Evolution of Hawaiian volcanoes
Encyclopedia
Hawaii
Hawaii is the newest of the 50 U.S. states , and is the only U.S. state made up entirely of islands. It is the northernmost island group in Polynesia, occupying most of an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of...
are the youngest in a chain of more than 129 volcanoes that stretch 5800 kilometres (3,604 mi) across the North Pacific Ocean, called the Hawaiian-Emperor seamount chain
Hawaiian-Emperor seamount chain
The Hawaiian–Emperor seamount chain is composed of the Hawaiian ridge, consisting of the islands of the Hawaiian chain northwest to Kure Atoll, and the Emperor Seamounts, a vast underwater mountain region of islands and intervening seamounts, atolls, shallows, banks and reefs along a line trending...
.
Hawaii's volcanoes rise an average of 4572 metres (15,000 ft) to reach sea level from their base. The largest and most famous, Mauna Loa
Mauna Loa
Mauna Loa is one of five volcanoes that form the Island of Hawaii in the U.S. state of Hawaii in the Pacific Ocean, and the largest on Earth in terms of volume and area covered. It is an active shield volcano, with a volume estimated at approximately , although its peak is about lower than that...
, has built itself up to a height of 4169 metres (13,678 ft). As shield volcano
Shield volcano
A shield volcano is a type of volcano usually built almost entirely of fluid lava flows. They are named for their large size and low profile, resembling a warrior's shield. This is caused by the highly fluid lava they erupt, which travels farther than lava erupted from more explosive volcanoes...
es, they are built by accumulated lava flows, growing no more than 3 metres (10 ft) at a time to form a broad and gently sloping shape.
Hawaiian volcanoes all follow a specific pattern of eruption, building, and erosion. Hawaiian islands undergo a systematic pattern of submarine and subaerial growth that is followed by erosion. An island's stage of development reflects its distance from the Hawaii hotspot
Hawaii hotspot
The Hawaii hotspot is the volcanic hotspot that created the Hawaiian Islands in the central Pacific Ocean, and is one of Earth's best-known and most heavily-studied hotspots....
.
Background
Hawaiian-Emperor seamount chain
The Hawaiian–Emperor seamount chain is composed of the Hawaiian ridge, consisting of the islands of the Hawaiian chain northwest to Kure Atoll, and the Emperor Seamounts, a vast underwater mountain region of islands and intervening seamounts, atolls, shallows, banks and reefs along a line trending...
is remarkable for its length and its number of volcanoes. The chain is split into two subsections across a break, separating the older Emperor Seamount Chain from the younger Hawaiian Ridge; the "V" shape bend of the chain is easily noticeable on maps. The volcanoes are progressively younger to the southeast; the oldest dated volcano, located at the northern end, is 81 million years old. The break between the two sub-chains is 43 million years- in comparison, the oldest of the principal islands, Kauai
Kauai
Kauai or Kauai, known as Tauai in the ancient Kaua'i dialect, is geologically the oldest of the main Hawaiian Islands. With an area of , it is the fourth largest of the main islands in the Hawaiian archipelago, and the 21st largest island in the United States. Known also as the "Garden Isle",...
, is little more than 5 million years.
The "assembly line" that forms the volcanoes is driven by a hotspot
Hotspot (geology)
The places known as hotspots or hot spots in geology are volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the mantle elsewhere. They may be on, near to, or far from tectonic plate boundaries. There are two hypotheses to explain them...
- a plume of magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
deep within the Earth procuring lava
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
at the surface. As the Pacific Plate
Pacific Plate
The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At 103 million square kilometres, it is the largest tectonic plate....
moves in a west-northwest direction, each volcano moves with it away from its place of origin above the hotspot. The age and location of the volcanoes are a record of the direction, rate of movement, and orientation of the Pacific Plate
Pacific Plate
The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At 103 million square kilometres, it is the largest tectonic plate....
. The pronounced 43 million year old break separating the Hawaiian Ridge from the Emperor Chain marks a dramatic change in direction of plate movement. Initial, deeper-water volcanic eruptions are characterized by pillow lava, so named for their shape, while shallow-water eruptions tend to be composed mainly of volcanic ash
Volcanic ash
Volcanic ash consists of small tephra, which are bits of pulverized rock and glass created by volcanic eruptions, less than in diameter. There are three mechanisms of volcanic ash formation: gas release under decompression causing magmatic eruptions; thermal contraction from chilling on contact...
. Once the volcano is high enough so as to eliminate interference from water, its lava flows become those of ropey pāhoehoe and blocky aā lava.
Our current understanding of the process of evolution originates from the first half of the 20th century. The understanding of the process was advanced by frequent observation of volcanic eruptions, study of contrasting rock types, and reconnaissance mapping. More recently our understanding has been aided by geophysical studies, offshore submersible studies, the advent of radioactive dating, advances in petrology and geochemistry, advanced surveillance and monitoring, and detailed geological studies. The ratio of magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
to silica in the lava is a sign of what stage the volcano is in, as over time the volcano's lavas shift from alkalic
Alkali basalt
Alkali basalt is a fine-grained, dark-coloured, volcanic rock characterized by phenocrysts of olivine, titanium-rich augite, plagioclase feldspar and iron oxides. For similar SiO2 concentrations, alkali basalts have a higher content of the alkalis, Na2O and K2O, than other basalt types such as...
to tholeiitic lava, and then back to alkalic.
Although volcanism and erosion are the chief factors in the growth and erosion of a volcano, other factors are also involved. Subsidence is known to occur. Changes in sea level, occurring mostly during the Pleistocene
Pleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
, have caused drastic changes; an example is the breakup of Maui Nui
Maui Nui
Maui Nui or Greater Maui, is a modern geologists' name given to a prehistoric Hawaiian Island built from seven shield volcanoes. Nui means "great/large" in the Hawaiian language....
, initially a seven-volcano island, which was transformed into five islands as a result of subsidence. High rainfall due the trade wind
Trade wind
The trade winds are the prevailing pattern of easterly surface winds found in the tropics, within the lower portion of the Earth's atmosphere, in the lower section of the troposphere near the Earth's equator...
effect impacts on the severity of erosion on many of the major volcanoes. Coastline collapses, a notable part of the history of many of the Hawaiian volcanoes, are often devastating and destroy large parts of the volcanoes.
Submarine preshield stage
Caldera
A caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption, such as the one at Yellowstone National Park in the US. They are sometimes confused with volcanic craters...
and has two or more rift zone
Rift zone
A rift zone is a feature of some volcanoes, especially the shield volcanoes of Hawaii, in which a linear series of fissures in the volcanic edifice allows lava to be erupted from the volcano's flank instead of from its summit...
s radiating from the summit. The type of lava erupted in this stage of activity is alkali
Alkali
In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...
basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
. Due to stretching forces, the development of two or more rift zone
Rift zone
A rift zone is a feature of some volcanoes, especially the shield volcanoes of Hawaii, in which a linear series of fissures in the volcanic edifice allows lava to be erupted from the volcano's flank instead of from its summit...
s is common. The lava accumulates in a shallow magma storage reservoir.
Because the eruptions occur with the volcano underwater, the form of lava typically erupted is pillow lava. Pillow lava is rounded balls of lava that was given very little time to cool due to immediate exposure to water. Water pressure prevents the lava from exploding upon contact with the cold ocean water, forcing it to simmer and solidify quickly. This stage is thought to last about 200,000 years, but lavas erupted during this stage make up only a tiny fraction of the final volume of the volcano. As time progresses, eruptions become stronger and more frequent.
The only example of a Hawaiian volcano in this stage is Lōihi Seamount
Loihi Seamount
Lōihi Seamount is an active undersea volcano located around off the southeast coast of the island of Hawaii about below sea level. It lies on the flank of Mauna Loa, the largest shield volcano on Earth...
, which is thought to be transitioning from the submarine preshield stage into the submarine phase of the shield stage. All older volcanoes have had their preshield stage lavas buried by younger lavas, so everything that is known about this stage comes from research done on Lōihi Seamount.
Shield stages
The shield stage of the volcano is subdivided into three phases: the submarine, explosive, and sub aerial. During this stage of growth, the volcano accumulates about 95 percent of its mass and it takes on the "shield" shape that shield volcanoShield volcano
A shield volcano is a type of volcano usually built almost entirely of fluid lava flows. They are named for their large size and low profile, resembling a warrior's shield. This is caused by the highly fluid lava they erupt, which travels farther than lava erupted from more explosive volcanoes...
es are named for. It is also the stage where the volcano's eruptive frequency reaches its peak.
Submarine phase
As eruptions become more and more frequent at the end of the preshield stage, the composition of the lava erupted from the Hawaiian volcano changes from alkalic basalt to tholeiitic basalt and the volcano enters the submarine phase of the shield stage. In this phase, the volcano continues to erupt pillow lava. Calderas form, fill, and reform at the volcano's summit and the rift zones remain prominent. The volcano builds its way up to sea level. The submarine phase ends when the volcano is only shallowly submerged.The only example of a volcano in this stage is Lōihi Seamount
Loihi Seamount
Lōihi Seamount is an active undersea volcano located around off the southeast coast of the island of Hawaii about below sea level. It lies on the flank of Mauna Loa, the largest shield volcano on Earth...
, which is currently transitioning into this phase from the preshield stage.
Explosive phase
Subaerial phase
Once a volcano has added enough mass and height to end frequent contact with water, the sub aerial substage begins. During this stage of activity, the explosive eruptions become much less frequent and the nature of the eruptions become much more gentle. Lava flows are a combination of pāhoehoe and aā. It is during this stage, that the low-profile "shieldShield
A shield is a type of personal armor, meant to intercept attacks, either by stopping projectiles such as arrows or redirecting a hit from a sword, mace or battle axe to the side of the shield-bearer....
" shape of Hawaiian volcanoes is formed, named for the shape of a warrior's shield. Eruption rates and frequencies peak, and about 95% of the volcano's eventual volume forms during a period of roughly 500,000 years.
The lava erupted in this stage form flows of pāhoehoe or aā. During this subaerial
Subaerial
The term subaerial is mainly used in geology to describe events or structures that are located at the Earth's surface...
stage, the flanks of the growing volcanoes are unstable and as a result, large landslide
Landslide
A landslide or landslip is a geological phenomenon which includes a wide range of ground movement, such as rockfalls, deep failure of slopes and shallow debris flows, which can occur in offshore, coastal and onshore environments...
s may occur. At least 17 major landslides have occurred around the major Hawaiian islands. This stage is arguably the most well-studied, as all eruptions that occurred in the 20th century on the island of Hawaii
Hawaii (island)
The Island of Hawaii, also called the Big Island or Hawaii Island , is a volcanic island in the North Pacific Ocean...
were produced by volcanoes in this phase.
Mauna Loa
Mauna Loa
Mauna Loa is one of five volcanoes that form the Island of Hawaii in the U.S. state of Hawaii in the Pacific Ocean, and the largest on Earth in terms of volume and area covered. It is an active shield volcano, with a volume estimated at approximately , although its peak is about lower than that...
and Kīlauea
Kilauea
Kīlauea is a volcano in the Hawaiian Islands, and one of five shield volcanoes that together form the island of Hawaii. Kīlauea means "spewing" or "much spreading" in the Hawaiian language, referring to its frequent outpouring of lava. The Puu Ōō cone has been continuously erupting in the eastern...
volcanoes are in this phase of activity.
Postshield stage
Eruptions in the postshield stage cap the volcano with a carapace of lava, containing low silica and high alkali contents, the reverse of the stage before it. Some Hawaiian volcanoes diverge from this, however. Lava is erupted as stocky, pasty aā flows along with a lot of cinder
Cinder
A cinder is a pyroclastic material. Cinders are extrusive igneous rocks. Cinders are similar to pumice, which has so many cavities and is such low-density that it can float on water...
. Caldera development stops, and the rift zones become more inactive. The new lava flows increase the slope grade, as the aā never reaches the base of the volcano. These lavas commonly fill and overflow the caldera
Caldera
A caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption, such as the one at Yellowstone National Park in the US. They are sometimes confused with volcanic craters...
. Eruption rate gradually decreases over a period of ca. 250,000 years, eventually stopping altogether as the volcano becomes dormant
Dormant
Dormant means lacking activity. It can refer to:*Dormancy in an organism's life cycle*Dormant volcano, a volcano that is inactive but may become active in the future...
.
Mauna Kea
Mauna Kea
Mauna Kea is a volcano on the island of Hawaii. Standing above sea level, its peak is the highest point in the state of Hawaii. However, much of the mountain is under water; when measured from its oceanic base, Mauna Kea is over tall—significantly taller than Mount Everest...
, Hualālai
Hualalai
Hualālai is a dormant shield volcano on the island of Hawaii in the Hawaiian Islands. It is the third-youngest and the third most active of the five volcanoes that form the island of Hawaii, following Kīlauea and the much larger Mauna Loa, and also the westernmost. Its peak is above sea...
, and Haleakalā
Haleakala
Haleakalā , or the East Maui Volcano, is a massive shield volcano that forms more than 75% of the Hawaiian Island of Maui. The western 25% of the island is formed by the West Maui Mountains.- History :...
volcanoes are in this stage of activity.
Erosional stage
After the volcano becomes dormantDormant
Dormant means lacking activity. It can refer to:*Dormancy in an organism's life cycle*Dormant volcano, a volcano that is inactive but may become active in the future...
, the forces of erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
gain control of the mountain. The volcano subsides into the oceanic crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
due to its immense weight and loses elevation. Meanwhile, rain also erodes the volcano, creating deeply incised valleys. Coral reef
Coral reef
Coral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps...
s grow along the shoreline. The volcano becomes a skeleton of its former self.
Kohala
Kohala (mountain)
Kohala is the oldest of five volcanoes that make up the island of Hawaii. Kohala is an estimated one million years old—so old that it experienced, and recorded, a reversal of magnetic field 780,000 years ago. It is believed to have breached sea level more than 500,000 years ago and to...
, Lānai
Lanai
Lānai or Lanai is the sixth-largest of the Hawaiian Islands. It is also known as the Pineapple Island because of its past as an island-wide pineapple plantation. The only town is Lānai City, a small settlement....
, and Waianae
Waianae Range
Waianae Range is the eroded remains of an ancient shield volcano that comprises the western half of the Hawaiian Island of Oahu...
volcanoes are examples of volcanoes in this stage of development.
Rejuvenated stage
After a long period of dormancy and erosion of the surface, the volcano might become active again, entering a final stage of activity called the rejuvenated stage. During this stage, the volcano erupts small volumes of lava very infrequently. These eruptions are often spread out over several millions of years. The composition of the lavas erupted in this stage is usually alkalic. The stage commonly occurs between 0.6 and 2 million years after it has entered the weathering cycle.Koolau
Koolau Range
Koolau Range is a name given to the fragmented remnant of the eastern or windward shield volcano of the Hawaiian island of Oahu. It is not a mountain range in the normal sense, because it was formed as a single mountain called Koolau Volcano...
and West Maui
West Maui Mountains
The West Maui Mountains or West Maui Volcano, known to the Hawaiians as Maui Komohana and to geologists as Mauna Kahalawai, forms a much eroded shield volcano that constitutes the western one-quarter of the Hawaiian Island of Maui....
volcanoes are examples of volcanoes in this stage of development. Note, however, that because in this stage eruptions are very infrequent (occurring thousands or even tens of thousands of years apart), erosion is still the primary factor controlling the volcano's development.
Coral atoll stage
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
and sand islands surrounding a lagoon
Lagoon
A lagoon is a body of shallow sea water or brackish water separated from the sea by some form of barrier. The EU's habitat directive defines lagoons as "expanses of shallow coastal salt water, of varying salinity or water volume, wholly or partially separated from the sea by sand banks or shingle,...
. All the Hawaiian islands west of the Gardner Pinnacles
Gardner Pinnacles
The Gardner Pinnacles are two barren rock outcrops surrounded by a reef and located in the Hawaiian Islands at , northwest of Honolulu and French Frigate Shoals. The total area of the two small islets—remnants of an ancient volcano—is . Its highest peak has a commanding height of 170...
in the Northwestern Hawaiian Islands
Northwestern Hawaiian Islands
The Northwestern Hawaiian Islands or the Leeward Islands are the small islands and atolls in the Hawaiian island chain located northwest of the islands of Kauai and Niihau. They are administered by the U.S. state of Hawaii except Midway Atoll, which has temporary residential facilities and is...
are in this stage.
Guyot stage
Atolls are the product of the growth of tropical marine organisms, so these islandIsland
An island or isle is any piece of sub-continental land that is surrounded by water. Very small islands such as emergent land features on atolls can be called islets, cays or keys. An island in a river or lake may be called an eyot , or holm...
s are only found in warm tropical waters. Eventually, the Pacific Plate
Pacific Plate
The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At 103 million square kilometres, it is the largest tectonic plate....
carries the volcanic atoll into waters too cold for these marine organisms to maintain a reef by growth. Volcanic islands located beyond the warm water temperature requirements of reef building organisms
Reef
In nautical terminology, a reef is a rock, sandbar, or other feature lying beneath the surface of the water ....
become seamounts as they subside and are eroded away at the surface. An island that is located where the ocean water temperatures
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
are just sufficiently warm for upward reef growth
Reef
In nautical terminology, a reef is a rock, sandbar, or other feature lying beneath the surface of the water ....
to keep pace with the rate of subsidence is said to be at the Darwin Point
Atoll
An atoll is a coral island that encircles a lagoon partially or completely.- Usage :The word atoll comes from the Dhivehi word atholhu OED...
. Islands in more northerly latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
s evolve towards seamounts or guyots; islands closer to the equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
evolve towards atolls (see Kure Atoll
Kure Atoll
Kure Atoll or Ocean Island is an atoll in the Pacific Ocean beyond Midway Atoll in the Northwestern Hawaiian Islands at . The only land of significant size is called Green Island and is habitat for hundreds of thousands of seabirds...
).
After the reef dies, the volcano subsides or erodes below sea level and becomes a coral-capped seamount. These flat-topped seamounts are called guyot
Guyot
A guyot , also known as a tablemount, is an isolated underwater volcanic mountain , with a flat top over 200 meters below the surface of the sea. The diameters of these flat summits can exceed ....
s. Most, if not all, the volcanoes west of Kure Atoll as well as most, if not all, the volcanoes in the Emperor Seamount chain are guyots or seamount
Seamount
A seamount is a mountain rising from the ocean seafloor that does not reach to the water's surface , and thus is not an island. These are typically formed from extinct volcanoes, that rise abruptly and are usually found rising from a seafloor of depth. They are defined by oceanographers as...
s.
Other patterns
Not all Hawaiian volcanoes go through all of these stages of activity. An example is Koolau volcanoKoolau Range
Koolau Range is a name given to the fragmented remnant of the eastern or windward shield volcano of the Hawaiian island of Oahu. It is not a mountain range in the normal sense, because it was formed as a single mountain called Koolau Volcano...
on Oahu
Oahu
Oahu or Oahu , known as "The Gathering Place", is the third largest of the Hawaiian Islands and most populous of the islands in the U.S. state of Hawaii. The state capital Honolulu is located on the southeast coast...
, which was prehistorically devastated by a cataclysmic landslide
Landslide
A landslide or landslip is a geological phenomenon which includes a wide range of ground movement, such as rockfalls, deep failure of slopes and shallow debris flows, which can occur in offshore, coastal and onshore environments...
, never underwent the postshield stage and went dormant for hundreds of thousands of years after the shield stage before coming back to life. Some volcanoes never made it above sea level; there is no evidence to suggest that West Molokai ever went through the rejuvenated stage, while its younger neighbors, East Molokai
Wailau
Wailau, also known as East Molokai Volcano, is an extinct shield volcano comprising the eastern two-thirds of the island of Molokai in the U.S. state of Hawaii.-Description:...
and West Maui, have evidently done so.
Application to other groups
In recent years research at other seamounts, for instance Jasper SeamountJasper Seamount
Jasper Seamount is a seamount located in the Fieberling-Guadalupe seamount track, west of Baja California, Mexico. Jasper is the site of detailed geophysical geological and geochemical studies which suggest that many seamounts, big and small, follow the same pattern of growth and death that was...
, has confirmed that the Hawaiian model applies to other seamounts as well.