Mauna Kea (/ˌmɔːnə ˈkeɪ.ə/ or /ˌmaʊnə ˈkeɪ.ə/, Hawaiian: [ˈmɐwnə ˈkɛjə]),
is a dormant volcano on the island of Hawaii.
Standing 4,207 m (13,802 ft) above sea level, its peak is the highest point in the state of Hawaii. Much of the mountain is under water; when
measured from its oceanic base, Mauna Kea is over 10,000 m
(33,000 ft) tall and is the tallest mountain on Earth. 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. Late volcanism has also given it a much rougher appearance
than its neighboring volcanoes; contributing factors include the construction
of cinder cones, the decentralization of its rift
zones, the glaciation on its peak, and the weathering effects of
the prevailing trade winds. Mauna Kea last erupted 6,000 to 4,000 years ago and is now
considered dormant.
In Hawaiian mythology, 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 mountain's
ecological balance. Mauna Kea can be ecologically divided into three sections:
an alpine climate at its summit, a Sophora chrysophylla–Myoporum sandwicense (or māmane–naio) forest on its flanks, and
an Acacia koa–Metrosideros polymorpha (or koa–ʻōhiʻa)
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 Hawai'i Department of Land and Natural Resources to eradicate all feral species on the
mountain.
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 from visible light
to radio, 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.
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 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.[14][19] 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 lava types 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.
Until
1993, artesian water was not known to be present in the Island of Hawaii.
Drilling by the University of Hawaii at
that time encountered an artesian groundwater aquifer at 300 meters below sea
level and 100 meters of hole depth within a compacted layer of soil and lava
where the flows of both Mauna Loa and Mauna Kea meet (Humuula saddle region).
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. Scientists believe there may be more water in Mauna Kea's fresh water
lens than current models may indicate. In 2012 two more bore holes were
drilled on Mauna Kea and water discovered at much higher elevations than
previously believed but shallower than expected. 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.”
Future activity
The
last eruption of Mauna Kea was about 4,600 years ago (about 2600
BCE); 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.
Despite
its dormancy, Mauna Kea is expected to erupt again, although there would be
sufficient warning to evacuate. The telescopes on Mauna Kea's summit would be
the first to detect the minute amounts of deformation resulting from the
volcano's swelling, acting like expensive tiltmeters. Based on prior
eruptions, such an event could occur anywhere on the volcano's upper flanks and
would likely produce extended lava flows, mostly of a'a, of 15–25 km
(9–16 mi) in length. Long periods of activity could build a cinder cone at
the source. Although not likely in the next few centuries, such an eruption
would probably result in little loss of life but significant damage to infrastructure