Hawaii hotspot

Coordinates: 18°55′N 155°16′W / 18.92°N 155.27°W / 18.92; -155.27
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The Hawaiʻi hotspot is a volcanic hotspot responsible for the creation of the Hawaiian Islands in the central Pacific Ocean, and is one of the best known and most studied hotspots on Earth.[2][3][4] While most volcanic activity occurs along the boundaries of tectonic plates, powered by the movement of the plates, hotspots can occur far from any geological boundaries, and require a completely different mechanism for maintaining volcanic activity.

Seafloor spreading pushes Hawaiʻi's volcanoes northwest about 10 centimeters (3.9 in) a year. 30 million years ago the Kure and Midway atolls were located where the island of Hawaiʻi is now. The oldest extant volcano in the chain, Meiji Seamount, began to form 86 million years ago; however, the hotspot may be older, if subduction of tectonic plates on the margin between the Pacific and Eurasian plates destroyed the older volcanoes.

The Hawaiʻi hotspot has created at least 129 volcanoes, arranged in an arc known as the Hawaiian–Emperor seamount chain. More than 123 are extinct volcanoes, seamounts, and atolls, four are active volcanoes, and two are dormant volcanoes.[5] Hawaiian volcanoes range in age from 300,000 to 86 million years, progressing from southeast to northwest. Most are heavily eroded. This chain includes the Hawaiian Ridge, consisting of the islands of the Hawaiian chain northwest to Kure Atoll, and the Emperor seamounts, a linear region of islands, seamounts, atolls, shallows, banks, and reefs along a line trending southeast to northwest beneath the northern Pacific Ocean. The chain stretches over 5,800 kilometers (3,604 mi) from the Aleutian Trench in the far northwest Pacific to Loiʻhi Seamount, the youngest volcano in the chain, lying about 35 kilometers (22 mi) southeast of the Island of Hawaiʻi.[2][6]

A bend corresponding to rocks between 41 and 43 million years old sharply divides the Hawaiian and Emperor sections. The bend was thought to result from sudden plate movement, but recent studies credit movement of the hotspot itself.

Ancient Hawaiian fishermen were the first to notice the increasing age of the islands, based on differences in erosion. 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 observance on the island of Hawaiʻi. 1946 saw the completion of the model for the evolution of Hawaiian volcanoes. J. Tuzo Wilson proposed the hotspot theory in 1963, using Hawaʻi island chain data. In the 1970s, a continuous effort began mapping Hawaiʻi's seafloor, to gain more information regarding the area's complex geology.

Hotspot theory

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

The Hawaiʻi hotspot is unusual, given that the vast majority of earthquakes and volcanic eruptions occur near plate boundaries, but the Hawaiian Islands are an exception, because the nearest plate boundary is more than 3,200 kilometers (1,988 mi) from the main island.[2] In 1963, Canadian geophysicist J. Tuzo Wilson proposed the hotspot theory to explain this anomaly.[7]

Wilson's stationary hotspot theory

Wilson claimed that certain small, long lasting, exceptionally hot areas of magma are located under Earth's surface, providing localized heat and energy systems, known as thermal or mantle plumes, that sustain long-lasting surface volcanic activity. This "mid-plate volcanism" builds mountains that rise from the relatively featureless sea floor, called 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 and the volcano becomes extinct, the island eventually erodes back below the waves as a seamount. Meanwhile, a new volcano forms over the hotspot repeating the process, this time forming a new island, continuing until the mantle plume collapses.[2]

According to Wilson's hotspot theory, the volcanoes of the Hawaiian chain should be progressively older and increasingly eroded northwestward from the hotspot. New volcanic rock is constantly being made at Hawaiʻi's main island. The oldest rocks in the main Hawaiian islands, the rocks on Kauai, are about 5.5 million years old and deeply eroded.[2] In contrast, the rocks of the "Big Island", and those of Loiʻhi, are under 0.7 million years old.[2][6]

Raised-relief map of Hawaiiʻn islands and surrounding waters, showing that the islands sit on a larger area of raised ocean bottom that connects them. Below the map, 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

This process of growth and later dormancy forms long chains of volcanic islands over many millions of years, and in the case of Hawaiʻi, it has left a trail of volcanic islands and seamounts across the Pacific Ocean floor. The group of Hawaiian volcanoes are part of a larger chain, dubbed the Hawaiian–Emperor seamount chain.

The chain's length, orientation and 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.[8] The change in direction was recorded 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.

Many geophysicists believe that hotspots originate from the interaction between Earth's deepest layer, the planetary core, and the overlying mantle (see also, structure of the Earth). This zone, about 2,900 kilometers (1,802 mi) thick, develops a small bump that protrudes slightly into the mantle from the deeper core layer of the Earth. The bump transfers the intense heat from Earth's center into the adjacent mantle, heating it. Convection currents raise the heat a few centimeters a year, because of molten rock's high viscosity. Because it is hotter than the surrounding mantle, the heat continues to rise, but does not melt the often highly metamorphosed silicate rock surrounding it, which is resistant to heat.[9] As the heat enters the less-resistant crust layer, it melts the rock, forming a mantle plume, from which the volcanoes get lava.[10]

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

  • The steady age progression of the Hawaiian islands and other seamount chains.[3]
  • Just south of the Hawaiian–Emperor chain, the Austral–Marshall Islands seamount chain shows a similar bend in the trail of the Macdonald hotspot.[3]
  • 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.[11][12]
  • Seismology studies of Hawaii seem to show increased temperatures down to the core–mantle boundary, suggesting evidence of a plume.[13]
Map marking of the Hawaiian islands showing topographic highs, Bouguer gravity anomalies, loc of shield volcanoes, and areas of closed low
The Loa and Kea volcanic trends

Shallow hotspot theory

Another hotspot theory speculates 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.[14] 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.[15]

Moving hotspot theory

The most 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 was not immobile, and that it may have drifted over time, at a relatively rapid pace of about 4 centimeters (1.6 in) per year during the late Cretaceous and early Tertiary times (81-47 Mya).[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.[16]

The Ocean Drilling Program (ODP) is 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.[17][18] The project drilled Detroit, Nintoku, and Koko seamounts, all of which are in the far northwest end of the chain, the oldest section.[18]

In 2003, a study used those lava samples to test the hotspot theory's validity. The findings suggested that the bend was caused by the motion of the Hawaiian hotspot itself.[19]

Raised-relief map of the Pacific basin
The chain of islands generated by the Hawaiʻi hotspot (bottom center), called the Hawaiian–Emperor seamount chain. Note the L-Shaped bend, a change in the direction of movement of the Pacific Plate or the hotspot itself.

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."[19]

The Emperor and Hawaiian chains differ in orientation by about 60°, and it has long been assumed that a major change in plate movement caused the bend; but new research suggests this did not occur. The change in direction was never recorded by magnetic declinations, fracture zone orientations or plate motion reconstruction. Also, a continental collision would not have been fast enough to have produced such a pronounced bend in the chain.[20]

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.[17]

The study indicated that 47 million years ago, the hotspot's southward motion greatly slowed, perhaps stopping.[19]

Some groups that do not believe in plate tectonics (see also, flood geology) cited the new discovery as one of many pieces of evidence against the hotspot theory.[19][20]

Crack and magma theory

The main alternative to the hotspot theory is the "crack and magma" theory, historically known as the "great fissure". Proponents postulate that cracks in the lithosphere are propagated by torsional stresses in, or stretching of, tectonic plates, which allows magma to leak from below. While a crack in the Earth's crust and magma leaking through it are both required to produce a volcano, the hotspot debate is over how and why the crack is produced. Norman H. Sleep, Professor of Geology at Stanford University stated, "the [question] is whether the crack is a secondary feature or the primary one."[21] Proponents also cite, as evidence against the hotspot theory, that most hotspots occur on young lithosphere (typically less than 30 million years old), which is thinner and weaker than older lithosphere.[22]

Other challenges

Arguments against the hotspot theory's validity generally center on several issues which either have yet to be explained by the hotspot theory, or directly challenge the theory:

Global map labeled Crustal Age with callouts for specific hotspots. There is an overall patern of younger crust in the East Pacific and younger in the West.
1. Mendocino Fracture Zone
2. Bend in Hawaii hotspot trail
3. Hawaii hotspot
Colors indicate age of seafloor, red is youngest which indicates areas of seafloor spreading
  • Age progressive volcanism can also occur along "leaky" transform faults.[22]
  • The bend between the Hawaiian and Emperor chains occurs close to where the chain crosses the Mendocino Fracture Zone.[20]
  • The possibility that the bend did not result from a change in direction of movement of the Pacific plate.[20]
  • Mantle temperature is inconsistent.[20]
  • Petrology has revealed the magma seems to originate from a very shallow chamber in the asthenosphere.[20]
  • Geochemistry is ambiguous regarding the depth of the magma's origin.[20]
  • Seismology has yet to conclusively detect the mantle plume.[20]
  • The apparent absence of an oceanic plume head or oceanic plateau (a flood basalt province or other large igneous province) that should have formed with the initial stages of mantle plume eruption. Other proposed hotspots also lack a plume head, while others lack a trail of volcanic activity, possibly indicating mantle plumes which produce only one volcanic event.[20]
  • The apparent absence of a heatflow anomaly (increased thinning and temperature of the lithosphere around the suspected hotspot). The lithosphere over a mantle plume is expected be thinner and hotter than the average for lithosphere of the same age elsewhere. An alternative model is that the plume head results from excess magma production rather than high temperatures. In the case of that model, no heatflow anomaly is expected.[20]
  • Large variations in the volcanism of the Hawaii hotspot, which is also three times more active than any other proposed hotspot. No thermal model has explained how high flux rates can occur beneath thick plates. The standard model predicts a large initial rate that declines subsequently, the opposite of that observed along the Hawaiian chain.[20]
  • The Emperor part of the chain (the oldest entities, especially Meiji Seamount) ends near a bend in the Kuril–Kamchatka Trench, where the seamounts on the Pacific Plate will be subducting under the Eurasian Plate. As of 2009, it is unknown whether the seamount chain has been subducting under the Eurasian Plate, and whether the hotspot is older than Meiji Seamount. A collision here may have provided a change in direction of the Pacific Plate, and created the bend in the Kuril–Kamchatka Trench. The relationship between these features is still being investigated.[20]

History of study

A depiction of a sharp-nosed, white-haired man
James Dwight Dana (1813–1895), an American geologist, mineralogist and zoologist, was the first geologist to study the Hawaiian island chain and its hotspot in detail.

In 1880 and 1881, James Dwight Dana directed the earliest geological study of the Hawaiian Islands. 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 Kilauea, Mauna Kea, Kohala, Haleakala, and West Maui. The Loa trend includes Loiʻhi, Mauna Loa, Hualalai, Kahoolawe, Lanai, and West Molokai.[20]

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.[20] His conclusions were based mostly on the fact that almost all of the Hawaiian volcanoes have two rift zones, but only one is usually active.[1]

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 the island of Hawaiʻre actually harbored five volcanoes, whereas Dana counted three. Dana had originally regarded Kilauea as a flank vent of Mauna Loa, and Kohala as part of Mauna Kea. Dutton also refined some of Dana's observations, and is credited with the naming of 'a'ā and pāhoehoe-type lavas, although Dana did previously notice a distinction. Stimulated by Dutton's expedition, Dana returned to the island in 1887, and published many accounts of his expedition in the American Journal of Science. In 1890 he published a manuscript that was the most detailed of its day, and remained the definitive guide to Hawaiian volcanism for decades. In 1909, two further large volumes were published, which extensively quoted from earlier works now out of circulation.[23]

A heavy-set, bald man
American geologist Thomas Jaggar, founder of the Hawaiian Volcano Observatory

In 1912 the study of Hawaiian volcanism was advanced by the foundation of the Hawaiian Volcano Observatory by geologist Thomas Jaggar. The facility was taken over in 1919 by the National Oceanic and Atmospheric Administration, which marked the start of continuous volcano observation on Hawaiʻi island. The next century was a period of thorough investigation, hallmarked by contributions from many top scientists and spearheaded by the volcanic observatory. The complete model for the evolution of Hawaiian volcanoes was first formulated in 1946, by United States Geological Survey (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).[24]

In the 1970s, the Hawaiian seafloor was mapped using sonar. More direct ship-based sonar data was compiled with math-based SYNBAPS (Synthetic Bathymetric Profiling System)[25] data, with the ship-based bathymetrics carrying the most weight.[26] In 1971, geologist W. Jason Morgan went to Hawaiʻi and gathered evidence against Dana's theory, which was first challenged in 1967 by geologists Jack Oliver and B. Isaacs.[16]

From 1994 to 1998 [27] the Japan Marine Science and Technology Center, mapped Hawaiʻi in detail and studied its ocean floor, making it one of the world's best-studied marine features. The project, a collaboration with the USGS and other scientific agencies, utilized manned submersibles, remotely operated underwater vehicles, dredge samplings, and core samples.[28] The Simrad EM300 multibeam side-scanning sonar system collected bathymetry and backscatter data.[27]

Characteristics

The immense size of the Hawaiian hotspot and its creations is just one of many fascinating aspects.

Size

Vegetation surrounded by lava
A kipuka that formed during the eruption of Pu'u 'O'o

The tallest mountain in the Hawaii chain, Mauna Kea, has raised itself to 4,205 meters (13,796 ft) above mean sea level. If measured from its base on the seafloor, this would make Mauna Kea the world's tallest mountain, at 10,203 meters (33,474 ft), compared to 8,848 meters (29,029 ft) for Mount Everest (measured from sea level).[29] Hawaiʻi is also surrounded by a myriad of seamounts; however, they were found to be unconnected to the hotspot and its vulcanism.[28] The amount of lava erupted from the hotspot is estimated to be approximately 750,000 cubic kilometers (180,000 cu mi), enough to cover California with a lava about 1.5 kilometers (1 mi) thick.[6]

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 Hawaiʻi'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 Hawaiʻi. Sea levels were lower than today, when glaciation during an ice age removed much of the Earth's water from the ocean. The volcanoes slowly subsided into the crust and along with erosion, the "saddles" connecting the volcanoes eventually flooded, and 200,000 years ago it subsided completely, forming the islands Maui, Molokaʻi, Lanaʻi, and Kahoʻolawe. Penguin Bank is a former island lying west of Molokaʻi that 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.[30]

Movement

Hawaiian volcanoes drift northwest from the hotspot at a rate of about 5–10 centimeters (2.0–3.9 in) a year.[1] The hotspot is known to have migrated south by about 800 kilometers (497 mi) relative to the Emperor seamount chain. This conclusion is supported by magnetic studies of volcanic rock from Emperor seamounts, which suggested that these seamounts formed at higher latitudes than present-day Hawaiʻi. 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.[20] What we know about Hawaiian drift comes mostly from the Ocean Drilling Program. The 2001[31] expedition drilled 6 of the Emperor seamounts, and tested the magnetic samples to determine their original latitude, and thus the characteristics and speed of the hotspot's drift pattern in total.[32]

Picture of several shiny black objects, several with one rounded end and the other thinner, elongated and squared off
Assorted shapes of Pele's tears from Mauna Ulu. U.S. dime for scale in lower right

The amount of time each volcano spends actively attached to the Hawaiian mantle plume has decreased. The large difference between the youngest and oldest lavas between Emperor and Hawaiian volcanoes provides evidence that the Hawaiʻi hotspot migrated far slower then than it does today; for example, Kohala, the oldest volcano on Hawaiʻi island) emerged from the sea 500,000 years ago, and last erupted 120,000 years ago, a period of only 380,000 years; in comparison to Detroit seamount's (second oldest in the chain) 18 million or more years of volcanic activity.[33]

Molten lava traversing land.
Glowing ʻaʻā lava flow advancing over pāhoehoe on the coastal plain of Kilauea

The oldest volcano in the chain, Meiji Seamount, perched on the edge of the Aleutian Trench, is believed to have formed 82 million years ago. The seamount will be destroyed within a few million years, at its current rate of motion, as the Pacific Plate slides under the Eurasian Plate. The existence of older seamounts that may have already been destroyed by subduction is currently, as of 2009, disputed.[34]

Topography and geoid

A detailed analysis of the topography and geoid of the Hawaiian–Emperor seamount chain reveals that while high near the hotspot, local elevation falls with distance, but most severely between the Murray and Molokai fracture zones. Both geoid and topography rapidly decrease westward of the intersection with the Murray. 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.[26]

In 1953, Robert S. Dietz and his colleagues first identified the swell behavior. It was suggested that the cause was an upwelling of the mantle. 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 reason. Understanding the Hawaiian swell has important implications for hotspot study, island formation, and inner Earth.[26]

Eruption frequency and scale

There is significant evidence that the volcanoes' eruption rates have been increasing, because the distance between volcanoes on the arc shrink towards the southeastern and newer end. At the time of its formation, the hotspot produced widely spaced volcanoes, such as the distance between Meiji and Detroit Seamount. It was not uncommon for the separation to reach 100 kilometers (62 mi) or even 200 kilometers (124 mi).[35][36] In the most recent times, the hotspot has produced a large island (Hawaiʻi) compounded from five volcanoes.[1][37] The eruption rate along the Emperor seamount chain averaged about 0.01 cubic kilometers (0.0024 cu mi) of lava per year. The eruption 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.

The eruption rate has 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 one 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.[20]

Lava

The volcanoes' lava composition has changed significantly, according to analysis of the Strontium–Niobium–Palladium elemental ratio. Data collected from the Emperor seamounts represents 43 million years of activity, with the oldest seamount 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 Kilauea, another 39 million years of activity, totaling 82 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 Hawaiʻi hotspot and the Pacific Plate moved away from one another.[38]

Lava shaped like hundreds of curving, parallel spaghetti strands
Pāhoehoe-type lava from Kilauea

Almost all magma created by the hotspot is igneous basalt; Hawaiian 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 have been found often on the older volcanoes, most prominently Detroit Seamount.[38] Most eruptions are runny because basaltic magma is more fluid than magmas typical in more explosive eruptions such as the andesitic magmas producing spectacular and dangerous eruptions around Pacific Basin margins.[39] Volcanoes are classified into several eruptive categories, and the eruptions at Hawaiian volcanoes are called "Hawaiian-type" after the typical Hawaiian volcanic eruption. Hawaiian lava spills out of craters and forms long streams of glowing molten rock, which spills down the slope, covering acres of land and forming new land where before there was ocean. The low gas and silica content of the lava keeps it runny for long periods of time.[40]

Hawaiian volcanoes produce predominantly two lava types, pāhoehoe and ʻaʻā. Pāhoehoe is a highly pliable, thin type of relatively fast-flowing lava. It can appear bulbous, fresh-looking, wrinkled, fibrous, or in some other shape depending on its temperature.[40] ʻAʻā flow, however, is characterized by a jagged, ruffled appearance compared to the smooth-flowing pāhoehoe flows. It is slightly thicker the pāhoehoe, but can move faster on an incline. The top cools and forms an insulating, jagged shell on the flow in the form of large basalt chunks, which insulates the flow and keeps it moving. Occasionally pāhoehoe converts to ʻaʻā while it is cooling or degassing.[40] In addition to the two types of lava, Hawaiian volcanoes produce unique volcanic forms, described below.

Indirect studies found that the magma chamber is located at about 90–100 kilometers (56–62 mi) 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 tested in two ways, by testing garnet's melting point in lava, and by adjusting the lava for olivine deterioration to find the temperature that best matches data. Both tests (carried out by scientists from the United States Geological Survey) 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 second 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).[41]

Eruption phenomena

A pile of long, straight strands of glassy material
Pele's hair on a pāhoehoe flow at Kilauea Volcano, Hawaiʻi

Hawaiian eruptions can produce Pele's hair, which are brownish threads of volcanic glass with a diameter of less than 0.5 millimeters (0.02 in), and may be as long as 2 meters (7 ft). The strands form when molten lava is overstretched; for example, when an ʻaʻā flow runs off a steep cliff. Pele's hair is often carried high into the air during eruptions. Wind can blow the glass threads tens of kilometers from their place of origin.[42]

Pele's tears are small droplet-shaped bits of volcanic glass ejected from volcanoes, which form when small bits of molten lava cool exceptionally quickly. Pele's tears are often black in color, and sometimes form on the tips of Pele's hair.[43]

Pele's seaweed are sheets of brownish volcanic glass that form when pāhoehoe lava pours into the ocean. Water may become trapped and begin to boil within the lava, creating steam-filled bubbles of lava. As the bubble cools and bursts, the walls shatter and form thin plates of glass which may resemble seaweed.[44]

A stream of molten lava spraying into the air.
A pāhoehoe lava fountain arching approximately 10 meters (33 ft) high near Kilauea

Sometimes, molten basaltic lava solidifies around trees, forming "lava trees". The tree is incinerated forming a mold inside the crust.[45] Lava Tree State Monument is located 2.7 miles (4.3 km) southeast of Pahoa on Hawaiʻi. It preserves molds that formed when a lava flow swept through a forested area in 1790. Tree molds often preserve both the shape and the structure of the tree. They are common in fluid and fast moving pāhoehoe flows, and occasionally found in blockier ʻaʻā flows. Sometimes, the lava drains away before it cools but after incinerating the tree, leaving a hole in the ground.[45]

The eruption of Hawaiian basaltic lavas results in the lava flowing down the volcano's slope, creating its own channel (or reusing existing channels), developing both pāhoehoe and ʻaʻā lava flows. Over time lava levees can develop in pāhoehoe by overflowing the channels, while in ʻaʻā they are caused by moving lava into blocks. On Hawaiʻi these channels can often surround a kipuka (Hawaiian for "island"), an island of mature vegetation surrounded by barren younger lava.[46][47] Kipuka form when lava surrounds a particular raised area, leaving its ecosystem intact.[40]

Photo of a smooth-walled natural tunnel through rock
Thurston Lava Tube in Hawaiʻi Volcanoes National Park. The step mark on the right wall shows where the lava flowed for a period of time.

Cooling of the lavas in a channel with pāhoehoe can result in the creation of a lava tube. The surrounding rock acts as an insulator for the interior lava preventing it from crystallizing.[47] The tube allows lava to travel far from the eruption center, and may allow the lava to flow at higher speeds; a 1984 flow through Mauna Loa was recorded at speeds of over 35 miles per hour (56 km/h).[47] The tube features include a flat floor and splatter (marking the flow's high point); an example is the Thurston lava tube, part of Hawaiʻi Volcanoes National Park. Inside the lava tubes, one may also occasionally find "lava stalactites". If the chamber refills with lava after it has drained, it may partially melt the roof of the tube (made up of pāhoehoe), and gravity does the rest. After the roof resolidifies, fragile lava stalactites cling to it. Unlike their mineral counterparts, lava stalactites do not grow after formation.[40]

Other structures include: dome lava fountains, essentially a hemispheric upwelling of lava;[48] lava lakes, which are large volumes of molten lava pooled in a crater or depression[49] (the one at Kilauea, Kupaianaha, is one of only five active lakes worldwide);[50] some of the highest lava fountains on Earth; lava falls (or lava cascade), where lava spills over a cliff or a steep descent;[51] and lava "skylights", which are holes in the roof of a lava tube or underground pools of lava.[40]

Evolution of Hawaiian volcanoes

Main article: Evolution of Hawaiian volcanoes
A hole in lava revealing molten lava and stalactites
Lava skylight with lava stalactites

Hawaiian volcanoes follow a well-established life cycle of growth and erosion. After a new volcano forms, its eruption rate gradually increases, peaking in both height and volcanic activity around 500,000 years of age, and then rapidly declines. The volcano's activity level fades with time until it goes dormant, and eventually extinct. At that point, erosion become the strongest factor, weathering the volcano until it sinks back below the waves, becoming a seamount.[52]

This life cycle consists of several stages. The first stage is the submarine preshield stage, currently occupied solely by Loiʻhi, the newest volcano. During this stage, the volcano starts building up height through increasingly frequent eruptions. The sea pressurizes the lava, preventing explosive eruptions (since Hawaiian volcanoes have typically runny lava that would not happen anyway). The cold water immediately contacts the lava, giving it extremely little time to solidify. For that reason, pillow lava is typical of underwater volcanic activity.[52]

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 of a volcano, and the formation of a coral reef around it—eventually resulting in an atoll

As the volcano slowly rises in height, it begins to go through the shield stages. The volcano forms many mature volcano features, such as a caldera, during the sub-surface part of the shield stage. The summit just breaches the surface, and a "battle" between the volcanic lava and ocean water begins, and the volcano enters the explosive subphase. This stage of development is exemplified by explosive vents of steam. The lava released during this stage is mostly volcanic ash, a result of the waves dampening the lava.[52] This conflict between lava and sea reverberates in Hawaiian mythology.[53]

The volcano next enters the subaerial subphase, once it is tall enough to end frequent contact with the water. During this stage the volcano enters its prime, when it puts on 95% of its height in a period of roughly 500,000 years. Thereafter eruptions become much less explosive and more gentle. The lava released in this stage often includes both pāhoehoe and ʻaʻā. The most impressive of the Hawaiian volcanoes, Mauna Loa and Kilauea, are in this phase of activity.[52] Because of the high growth rate, landslides are extremely common. Hawaiian lava is often runny, blocky, slow, and relatively easy to predict; the USGS tracks where lava will most likely run, and maintains a tourist site for viewing the lava.[52][54] Kilauea has erupted continuously for the last 26 years through Puʻu ʻŌʻō, a minor volcanic cone happily for vulcanologists (who get to study the lava) and tourists (who get to see it in person) alike.[55]

After the subaerial phase the volcano undergoes a series of postshield stages, during which erosion whittles it down. The volcano eventually sinks below the sea to become a seamount (or often a guyot) once more. Because of Hawaiʻi's location near the equator, as the volcano disintegrates, it develops into an atoll. Once the Pacific Plate moves it out of the 20 °C (68 °F) isotherm (the bounds of coral reef life), the reef mostly dies away, and the extinct volcano becomes one of an estimated 10,000 barren seamounts worldwide.[52][56] Every seamount in the Emperor element of the chain is a dead volcano.

Hawaiian mythology

A long-haired woman sitting in lava pool holding stone in one hand and torch in another
Humanized form of Pele, Goddess of Fire, painted and hanging in the Hawaiʻi Volcanoes National Park Visitor's Center
See also: Pele (deity), Halemaumau Crater, and Hawaiian mythology

The possibility that the Hawaiian islands became older as one moved northwest was suspected by ancient Hawaiians long before any scientific studies were conducted. During their voyages, sea-faring Hawaiians noticed differences in erosion, soil formation, and vegetation, allowing them to deduce that the islands to the north (Niihau and Kauai) were older then those to the southeast (Maui and Hawaiʻi).[2] The idea was handed down the generations through the legend of Pele, the fiery Hawaiian Goddess of Volcanoes.

Pele was born to the female spirit Haumea, or Hina, who, like all of the Hawaiʻi gods and goddesses, descended from the supreme beings, Papa, or Earth Mother, and Wakea, or Sky Father.[57][58] According to the myth, Pele originally lived on Kauaʻi, 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 she was forced by Nāmaka to flee again, Pele moved southeast to Maui and finally to Hawaiʻi, where she still lives in the Halemaumau Crater at the summit of Kilauea. There she was safe, as the slopes of the mighty volcano are so high that even Nāmaka's mighty waves cannot reach her.[2] The mythical flight of Pele from Kauaʻi to Hawaiʻi, 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, obtained centuries later by scientists using radiometric dating.[1][2]

Volcanoes of the hotspot

The well-documented major volcanoes of the chain are listed below, in chronological order. The chain also includes many other less-documented volcanoes. The main island of Hawaii comprises five volcanoes, with another, Loiʻhi, offshore. Hawaii is surrounded by large swarms of less significant seamounts.

Hawaiian archipelago
Name Last Eruption Coordinates Age Notes
Big Island
Loihi Seamount 1996 (Active)[1] 18°55′N 155°16′W / 18.92°N 155.27°W / 18.92; -155.27 > 400,000[1] Submarine volcano approximately 35 km (22 mi) southeast of Hawaii. It will eventually breach sea level and become the newest Hawaiian island.[1]
Kilauea Erupting[59] 19°25′N 155°17′W / 19.417°N 155.283°W / 19.417; -155.283 300,000–600,000 years[59] Kīlauea is currently the most active volcano on Earth.[59]

Puʻu ʻŌʻō, a cinder cone of Kīlauea, has been erupting continuously since January 3, 1983, making it the longest-lived rift-zone eruption of the last six centuries.[60]

Mauna Loa 1984 (Active)[61] 19°28′46.3″N 155°36′09.6″W / 19.479528°N 155.602667°W / 19.479528; -155.602667 ~1 million years[62] Largest volcano on Earth.[61]
Hualālai 1801 (Dormant)[63] 19°41′32″N 155°52′02″W / 19.69222°N 155.86722°W / 19.69222; -155.86722 > 300,000 years[63] Lies more or less due west of the much taller Mauna Kea and Mauna Loa mountains.[63]
Mauna Kea About 4460 BP (Dormant) 19°49′14.39″N 155°28′05.04″W / 19.8206639°N 155.4680667°W / 19.8206639; -155.4680667 ~ 375,000–1 million years[64][65] World's tallest mountain if below-sea elevation is counted.[29]
Kohala About 120,000 BP (Extinct)[66] 20°05′10″N 155°43′02″W / 20.08611°N 155.71722°W / 20.08611; -155.71722 ~ 430,000–1 million years[64][66] Believed to be the oldest volcano that makes up Hawaii Island.[66]
Māhukona 20°01′0″N 156°1′0″W / 20.01667°N 156.01667°W / 20.01667; -156.01667 Submerged, having long since dissapeared into the sea.[67]
Maui
Haleakalā 18th Century[68] 20°42′35″N 156°15′12″W / 20.70972°N 156.25333°W / 20.70972; -156.25333 ~ 0.75–2 million years[64][68] forms more than 75% of Maui.[68]
West Maui 20°54′N 156°37′W / 20.900°N 156.617°W / 20.900; -156.617 ~ 1.32 million years[64] Much eroded shield volcano which makes up the western quarter of Maui.
Kahoolawe
Kahoolawe 20°33′N 156°36′W / 20.550°N 156.600°W / 20.550; -156.600 > 1.03 million years[64][69] Smallest of the 8 principal Hawaiian islands.[66] Uninhabited.[70]
Lanai
Lanai 20°50′N 156°56′W / 20.833°N 156.933°W / 20.833; -156.933 ~ 1.28 million years[64] Sixth-largest island.[71] The only town is Lānaʻi City, a small settlement.
Molokai
East Molokai 21°7′N 156°51′W / 21.117°N 156.850°W / 21.117; -156.850 ~ 1.76 million years[64] Volcano is today only what remains of the southern half.[66]
West Molokai 21°9′N 157°14′W / 21.150°N 157.233°W / 21.150; -157.233 ~ 1.9 million years[64] Northern half suffered a large collapse 1.5 million years ago.[72]
Oahu
Koolau Range 21°19′N 157°46′W / 21.317°N 157.767°W / 21.317; -157.767 2.7 million[73] A fragmented remnant of the eastern or windward shield volcano which also suffered a large collapse sometime before the Molokai collapse.[72]
Waianae Range 2.5 million BP[74] 21°30′N 158°9′W / 21.500°N 158.150°W / 21.500; -158.150 3.7–3.9 million years[64][73] The eroded remains of a shield volcano that comprised the western half of the island.[74]
Kaʻula
Kaʻula 21°39′N 160°32′W / 21.650°N 160.533°W / 21.650; -160.533 ~ 4 million years[64] Tiny crescent-shaped barren island. Uninhabited but for divers & fishermen.[75]
Niihau
Niihau 21°54′N 160°10′W / 21.900°N 160.167°W / 21.900; -160.167 ~4.9 million[64][76] Smallest inhabited island.[77] Formed from a side vent of Kauai.
Kauai
Kauai 22°05′N 159°30′W / 22.083°N 159.500°W / 22.083; -159.500 >5 million[64][78] Oldest and fourth largest of the main islands, and home to Mount Waialeale, one of the wettest areas on Earth in terms of precipitation.[79]
Major Northwestern Hawaiian Islands
Name Stage Coordinates Age[80] Notes
Nihoa Extinct Island 23°03′38″N 161°55′19″W / 23.06056°N 161.92194°W / 23.06056; -161.92194 7.2 million ± 0.3[64] Small rocky island which supported a small population about 1000 CE; features over 80 cultural sites, including religious places, agricultural terraces, and burial caves.[81]
Necker Island Extinct Island 23°03′N 161°55′W / 23.050°N 161.917°W / 23.050; -161.917 10.3 million ± 0.4[64] Small deserted island with Hawaiian religious shrines and artifacts.[82]
French Frigate Shoals Atoll 23°52′08″N 166°17′10″W / 23.8689°N 166.2860°W / 23.8689; -166.2860 12 million[83] Largest atoll in the northwestern Hawaiian islands.[84]
Gardner Pinnacles Atoll Island 25°01′N 167°59′W / 25.017°N 167.983°W / 25.017; -167.983 12.3 million ± 1.0[64] Two barren rock outcrops surrounded by a reef.[85]
Maro Reef Atoll 25°25′N 170°35′W / 25.417°N 170.583°W / 25.417; -170.583 Largest coral reef of the northwestern Hawaiian islands.[86]
Laysan Atoll Island 25°46′03″N 171°44′00″W / 25.7675°N 171.7334°W / 25.7675; -171.7334 19.9 million ± 0.3[64] Originally named "Kauō" meaning egg, referring to its shape, and home to one of only five natural lakes in all of Hawaii.[87]
Lisianski Island Atoll Island 26°3′48.6564″N 173°57′57.346″W / 26.063515667°N 173.96592944°W / 26.063515667; -173.96592944 A small island surrounded by a huge coral reef nearly the size of Oahu.[88] Named after a Russian navy captain whose ship ran aground there in 1805.[89]
Pearl and Hermes Atoll Atoll Island 27°48′N 175°51′W / 27.800°N 175.850°W / 27.800; -175.850 20.6 million ± 2.7[64] A collection of small, sandy islands, with a lagoon and coral reef. Named after two whaling ships which wrecked on the reef in 1822.[90]
Midway Atoll Atoll Island 28°12′N 177°21′W / 28.200°N 177.350°W / 28.200; -177.350 27.7 million ± 0.6[64] Consists of a ring-shaped barrier reef and two large islets. Named "midway" because of its strategic location in the center of the Pacific Ocean, and was the site of a key battle during World War II.[91]
Kure Atoll Atoll 28°25′N 178°20′W / 28.417°N 178.333°W / 28.417; -178.333 Northern-most coral atoll in the world.[92]
Emperor Seamounts
Many are named after emperors or empresses of the Kufun dynasty of Japanese history.
Name Type Coordinates[93] Age Notes
Hancock Seamount 30°15′N 178°50′E / 30.250°N 178.833°E / 30.250; 178.833 Unknown
Colahan Seamount 31°15′N 176°0′E / 31.250°N 176.000°E / 31.250; 176.000 38.6 million ± 0.3[94]
Abbott Seamount 31°48′N 174°18′E / 31.800°N 174.300°E / 31.800; 174.300 38.7 million ± 0.9[94]
Daikakuji Guyot 32°5.00′N 172°18′E / 32.08333°N 172.300°E / 32.08333; 172.300 42.4 million ± 2.3[64] Also the name of a Japanese temple.
Kammu Guyot 32°10′N 173°0′E / 32.167°N 173.000°E / 32.167; 173.000 Unknown Named for former emperor of Japan Emperor Kammu.
Yuryaku Guyot 32°40.20′N 172°16.20′E / 32.67000°N 172.27000°E / 32.67000; 172.27000 43.4 million ± 1.6[64] Named after former emperor of Japan Emperor Yūryaku.
Kimmei Seamount 33°40.84′N 171°38.07′E / 33.68067°N 171.63450°E / 33.68067; 171.63450 ~ 39.9–50 million years[16][64] Named after former emperor of Japan Emperor Kimmei.
Koko Guyot 35°15.00′N 171°35.00′E / 35.25000°N 171.58333°E / 35.25000; 171.58333 48.1 million ± 0.8[64] Named after former emperor of Japan Emperor Kōkō.
Ojin Guyot 37°58.20′N 170°22.80′E / 37.97000°N 170.38000°E / 37.97000; 170.38000 55.2 million ± 0.7[64] Named after former emperor of Japan Emperor Ōjin.
Jingu Guyot 38°50′N 171°15′E / 38.833°N 171.250°E / 38.833; 171.250 55.4 million ± 0.9[95] Named after former empress of Japan Empress Jingū.
Nintoku Guyot 41°4.80′N 170°34.20′E / 41.08000°N 170.57000°E / 41.08000; 170.57000 56.2 million ± 0.6[64] Named after former emperor of Japan Emperor Nintoku.
Yomei Guyot 42°18′N 170°24′E / 42.300°N 170.400°E / 42.300; 170.400 Unknown Named for former emperor of Japan Emperor Yomei.
Suiko Guyot 44°35′N 170°20′E / 44.583°N 170.333°E / 44.583; 170.333 59.6 million ± 0.6 –64.7 million ± 1.1[96] Named after former empress of Japan Empress Suiko.
Detroit Seamount 51°28.80′N 167°36′E / 51.48000°N 167.600°E / 51.48000; 167.600 76–81 million years[18][97] Well documented seamount, second oldest.
Meiji Seamount 53°12′N 164°30′E / 53.200°N 164.500°E / 53.200; 164.500 81–86 million years[18][97] Named after former emperor of Japan Emperor Meiji. Oldest known seamount.
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See also

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Notes

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References

External links

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