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Mauna Loa

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics;
North American Geography

                  Mauna Loa
   Eruption of Mauna Loa, March 1984.
   Elevation     4,169  m (13,679  ft)
   Location      Hawaii, USA
   Range         Hawaiian Islands
   Coordinates   19°28′46.3″N, 155°36′09.6″W
   Topo map      USGS Mauna Loa
   Type          Shield volcano
   Age of rock   < 200  kyr
   Last eruption 1984
   Easiest route jeep trail

   Mauna Loa is an active shield volcano in the Hawaiian Islands, one of
   five volcanoes that form the Island of Hawaii. It is Earth's largest
   mountain, with a volume estimated at approximately 18,000  cubic miles
   (75,000   km³), although its peak is about 36  m (120  ft) lower than
   that of its neighbour, Mauna Kea. In Hawaiian, mauna loa means "long
   mountain". Lava erupted from Mauna Loa is very fluid, and the volcano
   has extremely shallow slopes as a result.

   The volcano has probably been erupting for at least 700,000 years and
   may have emerged from the sea about 400,000 years ago, although the
   oldest known dated rocks do not extend beyond 200,000 years. Its magma
   comes from a hotspot in the Earth's mantle far beneath the island that
   has been responsible for the creation of the Hawaiian island chain for
   tens of million of years. The slow drift of the Pacific Plate will
   eventually carry the volcano away from the hotspot, and the volcano
   will thus become extinct within 500,000 to one million years from now.

   Mauna Loa's most recent eruption occurred in 1984. No recent eruptions
   of the volcano have caused fatalities, but eruptions in 1926 and 1950
   destroyed villages, and the city of Hilo is partly built on lava flows
   from the late 19th century. In view of the hazards it poses to
   population centers, Mauna Loa is part of the Decade Volcanoes program,
   which encourages studies of the most dangerous volcanoes. Mauna Loa has
   been intensively monitored by the Hawaiian Volcano Observatory (HVO)
   since 1912. Observations of the atmosphere are undertaken at the Mauna
   Loa Observatory, and of the Sun at the Mauna Loa Solar Observatory,
   both located near its summit. Hawaii Volcanoes National Park covers the
   summit and the south-eastern flank of the volcano, including a separate
   volcano, Kilauea.

Eruption history

Origins

   Mauna Loa probably began erupting between 700,000 and one million years
   ago, and has grown steadily since then. Like all of the Hawaiian
   islands, Mauna Loa has its origins in a hotspot—a plume of magma rising
   from deep in the Earth's mantle. The hotspot remains in a fixed
   position, while the Pacific Plate drifts over it at a rate of about
   10 cm per year. The upwelling of the hot magma creates volcanoes, and
   each individual volcano erupts for a few million years before the
   movement of the plate carries it away from the rising magma.

   The hotspot has existed for at least 80 million years, and the Emperor
   Seamounts chain of old volcanoes stretches almost 5,800 km
   (3,600 miles) away from the hotspot. Currently, the hotspot feeds
   activity at five volcanoes: Mauna Loa, Kilauea, and Hualalai on the Big
   Island, Haleakala on Maui, and Loihi, a submarine volcano south of the
   Big Island and the youngest Hawaiian volcano. Mauna Loa is the largest
   of these, although currently Kilauea is the site of the most intense
   volcanic activity.

Prehistoric eruptions

   Mauna Loa erupting in 1984.
   Enlarge
   Mauna Loa erupting in 1984.

   Prehistoric eruptions of Mauna Loa have been extensively analysed by
   carrying out radiocarbon dating on fragments of charcoal found beneath
   lava flows. The mountain's prehistoric activity is probably the best
   known of any volcano. Studies have shown that a cycle occurs in which
   volcanic activity at the summit is dominant for several hundred years,
   after which activity shifts to the rift zones for several more
   centuries, and then back to the summit again. Two cycles have been
   clearly identified, each lasting 1,500–2,000 years. This cyclical
   behaviour is unique to Mauna Loa among the Hawaiian volcanoes.

   Further back, records show that between about 7,000 and 6,000 years
   ago, Mauna Loa was largely inactive. The cause of this cessation in
   activity is not known, and no known similar hiatus has been found at
   other Hawaiian volcanoes except for those currently in the post-shield
   stage. Between 11,000 and 8,000 years ago, activity was more intense
   than it is today. However, Mauna Loa's overall rate of growth has
   probably begun to slow over the last 100,000 years, and in fact may be
   nearing the end of its tholeiitic basalt shield-building phase.

Historic eruptions

   Although native Hawaiians have witnessed eruptions for many centuries,
   written records only exist for eruptions that have occurred since the
   early 1800s. The first historical eruption occurred in 1832, and since
   then 39 eruptions have been documented. In total, these eruptions have
   covered over 800  km² of the volcano's flanks with lava flows.
   Typically, eruptions have been brief but intense, with 0.25-0.5 km³ of
   lava erupted over a few weeks. Until 1950, eruptions occurred roughly
   every 3-4 years, but since then the repose periods have dramatically
   lengthened with the only eruptions occurring in 1975 and 1984.

   The two most recent eruptions have been the most extensively studied.
   The 1975 eruption lasted only two days, and occurred at the summit of
   the mountain. The 1984 eruption saw fissures open up to the northwest
   and southeast, from the summit down to 2,900 m above sea level. Flows
   from this eruption headed rapidly towards the city of Hilo, but stopped
   about 4 km from the outskirts when the eruption ended after three
   weeks.

Current activity

   Lava fountains feed an aa lava flow in March 1984.
   Enlarge
   Lava fountains feed an aa lava flow in March 1984.

   The volcano has been dormant since 1984. Seismic activity remained low
   until 2002, when there was a sudden onset of inflation and the caldera
   walls started to move apart at a rate of 5 cm per year. This is thought
   to indicate that magma is filling a reservoir about 5 km beneath the
   summit. The inflation has been intermittent, sometimes slowing, and
   sometimes stopping for several weeks. Thus far, though, it has always
   restarted, and this is likely to indicate an increased probability of
   an eruption in the next few years.

   The inflation has been accompanied by increased seismic activity. A
   swarm of deep earthquakes began in July 2004, and continued until the
   end of the year. Earthquakes were detected at a rate of one per day for
   the first three weeks, increasing steadily over subsequent months to 15
   or so per day by the end of the year. The swarm ended in December 2004,
   and earthquake levels have been only moderately elevated since then.

Relationship with Kilauea

   Diagram showing an oblique view of the Big Island. Mauna Loa is the
   largest peak in the center; Kilauea's crater can be seen below and to
   the right.
   Enlarge
   Diagram showing an oblique view of the Big Island. Mauna Loa is the
   largest peak in the centre; Kilauea's crater can be seen below and to
   the right.

   Kilauea lies on the southern flank of Mauna Loa and was originally
   thought to be a satellite vent of Mauna Loa. However, chemical
   differences between the lavas from the two volcanoes show that they
   have separate shallow magma chambers, and so they are now considered
   separate volcanoes. Nevertheless, activity patterns at the two
   volcanoes do appear to be correlated.

   The most apparent relation between the two mountains is that,
   generally, periods of frequent activity at one volcano coincide with
   periods of low activity at the other. For example, between 1934 and
   1952, Kilauea was dormant and only Mauna Loa was active, while from
   1952 to 1974, only Kilauea was active while Mauna Loa lay dormant.

   The 1984 eruption of Mauna Loa started during an eruption at Kilauea,
   but had no discernible effect on the Kilauea eruption. Occasionally,
   though, eruptions at one volcano do seem to influence activity at the
   other. The recent inflation of Mauna Loa's summit began on the same day
   as a new large lava flow broke out at Kilauea's Puu Oo crater, and
   geologists have suggested that a 'pulse' of magma entering Mauna Loa's
   deep plumbing system could have increased pressure inside Kilauea and
   triggered the eruption.

Hazards

   Volcanic eruptions in Hawaii rarely cause fatalities—the only fatality
   due to volcanic activity there in the last century occurred at Kilauea
   in 1924 when an unusually explosive eruption hurled rocks at onlookers,
   killing one . However, property damage is common. Mauna Loa is a Decade
   Volcano, which means it has been identified as worthy of particular
   research in light of its frequent eruptions and proximity to populated
   areas. Many towns and villages near the volcano are built on lava which
   has erupted in the last two hundred years, and there is a very strong
   likelihood that future eruptions will cause damage to populated areas.

Lava flows

   An aa lava flow overruns vegetation during the 1984 eruption.
   Enlarge
   An aa lava flow overruns vegetation during the 1984 eruption.

   The main volcanic hazard at Mauna Loa is lava flows. Most flows advance
   at about walking pace, and so present little danger to human life, but
   eruptions at Mauna Loa can be more intense than those at Kilauea; for
   example, the 1984 eruption emitted as much lava in three weeks as
   Kilauea's current eruption produces in three years. Such high emission
   rates can generate comparatively fast-moving flows.

   Two eruptions of Mauna Loa have destroyed villages. In 1926, the
   village of Hoopuloa Makai was overrun by lava flows, while in 1950, the
   most voluminous eruption ever seen at Mauna Loa sent lava flows racing
   towards the sea, and the village of Hookena Mauka was destroyed on 2
   June 1950 by the advancing flows. Hilo is partly built on lava from an
   1880 eruption, and is at risk from further lava flows. The brief but
   intense 1984 eruption saw lava flow towards Hilo but it had not reached
   any buildings when the eruption stopped.

Flank collapse

   A greater but rarer hazard at Mauna Loa is the possibility of a sudden
   massive collapse of the volcano's flanks. Deep faults allow large
   portions of the sides of Hawaiian mountains to slide gradually
   downwards, the best known example being the Hilina Slump. Occasionally,
   a large earthquake can trigger a collapse of the flank, creating a
   massive landslide which may trigger a tsunami. Kealakekua Bay, on the
   western slope of Mauna Loa, was created by such an event. Undersea
   surveying has revealed numerous landslides along the Hawaiian chain,
   and two giant tsunamis are known to have occurred: 200,000 years ago,
   Molokai experienced a 75 m tidal wave, and 100,000 years ago, a tsunami
   325 m high struck Lanai.

   A recent example of the risks associated with slumps occurred in 1975,
   when the Hilina Slump suddenly moved forward by several meters. A
   magnitude-7.2 earthquake resulted which triggered a small tsunami with
   a wave height of a few meters.

Monitoring

   A tiltmeter on Mauna Loa, used to predict eruptions by measuring very
   small changes in the profile of the mountain.
   Enlarge
   A tiltmeter on Mauna Loa, used to predict eruptions by measuring very
   small changes in the profile of the mountain.

   Mauna Loa is an intensively monitored volcano. The Hawaiian Volcano
   Observatory (HVO) was established in 1912 to observe the Hawaiian
   volcanoes, and the HVO has developed many techniques to help predict
   when eruptions at Mauna Loa and other volcanoes are imminent.

   One of the most important tools is seismology. More than 60
   seismometers around the Big Island allow scientists to measure the
   intensities and locations of hundreds of small earthquakes every week.
   Earthquakes can begin to increase years before an eruption actually
   starts: the 1975 and 1984 eruptions were both preceded by one to two
   years of increased seismic activity at depths of less than 13 km.

   Another type of seismic activity occurs in the hours preceding an
   eruption. So-called volcanic tremor is a continuous 'rumble' which
   contrasts with the normal seismic activity of sudden shocks, and is
   believed to be caused by the rapid movement of magma underground.
   Volcanic tremor normally indicates an imminent eruption, although it
   may also be caused by shallow intrusions of magma which do not reach
   the surface.

   Another important indicator of what is happening underground is the
   shape of the mountain. Tiltmeters measure very small changes in the
   profile of the mountain, and sensitive equipment measures distances
   between points on the mountain. As magma fills the shallow reservoirs
   below the summit and rift zones, the mountain inflates. A survey line
   across the caldera measured a 75  mm increase in its width over the
   year preceding the 1975 eruption, and a similar increase before the
   1984 eruption.

Observatories

   Atmospheric CO2 concentrations measured at the Mauna Loa Observatory.
   Enlarge
   Atmospheric CO[2] concentrations measured at the Mauna Loa Observatory.

   The elevation and location of Mauna Loa have made it an important
   location for atmospheric monitoring by the Global Atmosphere Watch, and
   other scientific observations. The Mauna Loa Solar Observatory (MLSO),
   located at 3,400 m (11,155 ft) on the northern slope of the mountain,
   has long been prominent in observations of the Sun. The NOAA Mauna Loa
   Observatory (MLO), located close by, monitors the global atmosphere at
   its location well above local influences. Atmospheric carbon dioxide
   has been measured regularly since 1958 and shows the steadily
   increasing trend associated with the greenhouse effect and global
   warming.

   Retrieved from " http://en.wikipedia.org/wiki/Mauna_Loa"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
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