Shield volcanoes
Shield volcanoes are a type of volcano built almost entirely of fluid lava flows. They are so
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.
Etymology
Shield volcanoes are built up by effusive eruptions, which flow out in all directions to create a
shield like that of a warrior.[1] 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.[2]
Geology
Diagram of a shield volcano, showing the multiple
layers of lava flows in a mature shield volcano
Shield volcanoes are one of the three major types of volcanoes, distinguished from the two other
major volcanic types, stratovolcanoes and cinder cones, 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 basaltic lavas that erupts in longer cycles than
that of a stratovolcano.[3] Shield volcanoes are distinctive products of hotspot volcanism, but can
form at rift and subduction zones as well.[4]
The types of eruptions that occur at shield volcanoes have been named Hawaiian eruptions, after
the Hawaiian chain in which they are most prominent. Hawaiian eruptions are characterized by
the effusive emission of fluid lavas.[5] The mobile nature of these lavas allows shield volcano
flows to travel a longer distance then those of other volcanic types, resulting in a larger and
thinner sheets of lava,[1] often just 1 m (3 ft) thick.[3] 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.[1]
Because of their gradual buildup and near-continuous eruptive characteristics, shield volcanoes
vary widely in size with their age.[3] Mature shield volcanoes are the largest volcanoes on
Earth.[4][n 1] Shield volcanoes often measure 3 to 4 mi (5 to 6 km) in diameter and surpass 1,500
to 2,000 ft (460 to 610 m) in height. The largest shield volcano (and the largest active volcano) in
the world is Mauna Loa in Hawaiʻi, which projects 13,677 ft (4,169 m) above sea level,[1] and is
over 60 mi (97 km) wide.[3] The volcano is estimated to contain 80,000 km3 (19,000 cu mi) of
basalt,[6] a mass is so great that it has slumped into the Earth's crust.[7] 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.[3] The height of a shield volcano is
typically 1/20th of its width.[3]
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.[8]
Rift zones 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 Hawaiʻi;
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.[3][1][7][5]
Another hallmark of shield volcanism are lava tubes.[6] 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 insulates the lava within.[9] 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.[6]
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.[5][4] A prominent example of a shield volcano-bound cinder cone is is Puʻu ʻŌʻō on
Kīlauea.[7] 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 2,290 ft (698 m) by over 25 years of
activity.[10]
In some shield volcano eruptions, basaltic 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.[1] 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.[5]
Calderas 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.[4]
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,[4] and are especially prevalent at the waterbound volcanoes of the Hawaiian Isles.[5]
Distribution
Main article: List of shield volcanoes
Clickable image of the sites of active and dormant shield volcanoes around the world.
Shield volcanoes are found worldwide. They can form over hotspots (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 zones, 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.[11]
Hawaiian islands
Mauna Kea, Hawaiʻi, the largest volcano on Earth.
The largest and most prominent shield volcano chain in the world are the Hawaiian Islands, a
chain of hotspot volcanoes in Pacific Ocean. The Hawaiian volcanoes are characterized by
frequent rift eruptions, their large size (thousands of km3 in volume), and their rough,
decentralized shape. Rift zones are a prominent feature on these volcanoes, and account for their
seemingly random volcanic structure.[3] Theu are fueled by the movement of the Pacific Plate
over the Hawaii hotspot, and form a long chain of volcanoes, atolls, and seamounts 2,600 km
(1,616 mi) long with a total volume of over 750,000 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.[12][13] The volcanoes follow a distinct evolutionary pattern of growth and
death.[14]
The chain includes the largest volcano on Earth, Mauna Loa, which stands 4,170 m (13,680 ft)
above sea level and reaches a further 13 km (8 mi) below the waterline and into the crust,
approximately 80,000 km3 (19,000 cu mi) of rock.[6] Kilauea, meanwhile, is one of the most
active volcanoes on Earth, with the current ongoing eruption having begun in January 1983.[1]
Galápagos islands
The Galápagos Islands are an isolated set of volcanoes, consisting of shield volcanoes and lava
plateaus, located 1,200 km (746 mi) east of Ecuador. They are driven by the Galápagos hotspot,
and are between approximately 4.2 million and 700,000 years of age.[8] 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.[15][16][17] 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 to 2,953 ft) at the base of the islands, which stretch over a 174 mi (280 km)long diameter.[18] 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.[15][17] Of the 21
emergent volcanoes, 13 are considered active.[8]
The Galápagos islands are geologically young for such a large chain, and the pattern of their rift
zones 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.[19]
Iceland
Skjaldbreiður, Iceland, is eponymous for shield volcanoes.
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.[7] 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.[18]
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.[18] They
are composed of either tholeiitic olivine or picritic basalt. The tholeiitic shields tend to be wider
and shallower then the picritic shields.[20] 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.[18][3] None of the shield volcanoes in the region are known to have erupted within the
last 5000 years.[21]
East Africa
East Africa is the site of volcanic activity generated by the development of the East African Rift,
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 lakes; these semi-permanent lava
bodies, extremely rare elsewhere, form in about 9% of African eruptions.[22]
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.[22] Other volcanic centers
include Menengai, a massive shield caldera.[23]
Extraterrestrial
Shield volcanoes are not only limited to Earth, but have been found on Mars. Though Mars'
shields are much more massive than those of Earth, reaching more than 17 miles (27 km) in
height and 350 miles (563 km) in diameter, they bear a resemblance to the volcanoes of the
Hawaiian Islands because of their shape and features.[24] The most famous example is Olympus
Mons, a shield volcano that is the highest known mountain in the solar system.[6]
Dangers
The Hawaiian eruptions 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.
Pyroclastic shields
Main article: Pyroclastic shield
Rarer pyroclastic shield volcanoes are similar to normal mafic shields in shape. But rather than
being formed entirely by basalt lavas, pyroclastic shields are mainly formed from explosive
eruptions of ignimbrite