A former summit caldera may have been filled and buried by later summit eruption deposits.
Mauna Kea is over 3,200 km The volcano continues to slip and flatten under its own weight at a rate of less than 0.2 mm (0.01 in) per year. Mauna Kea stands 4,205 m (13,800 ft) above sea level, just 35 m (110 ft) higher than its neighbor Mauna Loa, Lava flows from Mauna Kea overlapped in complex layers with those of its neighbors during its growth.
The lake is very small and shallow, with a surface area of 0.73 ha (1.80 acres) and a depth of 3 m (10 ft).
Radiocarbon dating of samples at the base of the lake indicates that it was clear of ice 12,600 years ago.
Mauna Kea is about a million years old, and has thus passed the most active shield stage of life hundreds of thousands of years ago.
In its current post-shield state, its lava is more viscous, resulting in a steeper profile.
Isotopic composition shows the water present to have been derived from rain coming off Mauna Kea at an elevation higher than 2000 meters above mean sea level.
Its presence is attributed to a freshwater head within Mauna Kea's basal lens.
An ancient law allowed only high-ranking aliʻi to visit its peak.
They are covered by the oldest exposed rock strata on Mauna Kea, the post-shield alkali basalts of the Hāmākua Volcanics, which erupted between 250,000 and 70–65,000 years ago.
The most recent volcanic flows are hawaiites and mugearites: they are the post-shield Laupāhoehoe Volcanics, erupted between 65,000 and 4,000 years ago.
Donald Thomas, director of the University of Hawaii's Center for the Study of Active Volcanoes believes one reason to continue study of the aquifers is due to use and ‘occupancy’ of the higher elevation areas, stating: "Nearly all of these activities depend on the availability of potable water that, in most cases, must be trucked to the Saddle from Waimea or Hilo — an inefficient and expensive process that consumes a substantial quantity of our scarce liquid fuels.” because of this inactivity, Mauna Kea is assigned a United States Geological Survey hazard listing of 7 for its summit and 8 for its lower flanks, out of the lowest possible hazard rating of 9 (which is given to the extinct volcano Kohala).
Twenty percent of the volcano's summit has seen lava flows in the past 10,000 years, and its flanks have seen virtually no lava flows during that time.