Shield volcano
Encyclopedia
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
. This results in the steady accumulation of broad sheets of lava, building up the shield volcano's distinctive form.
s, which flow out in all directions to create a shield like that of a warrior. The word "shield" has a long history, and is derived from the Old English scield or scild, which is in turn taken from the Proto-Germanic skeldus taken from various derivations of the Gothic skildus, meaning "to divide, split, or separate". Shield volcano itself is taken from the German term schildvulkan.
es and cinder cone
s, by distinct differences in structure and composition. Stratovolcanoes are built up by the accumulation of thick, viscous
lavas, whereas cinder cones are constructed of tephra
ejected in explosive eruptions. In comparison, shield volcanoes are built of relatively weakly viscous basalt
ic lavas that erupts in longer cycles than that of a stratovolcano. Shield volcanoes are distinctive products of hotspot
volcanism, but can form at rift
and subduction zones as well.
The types of eruptions that occur at shield volcanoes have been named Hawaiian eruption
s, after the Hawaiian chain
in which they are most prominent. Hawaiian eruptions are characterized by the effusive
emission of fluid lavas. The mobile nature of these lavas allows shield volcano flows to travel a longer distance than those of other volcanic types, resulting in larger and thinner sheets of lava, often just 1 m (3 ft) thick. Over long periods of time, the gradual buildup of thousands of these flows slowly constructs the characteristically low, broad profile of a mature shield volcano.
Because of their gradual buildup and near-continuous eruptive characteristics, shield volcanoes vary widely in size with their age. Mature shield volcanoes are the largest volcanoes on Earth.This excludes flood basalt
s, large sheets of volcanic material that do not actually resemble volcanoes. Shield volcanoes often measure 3 to 4 mi (4.8 to 6.4 km) in diameter and surpass 1500 to 2000 ft (457.2 to 609.6 m) in height. The largest shield volcano (and the largest active volcano) in the world is Mauna Loa
in Hawaii
, which projects 13677 ft (4,169 m) above sea level
, and is over 60 mi (97 km) wide. The volcano is estimated to contain 80000 km³ (19,193 cu mi) of basalt, a mass so great that it has slumped into the Earth's crust. Their lower slopes are generally gentle (~2 degrees), but steepen with elevation (reaching ~10 degrees) before flattening near the summit, giving the volcanoes a convex shape. The height of a shield volcano is typically 1/20th of its width.
The Hawaiian shield volcanoes and the Galápagos islands are unique in that they are not located near any plate boundaries
; instead, the two chains are fed by the movement of oceanic plates over an upwelling of magma known as a hotspot
. Over millions of years, the tectonic movement that moves continents also creates long volcanic trails across the seafloor. The Hawaiian and Galápagos shields, and other hotspot shields like them, are both constructed of oceanic island basalt. Their lavas are characterized by high levels of sodium
, potassium
, and aluminum.
Rift zone
s are a prevalent feature on shield volcanoes that is rare on other volcanic types. The large, decentralized shape of Hawaiian volcanoes as compared to their smaller, symmetrical Icelandian cousins can be attributed to rift eruptions. Fissure venting is common in Hawaii; most Hawaiian eruptions begin with a so-called "wall of fire" along a major fissure line before centralizing to a small number of points. This accounts for their asymmetrical shape, whereas Icelandian volcanoes follow a pattern of central eruptions dominated by summit calderas
, causing much evener lava.
Another hallmark of shield volcanism are lava tube
s. Lava tubes are cave-like volcanic straights formed by the hardening of overlaying lava. These structures help further the propagation of lava, as the walls of the tube insulate
s the lava within. Lava tubes can account for a large chunk of shield volcano activity; for example, an estimated 58% of the lava forming Kilauea comes from lava tubes.
Most mature shield volcanoes have multiple splatter (or cinder) cones on their flanks. The cones are a result of tephra ejection during incessant activity, building up a volcanic cone at the eruption site, and thus marking the site of former and current eruptive sites on the shield volcano. A prominent example of a shield volcano-bound cinder cone is Puʻu ʻŌʻō on Kīlauea
. The cone has been erupting continuously since 1983 in one of the longest-lasting rift eruptions in history, and was built up to its present height of 2290 ft (698 m) by over 25 years of activity.
In some shield volcano eruptions, basalt
ic lava pours out of a long fissure
instead of a central vent, and shrouds the countryside with a long band of volcanic material in the form of a broud plateau
. Plateaus of this type exist in Iceland
, Washington, Oregon
, and Idaho
; the most prominent ones are situated along the Snake River
in Idaho and the Columbia River
in Washington and Oregon, where they have been measured to be over a 1 mi (2 km) in thickness. Many eruptions start as a so-called "curtain of fire"—a long eruptive chain along a fissure vent
on the volcano. Eventually these eruptions die down and start to focus around a few points on the fissure, where activity is concentrated.
Calderas are a common feature on shield volcanoes. They are formed and reformed over the volcano's lifespan. Long eruptive periods form cinder cones, which then collapse over time to form calderas. The calderas are often filled up by future eruptions, or formed elsewhere, and this cycle of collapse and regeneration takes place throughout the volcano's lifespan.
Interactions between water and lava at shield volcanoes can cause some eruptions to become hydrovolcanic. These explosive
eruptions are drastically different from the usual shield volcanic activity, and are especially prevalent at the waterbound volcanoes of the Hawaiian Isle
s.
s (points where magma
from below the surface wells up), such as the Hawaiian-Emperor seamount chain
and the Galápagos Islands
, or over more conventional rift zone
s, such as the Icelandic shields and the shield volcanoes of the shield volcanoes of East Africa
. Many shield volcanoes are found in ocean basins, although they can be found inland as well—East Africa being one example of this.
, a chain
of hotspot volcanoes
in Pacific Ocean
. The Hawaiian volcanoes are characterized by frequent rift eruption
s, their large size (thousands of km3 in volume), and their rough, decentralized shape. Rift zone
s are a prominent feature on these volcanoes, and account for their seemingly random volcanic structure. They are fueled by the movement of the Pacific Plate
over the Hawaii hotspot
, and form a long chain of volcanoes, atoll
s, and seamount
s 2600 km (1,616 mi) long with a total volume of over 750000 km³ (179,935 cu mi). The chain contains at least 43 major volcanoes, and Meiji Seamount
at its terminus near the Kuril-Kamchatka Trench
is 85 million years old. The volcanoes follow a distinct evolutionary pattern
of growth and death.
The chain includes the largest volcano on Earth, Mauna Loa
, which stands 4170 m (13,681.1 ft) above sea level
and reaches a further 13 km (8 mi) below the waterline and into the crust, approximately 80000 km³ (19,193 cu mi) of rock. Kilauea
, meanwhile, is one of the most active volcanoes on Earth, with the current ongoing eruption having begun in January 1983.
are an isolated set of volcanoes, consisting of shield volcanoes and lava plateaus, located 1200 km (746 mi) west of Ecuador
. They are driven by the Galápagos hotspot
, and are between approximately 4.2 million and 700,000 years of age. The largest island, Isabela Island, consists of six coalesced shield volcanoes, each delineated by a large summit caldera. Española
, the oldest island, and Fernandina
, the youngest, are also shield volcanoes, as are most of the other islands in the chain. The Galápagos Islands are perched on a large lava plateau known as the Galápagos Platform. This platform creates a shallow water depth of 360 to 900 m (1,181.1 to 2,952.8 ft) at the base of the islands, which stretch over a 174 mi (280 km)-long diameter. Since Charles Darwin
's famous visit
to the islands in 1835, there have been over 60 recorded eruptions in the islands, from six different shield volcanoes. Of the 21 emergent volcanoes, 13 are considered active.
The Galápagos islands are geologically young for such a large chain, and the pattern of their rift zone
s follows one of two trends, one north-northwest, and one east-west. The composition of the lavas of the Galápagos shields are strikingly similar to those of the Hawaiian volcanoes. Curiously, they do not form the same volcanic "line" associated with most hotspots. They are not alone in this regard; the Cobb-Eickelberg Seamount chain
in the North Pacific is another example of such a delineated chain. In addition, there is no clear pattern of age between the volcanoes, suggesting a complicated, irregular pattern of creation. How exactly the islands were formed remains a geological mystery, although several theories have been fronted.
Another major center of shield volcanic activity is Iceland
. Located over the Mid-Atlantic Ridge
, a divergent tectonic plate
in the middle of the Atlantic Ocean
, Iceland is the site of about 130 volcanoes of various types. Icelandic shield volcanoes are generally of Holocene
age, between 5000 and 10000 years old, except for the island of Surtsey
, a Surtseyan shield
. The volcanoes are also very narrow in distribution, occurring in two bands in the West and North Volcanic Zones. Like Hawaiian volcanoes, their formation initially begins with several eruptive centers before centralizing and concentrating at a single point. The main shield then forms, burying the smaller ones formed by the early eruptions with its lava.
Icelandic shields are generally small (~15 km³ (4 cu mi)), symmetrical (although this can affected by surface topography), and characterized by eruptions from summit calderas
. They are composed of either tholeiitic olivine
or picritic basalt
. The tholeiitic shields tend to be wider and shallower than the picritic shields. They do not follow the pattern of caldera growth and destruction that other shield volcanoes do; caldera may form, but they generally do not disappear. None of the shield volcanoes in the region are known to have erupted within the last 5000 years.
, a developing plate boundary in Africa, and from nearby hotspots. Some volcanoes interact with both. Shield volcanoes are found near the rift and off the coast of Africa, although stratovolcanoes are more common. Although sparsely studied, the fact that all of its volcanoes are of Holocene age reflects how young the volcanic center is. One interesting characteristic of East African volcanism is a penchant for the formation of lava lake
s; these semi-permanent lava bodies, extremely rare elsewhere, form in about 9% of African eruptions.
The most active shield volcano in Africa is Nyamuragira. Eruptions at the shield volcano are generally centered within the large summit caldera or on the numerous fissures and cinder cones on the volcano's flanks. Lava flows from the most recent century extend down the flanks more than 30 km (19 mi) from the summit, reaching as far as Lake Kivu
. Erta Ale
in Ethiopia
is another active shield volcano, and one of the few places in the world with a permanent lava lake, which has been active since at least 1967, and possibly since 1906. Other volcanic centers include Menengai
, a massive shield caldera, and Mount Marsabit
, near the town of Marsabit
.
large or active enough to have a molten
inner core
, and since probes were first launched in the 1960s, volcanoes have been found across the solar system. Shield volcanoes and volcanic vents have been found on Mars
, Venus
, and Io
; cryovolcano
es on Triton
; and subsurface hotspots on Europa
.
The volcanoes of Mars
are very similar to the shield volcanoes on Earth. Both have gently sloping flanks, collapse craters along their central structure, and are built of highly fluid lavas. Volcanic features on Mars were observed long before they were first studied in detail during the 1976-1979 Viking mission
. The principal difference between the volcanoes of Mars and those on Earth is in terms of size; Martian volcanoes are over 17 mi (27 km) high and 350 mi (563 km) in diameter, far larger than the 6 mi (10 km) high, 74 mi (119 km) wide Hawaiian shields.
The most famous example is Olympus Mons
, a shield volcano that is the highest known mountain in the solar system.
Venus
also has over 150 shield volcanoes which are much flatter, with a larger surface area than those found on Earth, some having a diameter of more than 700 km (435 mi). Although the majority of these are long extinct it has been suggested, from observations by the Venus Express spacecraft
that many may still be active.
s of shield volcanoes do not pose much threat to humans, as they emit large amounts of slow moving lava over long periods of time. However, they are hazardous to agriculture
and infrastructure
; the ongoing 1983 eruption of Kīlauea
has destroyed over 200 structures and buried kilometers of highways.
lavas, pyroclastic shields are mainly formed from explosive eruptions of ignimbrite
.
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
usually built almost entirely of fluid
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
lava flows. They are named for their large size and low profile, resembling a warrior's shield
Shield
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....
. This is caused by the highly fluid lava they erupt, which travels farther than lava erupted from more explosive volcanoes
Explosive eruption
An explosive eruption is a volcanic term to describe a violent, explosive type of eruption. Mount St. Helens in 1980 was an example. Such an eruption is driven by gas accumulating under great pressure. Driven by hot rising magma, it interacts with ground water until the pressure increases to the...
. This results in the steady accumulation of broad sheets of lava, building up the shield volcano's distinctive form.
Etymology
Shield volcanoes are built up by effusive eruptionEffusive eruption
An effusive eruption is a volcanic eruption characterized by the outpouring of lava onto the ground...
s, which flow out in all directions to create a shield like that of a warrior. The word "shield" has a long history, and is derived from the Old English scield or scild, which is in turn taken from the Proto-Germanic skeldus taken from various derivations of the Gothic skildus, meaning "to divide, split, or separate". Shield volcano itself is taken from the German term schildvulkan.
Geology
Shield volcanoes are one of the three major types of volcanoes, distinguished from the two other major volcanic types, stratovolcanoStratovolcano
A stratovolcano, also known as a composite volcano, is a tall, conical volcano built up by many layers of hardened lava, tephra, pumice, and volcanic ash. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile and periodic, explosive eruptions...
es and cinder cone
Cinder cone
According to the , Cinder Cone is the proper name of 1 cinder cone in Canada and 7 cinder cones in the United States:In Canada: Cinder Cone In the United States:...
s, by distinct differences in structure and composition. Stratovolcanoes are built up by the accumulation of thick, viscous
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
lavas, whereas cinder cones are constructed of tephra
Tephra
200px|thumb|right|Tephra horizons in south-central [[Iceland]]. The thick and light coloured layer at center of the photo is [[rhyolitic]] tephra from [[Hekla]]....
ejected in explosive eruptions. In comparison, shield volcanoes are built of relatively weakly viscous 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...
ic lavas that erupts in longer cycles than that of a stratovolcano. Shield volcanoes are distinctive products of 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...
volcanism, but can form at rift
Rift
In geology, a rift or chasm is a place where the Earth's crust and lithosphere are being pulled apart and is an example of extensional tectonics....
and subduction zones as well.
The types of eruptions that occur at shield volcanoes have been named Hawaiian eruption
Hawaiian eruption
A Hawaiian eruption is a type of volcanic eruption where lava flows from the vent in a relative gentle, low level eruption, so called because it is characteristic of Hawaiian volcanoes. Typically they are effusive eruptions, with basaltic magmas of low viscosity, low content of gases, and high...
s, after the Hawaiian chain
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....
in which they are most prominent. Hawaiian eruptions are characterized by the effusive
Effusive eruption
An effusive eruption is a volcanic eruption characterized by the outpouring of lava onto the ground...
emission of fluid lavas. The mobile nature of these lavas allows shield volcano flows to travel a longer distance than those of other volcanic types, resulting in larger and thinner sheets of lava, often just 1 m (3 ft) thick. Over long periods of time, the gradual buildup of thousands of these flows slowly constructs the characteristically low, broad profile of a mature shield volcano.
Because of their gradual buildup and near-continuous eruptive characteristics, shield volcanoes vary widely in size with their age. Mature shield volcanoes are the largest volcanoes on Earth.This excludes flood basalt
Flood basalt
A flood basalt or trap basalt is the result of a giant volcanic eruption or series of eruptions that coats large stretches of land or the ocean floor with basalt lava. Flood basalts have occurred on continental scales in prehistory, creating great plateaus and mountain ranges...
s, large sheets of volcanic material that do not actually resemble volcanoes. Shield volcanoes often measure 3 to 4 mi (4.8 to 6.4 km) in diameter and surpass 1500 to 2000 ft (457.2 to 609.6 m) in height. The largest shield volcano (and the largest active volcano) in the world is 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...
in Hawaii
Hawaii (island)
The Island of Hawaii, also called the Big Island or Hawaii Island , is a volcanic island in the North Pacific Ocean...
, which projects 13677 ft (4,169 m) above sea level
Sea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...
, and is over 60 mi (97 km) wide. The volcano is estimated to contain 80000 km³ (19,193 cu mi) of basalt, a mass so great that it has slumped into the Earth's crust. Their lower slopes are generally gentle (~2 degrees), but steepen with elevation (reaching ~10 degrees) before flattening near the summit, giving the volcanoes a convex shape. The height of a shield volcano is typically 1/20th of its width.
The Hawaiian shield volcanoes and the Galápagos islands are unique in that they are not located near any plate boundaries
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
; instead, the two chains are fed by the movement of oceanic plates over an upwelling of magma known as 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...
. Over millions of years, the tectonic movement that moves continents also creates long volcanic trails across the seafloor. The Hawaiian and Galápagos shields, and other hotspot shields like them, are both constructed of oceanic island basalt. Their lavas are characterized by high levels of sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
, potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
, and aluminum.
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 are a prevalent feature on shield volcanoes that is rare on other volcanic types. The large, decentralized shape of Hawaiian volcanoes as compared to their smaller, symmetrical Icelandian cousins can be attributed to rift eruptions. Fissure venting is common in Hawaii; most Hawaiian eruptions begin with a so-called "wall of fire" along a major fissure line before centralizing to a small number of points. This accounts for their asymmetrical shape, whereas Icelandian volcanoes follow a pattern of central eruptions dominated by summit calderas
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...
, causing much evener lava.
Another hallmark of shield volcanism are lava tube
Lava tube
Lava tubes are natural conduits through which lava travels beneath the surface of a lava flow, expelled by a volcano during an eruption. They can be actively draining lava from a source, or can be extinct, meaning the lava flow has ceased and the rock has cooled and left a long, cave-like...
s. Lava tubes are cave-like volcanic straights formed by the hardening of overlaying lava. These structures help further the propagation of lava, as the walls of the tube insulate
Thermal insulation
Thermal insulation is the reduction of the effects of the various processes of heat transfer between objects in thermal contact or in range of radiative influence. Heat transfer is the transfer of thermal energy between objects of differing temperature...
s the lava within. Lava tubes can account for a large chunk of shield volcano activity; for example, an estimated 58% of the lava forming Kilauea comes from lava tubes.
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...
. The cone has been erupting continuously since 1983 in one of the longest-lasting rift eruptions in history, and was built up to its present height of 2290 ft (698 m) by over 25 years of activity.
In some shield volcano eruptions, 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...
ic lava pours out of a long fissure
Fissure vent
A fissure vent, also known as a volcanic fissure or simply fissure, is a linear volcanic vent through which lava erupts, usually without any explosive activity. The vent is usually a few meters wide and may be many kilometers long. Fissure vents can cause large flood basalts and lava channels...
instead of a central vent, and shrouds the countryside with a long band of volcanic material in the form of a broud plateau
Plateau
In geology and earth science, a plateau , also called a high plain or tableland, is an area of highland, usually consisting of relatively flat terrain. A highly eroded plateau is called a dissected plateau...
. Plateaus of this type exist in Iceland
Iceland
Iceland , described as the Republic of Iceland, is a Nordic and European island country in the North Atlantic Ocean, on the Mid-Atlantic Ridge. Iceland also refers to the main island of the country, which contains almost all the population and almost all the land area. The country has a population...
, Washington, Oregon
Oregon
Oregon is a state in the Pacific Northwest region of the United States. It is located on the Pacific coast, with Washington to the north, California to the south, Nevada on the southeast and Idaho to the east. The Columbia and Snake rivers delineate much of Oregon's northern and eastern...
, and Idaho
Idaho
Idaho is a state in the Rocky Mountain area of the United States. The state's largest city and capital is Boise. Residents are called "Idahoans". Idaho was admitted to the Union on July 3, 1890, as the 43rd state....
; the most prominent ones are situated along the Snake River
Snake River
The Snake is a major river of the greater Pacific Northwest in the United States. At long, it is the largest tributary of the Columbia River, the largest North American river that empties into the Pacific Ocean...
in Idaho and the Columbia River
Columbia River
The Columbia River is the largest river in the Pacific Northwest region of North America. The river rises in the Rocky Mountains of British Columbia, Canada, flows northwest and then south into the U.S. state of Washington, then turns west to form most of the border between Washington and the state...
in Washington and Oregon, where they have been measured to be over a 1 mi (2 km) in thickness. Many eruptions start as a so-called "curtain of fire"—a long eruptive chain along a fissure vent
Fissure vent
A fissure vent, also known as a volcanic fissure or simply fissure, is a linear volcanic vent through which lava erupts, usually without any explosive activity. The vent is usually a few meters wide and may be many kilometers long. Fissure vents can cause large flood basalts and lava channels...
on the volcano. Eventually these eruptions die down and start to focus around a few points on the fissure, where activity is concentrated.
Calderas are a common feature on shield volcanoes. They are formed and reformed over the volcano's lifespan. Long eruptive periods form cinder cones, which then collapse over time to form calderas. The calderas are often filled up by future eruptions, or formed elsewhere, and this cycle of collapse and regeneration takes place throughout the volcano's lifespan.
Interactions between water and lava at shield volcanoes can cause some eruptions to become hydrovolcanic. These explosive
Explosive eruption
An explosive eruption is a volcanic term to describe a violent, explosive type of eruption. Mount St. Helens in 1980 was an example. Such an eruption is driven by gas accumulating under great pressure. Driven by hot rising magma, it interacts with ground water until the pressure increases to the...
eruptions are drastically different from the usual shield volcanic activity, and are especially prevalent at the waterbound volcanoes of the Hawaiian Isle
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...
s.
Distribution Around the World
Shield volcanoes are found worldwide. They can form over hotspotHotspot (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...
s (points where 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...
from below the surface wells up), such as 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...
and the Galápagos Islands
Galápagos Islands
The Galápagos Islands are an archipelago of volcanic islands distributed around the equator in the Pacific Ocean, west of continental Ecuador, of which they are a part.The Galápagos Islands and its surrounding waters form an Ecuadorian province, a national park, and a...
, or over more conventional 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, such as the Icelandic shields and the shield volcanoes of the shield volcanoes of East Africa
East Africa
East Africa or Eastern Africa is the easterly region of the African continent, variably defined by geography or geopolitics. In the UN scheme of geographic regions, 19 territories constitute Eastern Africa:...
. Many shield volcanoes are found in ocean basins, although they can be found inland as well—East Africa being one example of this.
Hawaiian islands
The largest and most prominent shield volcano chain in the world are the Hawaiian IslandsHawaiian Islands
The Hawaiian Islands are an archipelago of eight major islands, several atolls, numerous smaller islets, and undersea seamounts in the North Pacific Ocean, extending some 1,500 miles from the island of Hawaii in the south to northernmost Kure Atoll...
, a 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...
of hotspot volcanoes
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....
in Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
. The Hawaiian volcanoes are characterized by frequent rift eruption
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, their large size (thousands of km3 in volume), and their rough, decentralized shape. 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 are a prominent feature on these volcanoes, and account for their seemingly random volcanic structure. They are fueled by the movement 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....
over 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....
, and form a long chain of volcanoes, atoll
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...
s, and 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 2600 km (1,616 mi) long with a total volume of over 750000 km³ (179,935 cu mi). The chain contains at least 43 major volcanoes, and Meiji Seamount
Meiji Seamount
Meiji Seamount is the oldest seamount in the Hawaiian-Emperor seamount chain, with an estimated age of 82 million years. It lies at the northernmost end of the chain, and is perched at the outer slope of the Kuril-Kamchatka Trench...
at its terminus near the Kuril-Kamchatka Trench
Kuril-Kamchatka Trench
The Kuril–Kamchatka Trench or Kuril Trench is an oceanic trench in the northwest Pacific Ocean. It lies off the southeast coast of Kamchatka and parallels the Kuril Island chain to meet the Japan Trench east of Hokkaido...
is 85 million years old. The volcanoes follow a distinct evolutionary pattern
Evolution of Hawaiian volcanoes
The fifteen volcanoes that make up the eight principal islands of Hawaii are the youngest in a chain of more than 129 volcanoes that stretch across the North Pacific Ocean, called the Hawaiian-Emperor seamount chain....
of growth and death.
The chain includes the largest volcano on Earth, 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...
, which stands 4170 m (13,681.1 ft) above sea level
Sea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...
and reaches a further 13 km (8 mi) below the waterline and into the crust, approximately 80000 km³ (19,193 cu mi) of rock. Kilauea
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...
, meanwhile, is one of the most active volcanoes on Earth, with the current ongoing eruption having begun in January 1983.
Galápagos islands
The Galápagos IslandsGalápagos Islands
The Galápagos Islands are an archipelago of volcanic islands distributed around the equator in the Pacific Ocean, west of continental Ecuador, of which they are a part.The Galápagos Islands and its surrounding waters form an Ecuadorian province, a national park, and a...
are an isolated set of volcanoes, consisting of shield volcanoes and lava plateaus, located 1200 km (746 mi) west of Ecuador
Ecuador
Ecuador , officially the Republic of Ecuador is a representative democratic republic in South America, bordered by Colombia on the north, Peru on the east and south, and by the Pacific Ocean to the west. It is one of only two countries in South America, along with Chile, that do not have a border...
. They are driven by the Galápagos hotspot
Galápagos hotspot
The Galápagos hotspot is a volcanic hotspot in the East Pacific Ocean responsible for the creation of the Galapagos Islands as well as three major aseismic ridge systems, Carnegie, Cocos and Malpelso which are on two tectonic plates. The hotspot is located near the Equator on the Nazca Plate not...
, and are between approximately 4.2 million and 700,000 years of age. The largest island, Isabela Island, consists of six coalesced shield volcanoes, each delineated by a large summit caldera. Española
Española Island
Española Island is part of the Galápagos Islands. The English named it Hood Island after Viscount Samuel Hood. It is located in the extreme southeast of the archipelago and is considered, along with Santa Fe, one of the oldest, at approximately four million years...
, the oldest island, and Fernandina
Fernandina Island
Fernandina Island is the third largest, and youngest, island of the Galápagos Islands. Like the others, the island was formed by the Galápagos hotspot...
, the youngest, are also shield volcanoes, as are most of the other islands in the chain. The Galápagos Islands are perched on a large lava plateau known as the Galápagos Platform. This platform creates a shallow water depth of 360 to 900 m (1,181.1 to 2,952.8 ft) at the base of the islands, which stretch over a 174 mi (280 km)-long diameter. Since Charles Darwin
Charles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...
's famous visit
Second voyage of HMS Beagle
The second voyage of HMS Beagle, from 27 December 1831 to 2 October 1836, was the second survey expedition of HMS Beagle, under captain Robert FitzRoy who had taken over command of the ship on its first voyage after her previous captain committed suicide...
to the islands in 1835, there have been over 60 recorded eruptions in the islands, from six different shield volcanoes. Of the 21 emergent volcanoes, 13 are considered active.
The Galápagos islands are geologically young for such a large chain, and the pattern of their 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 follows one of two trends, one north-northwest, and one east-west. The composition of the lavas of the Galápagos shields are strikingly similar to those of the Hawaiian volcanoes. Curiously, they do not form the same volcanic "line" associated with most hotspots. They are not alone in this regard; the Cobb-Eickelberg Seamount chain
Cobb-Eickelberg Seamount chain
The Cobb-Eickelberg Seamount chain is a seamount chain stretching from the Aleutian Trench in the north to Axial Seamount, the youngest volcano in the chain, which lies approximately 300 miles west of Cannon Beach, Oregon. The chain was created by the Cobb hotspot as the Pacific Plate drifted in a...
in the North Pacific is another example of such a delineated chain. In addition, there is no clear pattern of age between the volcanoes, suggesting a complicated, irregular pattern of creation. How exactly the islands were formed remains a geological mystery, although several theories have been fronted.
Iceland
Iceland
Iceland , described as the Republic of Iceland, is a Nordic and European island country in the North Atlantic Ocean, on the Mid-Atlantic Ridge. Iceland also refers to the main island of the country, which contains almost all the population and almost all the land area. The country has a population...
. Located over the Mid-Atlantic Ridge
Mid-Atlantic Ridge
The Mid-Atlantic Ridge is a mid-ocean ridge, a divergent tectonic plate boundary located along the floor of the Atlantic Ocean, and part of the longest mountain range in the world. It separates the Eurasian Plate and North American Plate in the North Atlantic, and the African Plate from the South...
, a divergent tectonic plate
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
in the middle of the Atlantic Ocean
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
, Iceland is the site of about 130 volcanoes of various types. Icelandic shield volcanoes are generally of Holocene
Holocene
The Holocene is a geological epoch which began at the end of the Pleistocene and continues to the present. The Holocene is part of the Quaternary period. Its name comes from the Greek words and , meaning "entirely recent"...
age, between 5000 and 10000 years old, except for the island of Surtsey
Surtsey
Surtsey is a volcanic island off the southern coast of Iceland. At it is also the southernmost point of Iceland. It was formed in a volcanic eruption which began 130 metres below sea level, and reached the surface on 15 November 1963. The eruption lasted until 5 June 1967, when the island...
, a Surtseyan shield
Surtseyan eruption
A Surtseyan eruption is a type of volcanic eruption that takes place in shallow seas or lakes. It is named after the island of Surtsey off the southern coast of Iceland....
. The volcanoes are also very narrow in distribution, occurring in two bands in the West and North Volcanic Zones. Like Hawaiian volcanoes, their formation initially begins with several eruptive centers before centralizing and concentrating at a single point. The main shield then forms, burying the smaller ones formed by the early eruptions with its lava.
Icelandic shields are generally small (~15 km³ (4 cu mi)), symmetrical (although this can affected by surface topography), and characterized by eruptions from summit calderas
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...
. They are composed of either tholeiitic olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
or picritic basalt
Picrite basalt
Picrite basalt, picrobasalt or oceanite is a variety of high-magnesium olivine basalt that is very rich in the mineral olivine. It is dark with yellow-green olivine phenocrysts and black to dark brown pyroxene, mostly augite....
. The tholeiitic shields tend to be wider and shallower than the picritic shields. They do not follow the pattern of caldera growth and destruction that other shield volcanoes do; caldera may form, but they generally do not disappear. None of the shield volcanoes in the region are known to have erupted within the last 5000 years.
East Africa
East Africa is the site of volcanic activity generated by the development of the East African RiftEast African Rift
The East African Rift is an active continental rift zone in eastern Africa that appears to be a developing divergent tectonic plate boundary. It is part of the larger Great Rift Valley. The rift is a narrow zone in which the African Plate is in the process of splitting into two new tectonic plates...
, a developing plate boundary in Africa, and from nearby hotspots. Some volcanoes interact with both. Shield volcanoes are found near the rift and off the coast of Africa, although stratovolcanoes are more common. Although sparsely studied, the fact that all of its volcanoes are of Holocene age reflects how young the volcanic center is. One interesting characteristic of East African volcanism is a penchant for the formation of lava lake
Lava lake
Lava lakes are large volumes of molten lava, usually basaltic, contained in a volcanic vent, crater, or broad depression. The term is used to describe both lava lakes that are wholly or partly molten and those that are solidified...
s; these semi-permanent lava bodies, extremely rare elsewhere, form in about 9% of African eruptions.
The most active shield volcano in Africa is Nyamuragira. Eruptions at the shield volcano are generally centered within the large summit caldera or on the numerous fissures and cinder cones on the volcano's flanks. Lava flows from the most recent century extend down the flanks more than 30 km (19 mi) from the summit, reaching as far as Lake Kivu
Lake Kivu
Lake Kivu is one of the African Great Lakes. It lies on the border between the Democratic Republic of the Congo and Rwanda, and is in the Albertine Rift, a part of the Great Rift Valley. Lake Kivu empties into the Ruzizi River, which flows southwards into Lake Tanganyika...
. Erta Ale
Erta Ale
Erta Ale is a continuously active basaltic shield volcano in the Afar Region of northeastern Ethiopia, the most active volcano in Ethiopia. It is in the Afar Depression, a badlands desert area spanning the border with Eritrea, and the volcano itself is surrounded completely by an area below sea...
in Ethiopia
Ethiopia
Ethiopia , officially known as the Federal Democratic Republic of Ethiopia, is a country located in the Horn of Africa. It is the second-most populous nation in Africa, with over 82 million inhabitants, and the tenth-largest by area, occupying 1,100,000 km2...
is another active shield volcano, and one of the few places in the world with a permanent lava lake, which has been active since at least 1967, and possibly since 1906. Other volcanic centers include Menengai
Menengai
Menengai Crater is a massive shield volcano inside one of the biggest calderas in the world, located in the Great Rift Valley, Kenya. Farmland occupies its flanks.Menengai is located north of Nakuru, the third-biggest city in Kenya....
, a massive shield caldera, and Mount Marsabit
Mount Marsabit
Marsabit is a 6300 km² basaltic shield volcano in Kenya, located 170 km east of the center of the East African Rift, in Marsabit District near the town of Marsabit. This was primarily built during the Miocene, but some lava flows and explosive maar-forming eruptions have occurred more...
, near the town of Marsabit
Marsabit
Marsabit is a town in northern Kenya, located 170 km east of the center of the East African Rift at 37°58' E, 2°19' N . It is located in the Eastern Province and is almost surrounded by the Marsabit National Park and Reserve...
.
Extraterrestrial volcanoes
Volcanoes are not limited to Earth; they can exist on any rocky planet or moonMoon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
large or active enough to have a molten
Mölten
Mölten is a comune in South Tyrol in the Italian region Trentino-Alto Adige/Südtirol, located about 60 km north of Trento and about 12 km northwest of Bolzano .-Geography:...
inner core
Inner core
The inner core of the Earth, its innermost hottest part as detected by seismological studies, is a primarily solid ball about in radius, or about 70% that of the Moon...
, and since probes were first launched in the 1960s, volcanoes have been found across the solar system. Shield volcanoes and volcanic vents have been found on Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
, Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
, and Io
Io (moon)
Io ) is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus....
; cryovolcano
Cryovolcano
A cryovolcano is a volcano that erupts volatiles such as water, ammonia or methane, instead of molten rock. Collectively referred to as cryomagma or ice-volcanic melt, these substances are usually liquids and form plumes, but can also be in vapour form...
es on Triton
Triton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...
; and subsurface hotspots on Europa
Europa (moon)
Europa Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This surface is striated by cracks and...
.
The volcanoes of Mars
Volcanism on Mars
Volcanic activity, or volcanism, has played a significant role in the geologic evolution of Mars. Scientists have known since the Mariner 9 mission in 1972 that volcanic features cover large portions of the Martian surface. These features include extensive lava flows, vast volcanic plains, and the...
are very similar to the shield volcanoes on Earth. Both have gently sloping flanks, collapse craters along their central structure, and are built of highly fluid lavas. Volcanic features on Mars were observed long before they were first studied in detail during the 1976-1979 Viking mission
Viking program
The Viking program consisted of a pair of American space probes sent to Mars, Viking 1 and Viking 2. Each spacecraft was composed of two main parts, an orbiter designed to photograph the surface of Mars from orbit, and a lander designed to study the planet from the surface...
. The principal difference between the volcanoes of Mars and those on Earth is in terms of size; Martian volcanoes are over 17 mi (27 km) high and 350 mi (563 km) in diameter, far larger than the 6 mi (10 km) high, 74 mi (119 km) wide Hawaiian shields.
The most famous example is Olympus Mons
Olympus Mons
Olympus Mons is a large volcanic mountain on the planet Mars. At a height of almost , it is one of the tallest mountains in the Solar System, three times as tall as Mount Everest and more than twice the height of Mauna Kea the tallest mountain on Earth. Olympus Mons is the youngest of the large...
, a shield volcano that is the highest known mountain in the solar system.
Venus
Volcanism on Venus
The surface of Venus is dominated by volcanism and has produced more volcanoes than any other planet in the solar system. It has a surface that is 90% basalt, and about 80% of the planet consists of a mosaic of volcanic lava plains, indicating that volcanism played a major role in shaping its surface...
also has over 150 shield volcanoes which are much flatter, with a larger surface area than those found on Earth, some having a diameter of more than 700 km (435 mi). Although the majority of these are long extinct it has been suggested, from observations by the Venus Express spacecraft
Venus Express
Venus Express is the first Venus exploration mission of the European Space Agency. Launched in November 2005, it arrived at Venus in April 2006 and has been continuously sending back science data from its polar orbit around Venus. Equipped with seven science instruments, the main objective of the...
that many may still be active.
Dangers
The Hawaiian eruptionHawaiian eruption
A Hawaiian eruption is a type of volcanic eruption where lava flows from the vent in a relative gentle, low level eruption, so called because it is characteristic of Hawaiian volcanoes. Typically they are effusive eruptions, with basaltic magmas of low viscosity, low content of gases, and high...
s of shield volcanoes do not pose much threat to humans, as they emit large amounts of slow moving lava over long periods of time. However, they are hazardous to agriculture
Agriculture
Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...
and infrastructure
Infrastructure
Infrastructure is basic physical and organizational structures needed for the operation of a society or enterprise, or the services and facilities necessary for an economy to function...
; the ongoing 1983 eruption of 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...
has destroyed over 200 structures and buried kilometers of highways.
Pyroclastic shields
Rarer pyroclastic shield volcanoes are similar to normal mafic shields in shape. But rather than being formed entirely by basaltBasalt
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...
lavas, pyroclastic shields are mainly formed from explosive eruptions of ignimbrite
Ignimbrite
An ignimbrite is the deposit of a pyroclastic density current, or pyroclastic flow, a hot suspension of particles and gases that flows rapidly from a volcano, driven by a greater density than the surrounding atmosphere....
.