Tall cinder cones atop the summit of Mauna Kea (4,205m) and lava flows that underlie its steep upper flanks have built the volcano a scant 35 m higher than nearby Mauna Loa (4,170 m). Mauna Kea, like Hawai`i's other older volcanoes, Hualalai and Kohala, has evolved beyond the shield-building stage, as indicated by (1) the very low eruption rates compared to Mauna Loa and Kilauea; (2) the absence of a summit caldera and elongated fissure vents that radiate its summit; (3) steeper and more irregular topography (for example, the upper flanks of Mauna Kea are twice as steep as those of Mauna Loa); and (4) different chemical compositions of the lava.

These changes in part reflect a low rate magma supply that causes the continuously active summit reservoir and rift zones of the shield stage to give way to small isolated batches of magma that rise episodically into the volcano, erupt briefly, and soon solidify. They also reflect greater viscosity and volatile content of the lava, which result in thick flows that steepen the edifice and explosive eruptions that build large cinder cones.

Most people don't think about snow or glaciers in Hawai`i, but geologists have long recognizd deposits formed by glaciers on Mauna Kea during recent ice ages. The latest work indicates that deposits of three glacial episodes since 150,000 to 200,000 years ago are preserved on the volcano. Glacial moraines on the volcano formed about 70,000 years ago and from approximately 40,000 to 13,000 years ago. If glacial deposits were formed on Mauna Loa, they have long since been buried by younger lava flows.

Even today, snow falls on both Mauna Kea and Mauna Loa. Both volcanoes are so high that snow falls during winter months, perhaps accumulating to a few meters depth. The seasonal snow cover on the steep slopes of Mauna Kea is easier to see from coastal areas than on the gentle, rounded slopes of Mauna Loa, whose summit cannot be seen from sea level.

Will Mauna Kea erupt again?
Mauna Kea is presently a dormant volcano, having last erupted about 4,500 years ago. However, Mauna Kea is likely to erupt again. Its quiescent periods between eruptions are long compared to those of the active volcanoes Hualalai (which erupts every few hundred years), Mauna Loa (which erupts every few years to few tens of years) and Kilauea (which erupts every few years). A swarm of earthquakes beneath Mauna Kea might signal that an eruption could occur within a short time, but such swarms do not always result in an eruption. Sensitive astronomical telescopes on top of Mauna Kea would, as a by product of their stargazing, detect minute ground tilts possibly foretelling a future eruption.

Most Recent Eruption(s)
At least 7 separate vents erupted between about 6,000 and 4,000 years ago

Number of Historical Eruptions
none

Oldest Dated Rocks
237,000 ± 31,000 years before present

Estimated Age of Mauna Kea
About 1 million years

Volcano Stage
Post-shield Stage (transition from shield stage to post-shield occurred before about 200,000 to 250,000 years ago)

Reference

Wolfe, E.W., Wise, S. W., and Dalrymple, B., 1997, The geology and petrology of Mauna Kea Volcano, Hawaii -- A study of postshield volcansim: U.S. Geological Survey Professional Paper 1557, p. 129, 4 plates.