Hawaii hotspot

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Coordinates: 18°55′N 155°16′W / 18.92°N 155.27°W / 18.92; -155.27
Hawaii hotspot
Hotspot
Raised-relief map of the Pacific basin, showing seamounts and islands trailing the Hawaii hotspot in a long line terminating near the Russian island of Kamchatka Peninsula in Russia.
Bathymetry of the Hawaiian – Emperor seamount chain, showing the long volcanic chain generated by the Hawaii hotspot, starting in Hawaiʻi and ending at the Aleutian Trench.
Country United States
State Hawaii
Region North Pacific Ocean
Coordinates 18°55′N 155°16′W / 18.92°N 155.27°W / 18.92; -155.27 —Loihi Seamount, actual hotspot lies about 40 km (25 mi) southeast
A diagram illustrates the hotspot area of the crust in cross-section and states that the motion of the overtopping Pacific Plate in the lithosphere expands the plume head in the asthenosphere by dragging it.
A diagram demonstrating the drift of the Earth's crust over the hotspot

The Hawaii hotspot is a volcanic hotspot located near the Hawaiian Islands in the central Pacific Ocean. One of the best-known and most studied hotspots in the world,[1][2] the Hawaii hotspot is responsible for the creation of the Hawaiian – Emperor seamount chain, a long chain of at least 129 volcanoes, more than 123 of which are extinct volcanoes, seamounts, and atolls, four active volcanoes and two dormant volcanoes. The chain extends from the island of Hawaiʻi to the Kuril–Kamchatka Trench, near Russia. While most volcanoes are created by geological activity at tectonic plate boundaries, the Hawaii hotspot was created by a mantle plume located far from any nearby plate boundaries.

The classic hotspot theory, first described in 1963 by John Tuzo Wilson, proposes that hotspot activity is generated by a single, fixed mantle plume that generates local volcanic activity; as the Pacific Plate moves over the hotspot, newly constructed volcanoes lose their connection to the hotspot, becoming inactive and eventually eroding below sea level over a period of millions of years. According to Wilson's theory, the nearly 60° bend separating the Emperor and Hawaiian segments of the chain would have been caused by a sudden shift in the movement of the Pacific Plate. In 2003, evidence for a mobile hotspot theory suggested that about 47 million years ago, the bend in the chain was caused by a shift in direction of motion.

Ancient Hawaiian fishermen were the first to recognize that the age and erosion of the islands increased as one progressed towards the northwest. James Dwight Dana directed the earliest formal geological study, from 1880 to 1881, and first confirmed the relationship between age and location, again based on erosion. In 1912, geologist Thomas Jaggar founded the Hawaiian Volcano Observatory, initiating continuous scientific volcano observation on the island of Hawai'i. The model for the evolution of Hawaiian volcanoes was completed in 1946. Wilson used Hawaii island chain data in 1963 to develop the hotspot theory. In the 1970s, a mapping project was initiated to gain more information about the complex geology of Hawaii's seafloor.

The hotspot has since been tomographically imaged, showing it to be 500 to 600 km (310 to 370 mi) wide and up to 2,000 km (1,200 mi) deep, and olivine and garnet-based studies have shown its magma chamber is approximately 1,500 °C (2,730 °F). In its at least 85 million years of activity the hotspot has produced an estimated 750,000 km3 (180,000 cu mi) of rock. The chain's rate of drift has slowly increased over time, causing the amount of time each individual volcano is active to decrease, from 18 million years for the 76 million year old Detroit Seamount, to just under 900,000 for the one million year-old Kohala; on the other hand, eruptive volume has increased from 0.01 km3 (0.002 cu mi) per year to about 0.21 km3 (0.050 cu mi). Overall, this has caused a trend towards more active but quickly-silenced, closely spaced volcanoes—whereas volcanoes on the near side of the hotspot overlap each other (forming such superstructures as Hawaiʻi island and the ancient Maui Nui), the oldest of the Emperor seamounts are spaced as far as 200 km (120 mi) apart.

Contents

[edit] Hotspot theory

See also: Hotspot (geology), Hawaiian – Emperor seamount chain, and Evolution of Hawaiian volcanoes

Tectonic plates generally focus deformation and volcanism at plate boundaries. However, the Hawaii hotspot is more than 3,200 kilometers (1,988 mi) from the nearest plate boundary;[1] in 1963, Canadian geophysicist J. Tuzo Wilson proposed the hotspot theory to explain how these zones of deformation and volcanism form in plate interiors.[3]

[edit] Wilson's stationary hotspot theory

Global map labeled Crustal Age with callouts for specific areas of interest. There is an overall pattern of younger crust in the East Pacific and younger in the West.
Map, color coded from red to blue to indicate the age of crust built by seafloor spreading. 2 indicates the position of the bend in the hotspot trail, and 3 points to the present location of the Hawaii hotspot.

Wilson claimed that certain small, long lasting, exceptionally hot areas of magma are located under the Earth's surface, create localized heat and energy systems, known as thermal or mantle plumes, that sustain long-lasting volcanic activity. This "mid-plate volcanism" builds mountains that rise from the relatively featureless sea floor, initially as seamounts, which eventually rise above the surface, forming volcanic islands. The hotspot remains relatively stationary, as Earth's tectonic plates slide over it, carrying the volcanoes with them. When the magma supply is cut by the plate's motion and the volcano becomes extinct, the island eventually erodes below the waves, again becoming a seamount. Meanwhile, a new volcano forms over the hotspot, forming another new island. The process continues until the mantle plume collapses.[1]

According to Wilson, the Hawaiian chain's volcanoes should be progressively older and increasingly eroded northwestward from the hotspot. The oldest rocks in the main Hawaiian islands, the rocks on Kauaʻi, are about 5.5 million years old and deeply eroded. In contrast, the rocks of the "Big Island", and those of Loʻihi, are under 0.7 million years old. New lava is constantly emitted from the Kilauea caldera on Hawaii's Big Island.[1][4]

This cycle of growth and dormancy forms long chains of volcanic islands over many millions of years, and in the case of Hawaii, it has left a trail of volcanic islands and seamounts across the Pacific Ocean floor. The chain's length, orientation and the distance between volcanoes records the direction and speed of the Pacific Plate's movement. Approximately 40–50 million years ago, it is thought to have suddenly changed direction, because of the subduction of the spreading ridge separating the Pacific and Izanagi plates, and the initiation of subduction along much of the Pacific Plate's western boundary.[5] The change in direction was marked by an L-shaped bend in the seamount chain, easily visible in raised-relief maps. However, this part of the theory has recently been challenged, and the bend might be attributed to the movement of the hotspot itself.[6]

Geophysicists generally believe that hotspots originate at one of two major boundaries deep in the Earth. The shallow option is an interface in the lower mantle between an upper convecting layer and a lower non-convecting layer. The deeper option is the D'' ("D double-prime") layer, which is approximately 200 kilometres (120 mi) thick and exists immediately above the core-mantle boundary.[7] A mantle plume would initiate at the interface when the warmer lower layer heats a portion of the cooler upper layer. This heated, buoyant, and less-viscous portion of the upper layer would become less dense due to thermal expansion, and rise towards the surface as a Rayleigh-Taylor instability or gravitational instability.[8] When the mantle plume reaches the base of the lithosphere, the plume heats it and produces melt. This magma then makes its way to the surface, where it is erupted as lava.[9]

Arguments for the validity of the hotspot theory generally center on:

  • The steady age progression of the Hawaiian islands and other seamount chains.[10]
  • Just south of the Hawaiian – Emperor chain, the Austral–Marshall Islands seamount chain shows a similar bend in the trail of the Macdonald hotspot.[11]
  • Other proposed hotspots (the Marquesas, Society, Pitcairn, Samoa and Macdonald hotspots) within the Pacific Basin, follow the same age-progressed trend from southeast to northwest, remaining in fixed relative positions.[12][13]
  • Seismologic studies of Hawaii seem to show increased temperatures down to the core–mantle boundary, suggesting evidence of a plume.[14]

[edit] Shallow hotspot theory

Another hypothesis is that hotspots are born from shallow tectonic interactions between the lithosphere and asthenosphere. Because of shifting plate boundaries, the area around Hawaii was vastly different 70–100 million years ago, and there may have been a spreading ridge (the Pacific–Kula Ridge) in the area that disappeared in the early Tertiary period, about 65 million years ago.[15] After changing plate dynamics moved the ridge away, the area may have established a continuing magma supply, thus forming a self-sustaining hotspot, possibly tapping into deeper mantle processes.[16] However, seismic tomography rejects this hypothesis, as it shows that the plume beneath the Hawaii hotspot extends to the core–mantle boundary.[17]

[edit] Moving hotspot theory

The most heavily disputed element of the theory is whether or not hotspots are fixed relative to the overlying tectonic plates, and to each other. Drill samples, collected by scientists as far back as 1963, suggested that the hotspot may have drifted over time, at the relatively rapid pace of about 4 centimeters (1.6 in) per year during the late Cretaceous and early Paleogene eras (81-47 Mya);[18] in comparison, the Mid-Atlantic Ridge spreads at a rate of 2.5 cm (1.0 in) per year.[1]

In 1987, Peter Molnar and Joann Stock found that the hotspot does move, at least relative to the Atlantic Ocean. However, this was believed to be a result of the relative motions of the North American and Pacific plates rather than the hotspot itself.[19]

The Ocean Drilling Program (since merged into the Integrated Ocean Drilling Program) was an international research effort designed to study the world's seafloors. In 2001, ODP funded a two-month excursion aboard the research vessel JOIDES Resolution to collect lava samples from four submerged Emperor seamounts. The project drilled Detroit, Nintoku, and Koko seamounts, all of which are in the far northwest end of the chain, the oldest section.[20][21]

In 2003, these lava samples were used to test which hotspot theory was correct. The findings suggested that the bend was caused by the motion of the Hawaiian hotspot itself.[22][6] Lead scientist John Tarduno told National Geographic:

The Hawaii bend was used as a classic example of how a large plate can change motion quickly. You can find a diagram of the Hawaii – Emperor bend entered into just about every introductory geological textbook out there. It really is something that catches your eye."[22]

Despite the large shift, the change in direction was never recorded by magnetic declinations, fracture zone orientations or plate reconstructions; nor could a continental collision have occurred fast enough to produce such a pronounced bend in the chain.[23] To test whether or not the bend was a result of a change in direction of the Pacific Plate, scientists analyzed the lava samples' geochemistry to determine where and when they formed. Age was determined by the radiometric dating of radioactive isotopes of potassium and argon. Researchers estimated that the volcanoes formed during a period 81 million to 45 million years ago. Tarduno and his team determined where the volcanoes formed by analyzing the rock for the magnetic mineral magnetite. While hot lava from a volcanic eruption cools, tiny grains within the magnetite align with the Earth's magnetic field, and lock in place once the rock solidifies. Researchers were able to verify the latitudes at which the volcanoes formed by measuring the grains' orientation within the magnetite. Paleomagnetists concluded that the Hawaiian hotspot had drifted southward sometime in its history, and that, 47 million years ago, the hotspot's southward motion greatly slowed, perhaps even stopping entirely.[20][22]

[edit] History of study

[edit] Ancient Hawaiian

See also: Pele (deity), Halemaumau Crater, and Hawaiian mythology

The possibility that the Hawaiian islands became older as one moved to the northwest was suspected by ancient Hawaiians long before Europeans arrived. During their voyages, sea-faring Hawaiians noticed differences in erosion, soil formation, and vegetation, allowing them to deduce that the islands to the northwest (Niʻihau and Kauaʻi) were older than those to the southeast (Maui and Hawaii).[1] The idea was handed down the generations through the legend of Pele, the fiery Hawaiian Goddess of Volcanoes. This dynamic view contrasted with the static Genesis creation myth taught by Europeans at the time.[24]

Pele was born to the female spirit Haumea, or Hina, who, like all Hawaiian gods and goddesses, descended from the supreme beings, Papa, or Earth Mother, and Wakea, or Sky Father.[25]:63[26] According to the myth, Pele originally lived on Kauai, when her older sister Nāmaka, the Goddess of the Sea, attacked her for seducing her husband. Pele fled southeast to the island of Oahu. When forced by Nāmaka to flee again, Pele moved southeast to Maui and finally to Hawaii, where she still lives in the Halemaumau Crater at the summit of Kīlauea. There she was safe, because the slopes of the volcano are so high that even Nāmaka's mighty waves could not reach her.Pele's mythical flight, which alludes to an eternal struggle between volcanic islands and ocean waves, is consistent with geologic evidence about the ages of the islands decreasing to the southeast.[1][18]

[edit] Modern studies

The Hawaiian islands with attention called to topographic highs, Bouguer gravity anomalies, locus of shield volcanoes, and areas of closed low. Two and sometimes three parallel paths of volcanic loci are shown trailing the hotspot for thousands of miles.
The Loa and Kea volcanic trends follow meandering parallel paths for thousands of miles.

Three of the earliest recorded observers of the volcanoes were the Scottish scientists Archibald Menzies in 1794,[27] James Macrae in 1825,[28] and David Douglas in 1834. Just reaching the summits proved daunting: Menzies took three attempts to ascend Mauna Loa, and Douglas died on the slopes of Mauna Kea. The United States Exploring Expedition spent several months studying the islands in 1840–1841.[29] American geologist James Dwight Dana was on that expedition, as was Lieutenant Charles Wilkes, who spent most of the time leading a team of hundreds that hauled a pendulum to the summit of Mauna Loa to measure gravity. Dana stayed with missionary Titus Coan, who would provide decades of first-hand observations.[30] Dana published a short paper in 1852.[31]

Dana remained interested in the origin of the Hawaiian Islands, and directed a more in-depth study in 1880 and 1881. He confirmed that the islands' age increased with their distance from the southeastern-most island by observing differences in their degree of erosion. He also suggested that many other island chains in the Pacific showed a similar general increase in age from southeast to northwest. Dana concluded that the Hawaiian chain consisted of two volcanic strands, located along distinct but parallel curving pathways. He coined the terms "Loa" and "Kea" for the two prominent trends. The Kea trend includes the volcanoes of Kīlauea, Mauna Kea, Kohala, Haleakalā, and West Maui. The Loa trend includes Loiʻhi, Mauna Loa, Hualālai, Kahoʻolawe, Lānaʻi, and West Molokaʻi. Dana proposed that the alignment of the Hawaiian Islands reflected localized volcanic activity along a major fissure zone. Dana's "great fissure" theory served as the working hypothesis for subsequent studies until the mid-20th century.[23]

Dana's work was followed up by geologist C. E. Dutton's 1884 expedition, who refined and expanded Dana's ideas. Most notably, Dutton established that the island of Hawaii actually harbored five volcanoes, whereas Dana counted three. This is because Dana had originally regarded Kīlauea as a flank vent of Mauna Loa, and Kohala as part of Mauna Kea. Dutton also refined others of Dana's observations, and is credited with the naming of 'a'ā and pāhoehoe-type lavas, although Dana had noted a distinction. Stimulated by Dutton's expedition, Dana returned in 1887, and published many accounts of his expedition in the American Journal of Science. In 1890 he published the most detailed manuscript of its day, and remained the definitive guide to Hawaiian volcanism for decades. 1909 saw the publication of two large volumes which extensively quoted from earlier works now out of circulation.[32]:154-155

In 1912 geologist Thomas Jaggar founded the Hawaiian Volcano Observatory. The facility was taken over in 1919 by the National Oceanic and Atmospheric Administration and in 1924 by the United States Geological Survey (USGS), which marked the start of continuous volcano observation on Hawaii island. The next century was a period of thorough investigation, marked by contributions from many top scientists. The first complete evolutionary model was first formulated in 1946, by USGS geologist and hydrologist Harold T. Stearns. Since that time, advances have enabled the study of previously limited areas of observation (e.g. improved rock dating methods and submarine volcanic stages).[32]:157

In the 1970s, the Hawaiian seafloor was mapped using ship-based sonar. Computed SYNBAPS (Synthetic Bathymetric Profiling System)[33] data filled holes between the ship-based sonar bathymetric measurements.[19][34] From 1994 to 1998[35] the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) mapped Hawaii in detail and studied its ocean floor, making it one of the world's best-studied marine features. The JAMSTEC project, a collaboration with USGS and other agencies, utilized manned submersibles, remotely operated underwater vehicles, dredge samples, and core samples.[36] The Simrad EM300 multibeam side-scanning sonar system collected bathymetry and backscatter data.[35]

[edit] Characteristics

[edit] Position

The Hawaii hotspot has been imaged through seismic tomography, and is estimated to be 500–600 km (310–370 mi) wide.[17][37] Recent diffraction tomography and high-resolution local tomography indicate a lower-mantle mantle plume, and a pond of plume material is evidenced by a large low-velocity zone in the upper mantle. A thin low-velocity zone extending downward from 670 to 1,500 km (420 to 930 mi) under Hawaii connects with a large low-velocity zone at 2,000 km (1,200 mi) on the boundary between the core and mantle north of Hawaiʻi, showing that the plume is tilted to a certain degree, deflected toward the south by mantle flow.[38] Uranium decay-series disequilibria data has shown that the actively flowing region of the melt zone is 220 ± 40 km (137 ± 25 mi) km at its base and 280 ± 40 km (174 ± 25 mi) at the upper mantle upwelling, consistent with tomographic measurements.[39]

[edit] Temperature

Indirect studies found that the magma chamber is located about 90–100 kilometers (56–62 mi) underground, which matches the estimated depth of the Cretaceous Period rock in the lithosphere. This seeming coincidence may indicate that the lithosphere acts as a lid on melting by arresting the magma's ascent. The lava's original temperature was found in two ways, by testing garnet's melting point in lava and by adjusting the lava for olivine deterioration. Both USGS tests seem to confirm the temperature at about 1,500 °C (2,730 °F). The olivine test also found that the temperature could not have been greater than 1,550 °C (2,820 °F) for the olivine to have retained that amount of clinopyroxene. The garnet test agreed, putting the temperature range at 1,500 °C (2,730 °F) to 1,560 °C (2,840 °F). In comparison, the estimated temperature for mid-ocean ridge basalt is about 1,325 °C (2,417 °F).[40]

The surface heat flow anomaly around the Hawaiian Swell is only of the order of 10 mW/m2.[41][42] This is unexpected for the classic model of a hot, buoyant plume in the mantle. However, it has been shown that other plumes display highly variable surface heat fluxes and that this variability may be due to variable hydrothermal fluid flow in the Earth's crust above the hotspots. This fluid flow advectively removes heat from the crust, and the measured conductive heat flow is therefore lower than the true total surface heat flux.[42] The low heat across the Hawaiian Swell indicates that it must be supported not by thermal buoyancy, but rather by the existence of a buoyant mass underneath the swell which causes the surface to rise;[41] this density-driven surface uplift is called "dynamic topography".

[edit] Movement

Hawaiian volcanoes drift northwest from the hotspot at a rate of about 5–10 centimeters (2.0–3.9 in) a year.[18] The hotspot has migrated south by about 800 kilometers (497 mi) relative to the Emperor chain. This conclusion is supported by magnetic studies, which suggest that these seamounts formed at higher latitudes than present-day Hawaii. Prior to the bend, the hotspot migrated an estimated 7 centimeters (2.8 in) per year; the rate of movement changed at the time of the bend to about 9 centimeters (3.5 in) per year.[23] The Ocean Drilling Program provided most of the current knowledge about the drift. The 2001[43] expedition drilled six seamounts and tested the samples to determine their original latitude, and thus the characteristics and speed of the hotspot's drift pattern in total.[44]

Each successive volcano spends less time actively attached to the plume. The large difference between the youngest and oldest lavas between Emperor and Hawaiian volcanoes indicates that the hotspot's velocity is increasing. For example, Kohala, the oldest volcano on Hawaii island, is one million years old and last erupted 120,000 years ago, a period of just under 900,000 years; whereas one of the oldest, Detroit Seamount, experienced 18 million or more years of volcanic activity.[45]

The oldest volcano in the chain, Meiji Seamount, perched on the edge of the Aleutian Trench, formed 85 million years ago.[46] At its current velocity, the seamount will be destroyed within a few million years, as the Pacific Plate slides under the Eurasian Plate. It is unknown whether the seamount chain has been subducting under the Eurasian Plate, and whether the hotspot is older than Meiji Seamount, as any older semounts have since been destroyed by the plate margin. It is also possible that a collision near the Aleutian Trench had changed the velocity of the Pacific Plate, explaining the hotspot chain's bend; the relationship between these features is still being investigated.[23][47]

[edit] Eruption frequency and scale

Bathymetric rendering of the Hawaiian island chain showing greater depths as blue, shallower depths as red, and exposed land as gray. The main island is the tallest, the ones in the middle sit on a raised plateau, and three more islands sit separately at the west end of the chain. A series of small elevation bumps (seamounts) sit south of the main landmass.
Bathymetry and topography of the southeastern Hawaiian Islands, with historic lava flows shown in red. Partly submerged Maui Nui (right of center) was as large as the main island (lower right) is now.

There is significant evidence that lava flow rates have been increasing. Over the last six million years it has been far higher than ever before, at over 0.095 km3 (0.023 cu mi) per year. The average for the last million years is even higher, at about 0.21 km3 (0.050 cu mi). In comparison, the average production rate at a mid-ocean ridge is about 0.02 km3 (0.0048 cu mi) for every 1,000 kilometers (621 mi) of ridge. The rate along the Emperor seamount chain averaged about 0.01 cubic kilometers (0.0024 cu mi) per year. The rate was almost zero for the initial five million or so years in the hotspot's life. The average lava production rate along the Hawaiian chain has been greater, at 0.017 km3 (0.0041 cu mi) per year.[23] In total, the hotspot has produced an estimated 750,000 cubic kilometers (180,000 cu mi) of lava, enough to cover California with a layer about 1.5 kilometers (1 mi) thick.[4][18][48][49][50]

The distance between individual volcanoes has shrunk. Although volcanoes have been drifting north faster and spending less time active, the far greater modern eruptive volume of the hotspot has generated more closely spaced volcanoes, and many of them overlap, forming such superstructures as Hawaiʻi island and the ancient Maui Nui. Meanwhile, many of the volcanoes in the Emperor seamounts are separated by 100 kilometers (62 mi) or even as much as 200 kilometers (124 mi).[49][50]

[edit] Topography and geoid

A detailed topographic analysis of the Hawaiian – Emperor seamount chain reveals the hotspot as the center of a topographic high, and that elevation falls with distance from the hotspot. The most rapid decrease in elevation and the highest ratio between the topography and geoid height are over the southeastern part of the chain, falling with distance from the hotspot, particularly at the intersection of the Molokai and Murray fracture zones. The most likely explanation is that the region between the two zones is more susceptible to reheating than most of the chain. Another possible explanation is that the hotspot strength swells and subsides over time.[34]

In 1953, Robert S. Dietz and his colleagues first identified the swell behavior. It was suggested that the cause was mantle upwelling. Later work pointed to tectonic uplift, caused by reheating within the lower lithosphere. However, normal seismic activity beneath the swell, as well as lack of detected heat flow, caused scientists to suggest a dynamic explanation. Understanding the Hawaiian swell has important implications for hotspot study, island formation, and inner Earth.[34]

[edit] Magma

A lava fountain at Pu'u 'O'o, a volcanic cone on the flank of Kilauea. Pu'u 'O'o is one of the most active volcano in the world, and has been continuously erupting since January 3, 1983.

The volcanoes' magma composition has changed significantly, according to analysis of the strontium–niobium–palladium elemental ratio. Emperor data represents 46 million years of activity, with the oldest lava dated to the late Mesozoic Era (Cretaceous Period) and the youngest to the early Cenozoic Era (Paleogene Period). This leads to the modern day with the eruptions on Loiʻhi and Kīlauea, another 39 million years of activity, totaling 85 million years. Data demonstrate a large upward variation in the amount of strontium present in both the alkalic (early stages) and tholeitic (later stages) lavas. The systematic increase slows drastically at the time of the bend. The change is partially associated with the thinning of the local plate as the hotspot and the Pacific Plate separated.[46]

Almost all magma created by the hotspot is igneous basalt; the volcanoes are constructed almost entirely of this or the similar, coarse-grained gabbro and diabase. Rarely, there are different igneous rocks, such as nephelinite; these occur often on the older volcanoes, most prominently Detroit Seamount.[46] Most eruptions are runny because basaltic magma is more fluid than magmas typical in more explosive eruptions such as the andesitic magmas that produce spectacular and dangerous eruptions around Pacific Basin margins.[6] Volcanoes fall into several eruptive categories. Hawaiian volcanoes are called "Hawaiian-type". Hawaiian lava spills out of craters and forms long streams of glowing molten rock, flowing down the slope, covering acres of land and replacing ocean with new land.[51]

[edit] Volcanoes

See also: List of volcanoes in the Hawaiian – Emperor seamount chain

Over its 85 million history, the Hawaii hotspot has created at least 129 volcanoes, more than 123 of which are extinct volcanoes, seamounts, and atolls, four of which are active volcanoes, and two of which are dormant volcanoes.[21][44][52] They can be organized into three general categories: the Hawaiian archipelago, which comprises most of the U.S. state of Hawaii and is the location of all modern volcanic activity; the Northwestern Hawaiian Islands, which consist of coral atolls, extinct islands, and atoll islands; and the Emperor Seamounts, all of which have since eroded and subsided to the sea and become seamounts and guyots (flat-topped seamounts).[53]

[edit] Volcanic characteristics

See also: Shield volcano and Maui Nui

Hawaiian volcanoes are characterized by frequent rift eruptions, their large size (thousands of cubic kilometers in volume), and their rough, decentralized shape. Rift zones are a prominent feature on these volcanoes, and account for their seemingly random volcanic structure.[54] The tallest mountain in the Hawaii chain, Mauna Kea, rises 4,205 meters (13,796 ft) above mean sea level. Measured from its base on the seafloor, it is the world's tallest mountain, at 10,203 meters (33,474 ft). Mount Everest rises 8,848 meters (29,029 ft) above sea level.[55] Hawaii is surrounded by a myriad of seamounts; however, they were found to be unconnected to the hotspot and its vulcanism.[36] Kīlauea has erupted continuously since 1983 through Puʻu ʻŌʻō, a minor volcanic cone, which has become an attraction for vulcanologists and tourists alike.[56]

A series of maps showing Maui's progressive submergence at intervals of 200,000 years beginning 1.2 millions years ago.
Maui Nui submergence history, showing extent of Maui Nui landmass at times indicated ("Ma" or million years ago). Panel labeled "Recent" represents latest glacial cycle, about 18,000 years ago.

Seven shield volcanoes created the island of Maui Nui. In Hawaiian, "Nui" means "great" or "large", and Maui is the name of Hawaii's second largest island (which formed Maui Nui's backbone). At its maximum, about 1.2 million years ago, Maui Nui was 14,600 square kilometers (5,600 sq mi) in size, 50% larger than the present-day island of Hawaii. Sea levels were lower than today during ice ages. Subsidence and erosion caused the "saddles" connecting the volcanoes to flood by 200,000 years ago, forming the islands of Maui, Molokai, Lānai, and Kahoolawe. Penguin Bank is a former island lying west of Molokai that is completely submerged and now hosts a cap of coral. The water between these four islands is relatively shallow, about 500 meters (1,600 ft) deep. Maui Nui's outer edges plummet quickly to the abyssal plain. A flank collapse along the steep slopes could produce massive landslides. One prior collapse removed much of the northern half of East Molokaʻi.[57]

[edit] Evolution and construction

Main article: Evolution of Hawaiian volcanoes

Hawaiian volcanoes follow a well-established life cycle of growth and erosion. After a new volcano forms, its lava output gradually increases. Height and activity both peak at about age 500,000 and then rapidly decline. Eventually it goes dormant, and eventually extinct. Erosion then weathers the volcano until it again becomes a seamount.[53]

Animation showing an intact volcano that gradually shrinks in size with some of the lava around its perimeter replaced by coral
An animated sequence showing the erosion and subsidence of a volcano, and the formation of a coral reef around it—eventually resulting in an atoll

This life cycle consists of several stages. The first stage is the submarine preshield stage, currently represented solely by Loiʻhi. During this stage, the volcano builds height through increasingly frequent eruptions. The sea's pressure prevents explosive eruptions. The cold water quickly solidifies the lava, producing the pillow lava that is typical of underwater volcanic activity.[53]

As the seamount slowly grows, it goes through the shield stages. It forms many mature features, such as a caldera while submerged. The summit eventually breaches the surface, and the lava and ocean water "battle" for control as the volcano enters the explosive subphase. This stage of development is exemplified by explosive steam vents. This stage produces mostly volcanic ash, a result of the waves dampening the lava.[53] This conflict between lava and sea influences Hawaiian mythology.[25]:8–11

The volcano enters the subaerial subphase once it is tall enough to escape the water. Now the volcano puts on 95% of its above-water height over roughly 500,000 years. Thereafter eruptions become much less explosive. The lava released in this stage often includes both pāhoehoe and ʻaʻā. The most impressive Hawaiian volcanoes, Mauna Loa and Kīlauea, are in this phase. Because of the high growth rate, landslides are common. Hawaiian lava is often runny, blocky, slow, and relatively easy to predict; the USGS tracks where it is most likely to run, and maintains a tourist site for viewing the lava.[53][58]

After the subaerial phase the volcano enters a series of postshield stages involving subsidence and erosion, becoming an atoll and eventually a seamount (or often a guyot). Once the Pacific Plate moves it out of the 20 °C (68 °F) tropics, the reef mostly dies away, and the extinct volcano becomes one of an estimated 10,000 barren seamounts worldwide.[53][59] Every Emperor seamount is a dead volcano.

[edit] See also

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[edit] References

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