcanic activity continued into glacial time, and that a few minor eruptions may have occurred since the recession of the ice.
In several places west and northwest of the Pu‘u Keonehehe‘e, aa flows overlap the glacial moraine lying along the east side of Waikahalulu Gulch. These flow lavas appear to have come from the small cone just northwest of Keonehehe‘e (12,000 feet). Certainly the lava flow and probably the entire mass of the adjacent cone are post-glacial. Another post-glacial flow...Pu‘u Poepoe, south-southwest of Pu‘u Makanaka.
...glacial ice moved over a terrain having substantially the same configuration as now, and the clearly post-glacial flows are few and small in the immediate summit area, it is certain that all the large cones and the mountain as a whole assumed their present forms in pre-glacial time.
Though the cone-dotted summit plateau of Mauna Kea lacks the ruggedness of most high mountains, the generally smoothed surface produced by glaciation has been much modified by the work of snow and ice.
Around the margins of cinder cones a thin layer of cinders had crept outward and downward into a zone evidently once covered by glacial ice. In places the cinders surround and nearly cover large glacial blocks which remained in place while the finer material crept downward under snow banks. Here and there, long trains of stones extend down the slopes. They are conspicuous at about 13,000 feet altitude north of the summit cluster of cones. These so-called "rock glaciers" are probably due to a slow, downhill creeping aided by freezing and thawing...
Another feature associated with nivation and frost work is the conspicuous "striped ground" or "hachured ground" observed in many places and particularly well developed on the non-glaciated lower slopes of cinder cones.
These stripes are believed to be due to frost heaving and sorting of rock debris in the surficial layers, and are evidently related to the "stone nets" or polygonboden developed in many regions where frost heaving is relatively active and other processes locally feeble.
Rock stripes and polygonboden are...due to freezing and thawing of the surface layers...the patterns are characteristic of high latitudes and high altitudes where soil and fine rock materials of fairly uniform constitution are subjected to freezing through much of the year.
Over the glaciated area of Mauna Kea the wedge-work of ice is conspicuous. The bed rock has been shattered, and spalls and slabs by thousands are strewn over the surface.
The temperature falls below the freezing point every night in the year, and, with the possible exception of short periods during the winter, it rises above freezing point each day. Thus alternate melting and freezing of ice in cracks is nearly a continuous process. Moreover, daytime melting of the snow banks that during most of the year lie in sheltered places permits water to trickle down rock surfaces and into all accessible crevices, only to be frozen at night. Observations indicate that the wedgework of ice on Mauna Kea is an exceptionally active process, perhaps ten times more effective than in most parts of the northern United States.