Mauna Kea - Geology

Geology

Mauna Kea is one of five hotspot volcanoes that form the island of Hawaii, the largest and youngest island of the Hawaiian-Emperor seamount chain. Of these five volcanoes, Mauna Kea is the fourth oldest and fourth most active. It began as a preshield volcano driven by the Hawaii hotspot around one million years ago, and became exceptionally active during its shield stage until 500,000 years ago. Mauna Kea entered its quieter post-shield stage 250,000 to 200,000 years ago, and is currently dormant. Mauna Kea does not have a visible summit caldera, but contains a number of small cinder and pumice cones near its summit. A former summit caldera may have been filled and buried by later summit eruption deposits.

Mauna Kea is over 3,200 km3 (770 cu mi) in volume, so massive that it and its neighbor, Mauna Loa, depress the ocean crust beneath it by 6 km (4 mi). 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. Much of its mass lies east of its present summit. Mauna Kea stands 4,205 m (13,800 ft) above sea level, just 35 m (110 ft) higher than its neighbor Mauna Loa, and is the highest point in the state of Hawaii. Measured from its base on the ocean floor, it rises over 10,000 m (33,000 ft), significantly greater than the elevation of Mount Everest above sea level.

Like all Hawaiian volcanoes, Mauna Kea has been created as the Pacific tectonic plate has moved over the Hawaiian hotspot in the Earth's underlying mantle. The Hawaii island volcanoes are merely the most recent evidence of this process that, over 70 million years, has created the 6,000 km (3,700 mi)-long Hawaiian Ridge – Emperor Seamount chain. The prevailing, though not completely settled, view is that the hotspot has been largely stationary within the planet's mantle for much, if not all of the Cenozoic Era. However, while Hawaiian volcanism is well-understood and extensively studied, there remains no definite explanation of the mechanism that causes the hotspot effect.

Lava flows from Mauna Kea overlapped in complex layers with those of its neighbors during its growth. Most prominently, Mauna Kea is built upon older flows from Kohala to the northwest, and intersects the base of Mauna Loa to the south. The original eruptive fissures (rift zones) in the flanks of Mauna Kea were buried by its post-shield volcanism. Hilo Ridge, a prominent underwater rift zone structure east of Mauna Kea, was once believed to be a part of the volcano; however, it is now understood to be a rift zone of Kohala that has been affected by younger Mauna Kea flows.

The shield-stage lavas that built the enormous main mass of the mountain are tholeiitic basalts, like those of Mauna Loa, created through the mixing of primary magma and subducted oceanic crust. 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. These changes in lava composition accompanied the slow reduction of the supply of magma to the summit, which led to weaker eruptions that then gave way to isolated episodes associated with volcanic dormancy. The Laupāhoehoe lavas are more viscous and contain more volatiles than the earlier tholeiitic basalts; their thicker flows significantly steepened Mauna Kea's flanks. In addition, explosive eruptions have built cinder cones near the summit. These cones are the most recent eruptive centers of Mauna Kea. Its present summit is dominated by lava domes and cinder cones up to 1.5 km (0.9 mi) in diameter and hundreds of meters tall.

Mauna Kea is the only Hawaiian volcano with distinct evidence of glaciation. Similar deposits probably existed on Mauna Loa, but have been covered by later lava flows. Despite Hawaii's tropical location, during several past ice ages a drop of only a degree in temperature allowed snow to remain at the mountain's summit through summer, triggering the formation of an ice cap. There are three episodes of glaciation that have been recorded from the last 180,000 years: the Pōhakuloa (180–130 ka), Wāihu (80–60 ka) and Mākanaka (40–13 ka) series. These have extensively sculpted the summit, depositing moraines and a circular ring of till and gravel along the mountain's upper flanks. Subglacial eruptions built cinder cones during the Mākanaka glaciation, most of which were heavily gouged by glacial action. The most recent cones were built between 9000 and 4500 years ago, atop the glacial deposits, although one study indicates that the last eruption may have been around 3600 years ago.

At their maximum extent, the glaciers extended from the summit down to between 3,200 and 3,800 m (10,500 and 12,500 ft) of elevation. A small body of permafrost, less than 25 m (80 ft) across, was found at the summit of Mauna Kea prior to 1974, and may still be present. Small gullies etch the summit, formed by rain- and snow-fed streams that flow only during winter melt and rain showers. On the windward side of the mountain, stream erosion driven by trade winds has accelerated erosion in a manner similar to that on older Kohala.

Mauna Kea is home to Lake Waiau, the highest lake in the Pacific Basin. At an altitude of 3,969 m (13,022 ft), it lies within the Puʻu Waiau cinder cone and is the only alpine lake in Hawaii. 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. Hawaiian lavas are typically permeable, preventing the formation of lakes due to infiltration. Here, either sulfur-bearing steam altered the volcanic ash to low-permeability clays, or explosive interactions between rising magma and groundwater or surface water (phreatic eruptions) formed exceptionally fine ash that also would reduce the permeability of the lake bed.

In November 2012, scientists analysing the mineral composition of Martian soil dug up by NASA's Curiosity rover said it was similar to soil found on Mauna Kea.