Mauna Kea
Mauna Kea is a dormant shield volcano on the island of Hawaiʻi. Its peak is above sea level, making it the highest point in Hawaii and the island with the second highest high point, behind New Guinea. The peak is about higher than Mauna Loa, its more massive neighbor. Mauna Kea is unusually topographically prominent for its height: its prominence from sea level is 15th in the world among mountains, at ; its prominence from under the ocean is, rivaled only by Mount Everest. This dry prominence is greater than Everest's height above sea level of, and some authorities have labeled Mauna Kea the tallest mountain in the world, from its underwater base. Mauna Kea is ranked 8th by topographic isolation.
It is about one million years old and 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. Late volcanism has also given it a much rougher appearance than its neighboring volcanoes due to construction of cinder cones, decentralization of its rift zones, glaciation on its peak, and weathering by the prevailing trade winds. Mauna Kea last erupted 6,000 to 4,000 years ago and is now thought to be dormant.
In Hawaiian religion, the peaks of the island of Hawaiʻi are sacred. An ancient law allowed only high-ranking aliʻi to visit its peak. Ancient Hawaiians living on the slopes of Mauna Kea relied on its extensive forests for food, and quarried the dense volcano-glacial basalts on its flanks for tool production. When Europeans arrived in the late 18th century, settlers introduced cattle, sheep, and game animals, many of which became feral and began to damage the volcano's ecological balance. Mauna Kea can be ecologically divided into three sections: an alpine climate at its summit, a Sophora chrysophylla–''Myoporum sandwicense forest on its flanks, and an Acacia koa–Metrosideros polymorpha'' forest, now mostly cleared by the former sugar industry, at its base. In recent years, concern over the vulnerability of the native species has led to court cases that have forced the Hawaiʻi Department of Land and Natural Resources to work towards eradicating all feral species on the volcano.
With its high elevation, dry environment, and stable airflow, Mauna Kea's summit is one of the best sites in the world for astronomical observation. Since the creation of an access road in 1964, thirteen telescopes funded by eleven countries have been constructed at the summit. The Mauna Kea Observatories are used for scientific research across the electromagnetic spectrum and comprise the largest such facility in the world. Their construction on a landscape considered sacred by Native Hawaiians continues to be a topic of debate to this day.
Topographic prominence
Mauna Kea is unusually topographically prominent for its height, with a wet prominence fifteenth in the world among mountains, and a dry prominence second in the world, after only Mount Everest. It is the highest peak on its island, so its wet prominence matches its height above sea level, at. Because the Hawaiian Islands slope deep into the ocean, Mauna Kea has a dry prominence of. This dry prominence is taller than Mount Everest's height above sea level of, so Everest would have to include whole continents in its foothills to exceed Mauna Kea's dry prominence.Given how much Mauna Kea protrudes from the Hawaiian Trough, some authorities have called it the tallest mountain in the world, as measured from base to peak. Unlike prominence, base is loosely defined, which has resulted in numbers ranging from to . Similar calculations have made contradictory claims for other mountains. The climb from the deepest known point on Earth's seabed to nearby Mount Lamlam is, and the Himalayan Mountains have tremendously deep roots. The climb between parts of the Atacama Trench and Andes Mountains has been described as "the highest slope on earth": the bottom of Richard's Deep to the peak of the nearby Llullaillaco is. Neither Mount Lamlam nor Llullaillaco have the dry prominence of Mauna Kea, because they do not extend into trenches in every direction.
Geology
Mauna Kea is one of five volcanoes that form the island of Hawaiʻi, the largest and youngest island of the Hawaiian–Emperor seamount chain. Of these five hotspot volcanoes, Mauna Kea is the fourth oldest and fourth most active. It began as a preshield volcano driven by the Hawaiʻi 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 active, having last erupted between 4,500 and 6,000 years ago. 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 in volume, so massive that it and its neighbor, Mauna Loa, depress the ocean crust beneath it by.
The volcano continues to slip and flatten under its own weight at a rate of less than per year. Much of its mass lies east of its present summit. It stands above sea level, about higher than its neighbor Mauna Loa, and is the highest point in the state of Hawaii.
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 the most recent evidence of this process that, over 70 million years, has created the -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 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 volcano 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 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 a degree in temperature allowed snow to remain at the volcano'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, Wāihu and Mākanaka series. These have extensively sculpted the summit, depositing moraines and a circular ring of till and gravel along the volcano'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 9,000 and 4,500 years ago, atop the glacial deposits, although one study indicates that the last eruption may have been around 3,600 years ago.
At their maximum extent, the glaciers extended from the summit down to between of elevation. A small body of permafrost, less than across, was found at the summit of Mauna Kea before 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 volcano, 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, it lies within the Puu Waiau cinder cone and is the only alpine lake in Hawaii. The lake is very small and shallow, with a surface area of and a depth of when fullest. Radiocarbon dating of samples at the base of the lake indicates that it was clear of ice 12,600 years ago. Hawaiian lava types are typically permeable, preventing lake formation due to infiltration. Either sulfur-bearing steam altered the volcanic ash to low-permeability clays, or explosive interactions between rising magma and groundwater or surface water during phreatic eruptions formed exceptionally fine ash that reduced the permeability of the lake bed.
No artesian water was known on the island of Hawaiʻi until 1993 when drilling by the University of Hawaiʻi tapped an artesian aquifer more than below sea level, that extended more than of the borehole's total depth. The borehole had drilled through a compacted layer of soil and lava where the flows of Mauna Loa had encroached upon the exposed Mauna Kea surface and had subsequently been subsided below sea level. Isotopic composition shows the water present to have been derived from rain coming off Mauna Kea at higher than above mean sea level. The aquifer's presence is attributed to a freshwater head within Mauna Kea's basal lens. Scientists believe there may be more water in Mauna Kea's freshwater lens than current models may indicate. Two more boreholes were drilled on Mauna Kea in 2012, with water being found at much higher elevations and shallower depths than expected. Donald Thomas, director of the University of Hawaiʻi'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."