Mauna Kea - A Postshield-Stage Volcano that Once Hosted Glaciers
Mauna Kea, the highest peak on the Island of Hawai‘i (4,207.3 m, 13,803 ft), is a shield volcano built upon the southern flank of Kohala Volcano's eastern rift zone. Mauna Kea is slightly higher than Mauna Loa (4,170 m, 13,681 ft), but its estimated volume (42,000 km3) is about 55% less than that of Mauna Loa (95,000 km3). The volcano is in the advanced-postshield stage (hawaiitic-substage) of Hawaiian volcanism; geologic research of Mauna Kea shows that eruptions have become less frequent and chemistry of lava erupted has changed as the volcano has moved steadily away from the source of magma generation—the Hawaiian hot spot.
Shield stage erupted most of Mauna Kea's lava
The oldest visible rocks on Mauna Kea are found near sea level in deep gulches on the northeast flank of the volcano. Chemistry of these rocks correlates with magma that typically erupts in the postshield stage. Age dating shows that they were erupted 200,000 to 250,000 years ago, which means the rapid-growth shield stage had ended by that time. Scientists estimate that eruptions during the shield stage lasted for about 800,000 years, so Mauna Kea is probably at least 1 million years old. Lava erupted during the shield stage built more than 90% of its overall estimated volume.
Post-shield stage: low eruption rates, tephra, and glaciers
Scientists have pieced together a detailed and rich geologic history of volcanism that erupted relatively stubby lava flows, cinder cones, and tephra during two different time periods—known as the Hamakua and Laupahoehoe volcanics. Due to differences in chemistry and mineral makeup, postshield lavas have a higher viscosity than those erupted during the shield stage. Higher viscosity lavas produce thick and rubbly flows and tall cinder cones, which is why Mauna Kea has a steeper and more irregular shaped surface than neighboring shield-stage Mauna Loa. Additionally, these two geologic units host evidence of repeated glaciation during the past 250,000 years.
Eruptions of Mauna Kea during the postshield stage produced about 850 km3 of lava between 250,000 and 70,000 years ago. These basalt lava flows and cinder cones, named the Hamakua Volcanics, completely cover the volcano but are most common at the summit and upper flanks. The eruption of these viscous postshield lavas from high-elevation vents formed a thick cap and contributed to the steepening of Manua Kea's slopes, which are twice as steep as the upper flanks of Mauna Loa.
The average lava supply during the Hamakua Volcanics was about 0.005 km3/yr, which is about 10% of the supply rate estimated during its shield stage (0.05km3/yr). For comparison, the supply rate at Kīlauea Volcano is about 0.1 km3/yr, based on its eruptions and intrusions during the past 200 years.
Beginning about 65,000 years ago, a change in lava chemistry of Mauna Kea's eruptions formed prominent cinder cones and elongate, rugged ‘a‘ā lava flows, named the Laupahoehoe Volcanics. These cones and flows create much of the topographic irregularities that make the upper slopes and summit of Mauna Kea in contrast with the much smoother slopes of the Mauna Loa shield. It is likely that these eruptions occurred as sporadic clusters of activity, the most recent between 6,000 and 4,000 years ago. Laupahoehoe eruptions produced large volumes of tephra that fell beyond the cinder cones. The ash is a dominant component of the surficial geology that blanket parts of Mauna Kea's flanks.
Scientists estimate that about 25 km3 of lava erupted during the Laupahoehoe activity, giving an average lava-supply rate of about 0.0004 km3/yr. This is about 8% of the supply rate calculated during the Hamakua Volcanics.