A shield volcano is a type of volcano usually built almost entirely of
fluid lava flows. They are named for their low profile, resembling a
warrior's shield lying on the ground. This is caused by the highly
fluid (low viscosity) lava they erupt which travels farther than lava
erupted from stratovolcanoes. This results in the steady accumulation
of broad sheets of lava, building up the shield volcano's distinctive
form. The shape of shield volcanoes is due to the low viscosity of
their mafic lava.
2.2 Eruptive characteristics
3.1 Hawaiian islands
3.2 Galápagos islands
3.4 East Africa
3.5 Extraterrestrial volcanoes
4 See Also
6 External links
Shield volcanoes are built by effusive eruptions, 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
*skelduz, and related to the Gothic skildus, meaning "to divide,
split, or separate".
Shield volcano itself is taken from the German
Diagram of the common structural features of a shield volcano.
Shield volcanoes are distinguished from the three other major volcanic
archetypes—stratovolcanoes, lava domes, and cinder cones—by their
structural form, a consequence of their unique magmatic composition.
Of these four forms shield volcanoes erupt the least viscous lavas:
where stratovolcanoes and especially lava domes are the product of
highly immotile flows and cinder cones are constructed by explosively
eruptive tephra, shield volcanoes are the product of gentle effusive
eruptions of highly fluid lavas that produce, over time, a broad,
gently sloped eponymous "shield". Although the term is generally
ascribed to basaltic shields it has also at times been appended to
rarer scutiform volcanoes of differing magmatic
composition—principally pyroclastic shields, formed by the
accumulation of fragmental material from particularly powerful
explosive eruptions, and rarer felsic lava shields formed by unusually
fluid felsic magmas. Examples of pyroclastic shields include Billy
Mitchell volcano in
Papua New Guinea
Papua New Guinea and the
Purico complex in
Chile; an example of a felsic shield is the Big
Obsidian Flow in
Shield volcanoes are also related in origination to vast
lava plateaus and flood basalts present in various parts of the world,
generalized eruptive features which occur along linear fissure vents
and are distinguished from shield volcanoes proper by the lack of an
identifiable primary eruptive center.
Active shield volcanoes experience near-continuous eruptive activity
over extremely long periods of time, resulting in the gradual build-up
of edifices that can reach extremely large dimensions. With the
exclusion of flood basalts mature shields are the largest volcanic
features on Earth: the summit of the largest subaerial volcano in
the world, Mauna Loa, lies 4,169 m (13,678 ft) above sea
level, and the volcano, over 60 mi (100 km) wide at its
base, is estimated to contain about 80,000 km3
(19,000 cu mi) of basalt. The mass of the volcano is
so great that it has slumped the crust beneath it a further 8 km
(5 mi); accounting for this subsidence and for the height of
the volcano above the sea floor the "true" height of
Mauna Loa from
the start of its eruptive history is about 17,170 m
(56,000 ft). Mount Everest, by comparison, is 8,848 m
(29,029 ft) in height. In September 2013 a team led by the
University of Houston's
William Sager announced the singular
origination of Tamu Massif, an enormous extinct submarine shield
volcano of previously unknown origin which, approximately 450 by
650 km (280 by 400 mi) in area, dwarfs all previously known
volcanoes on the planet. The research has not yet been confirmed.
Shield volcanoes feature a gentle (usually 2° to 3°) slope that
gradually steepens with elevation (reaching approximately 10°) before
eventually flattening near the summit, forming an overall upwardly
convex shape. In height they are typically about one twentieth their
width. Although the general form of a "typical" shield volcano
varies little worldwide regional differences exist in their size and
morphological characteristics. Typical shield volcanoes present in
Oregon measure 3 to 4 mi (5 to 6 km) in
diameter and 1,500 to 2,000 ft (500 to 600 m) in height;
shield volcanoes in the central Mexican Michoacán–Guanajuato
volcanic field, by comparison, average 340 m (1,100 ft) in
height and 4,100 m (13,500 ft) in width, with an average
slope angle of 9.4° and an average volume of 1.7 km3
(0.4 cu mi).
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 Icelandic
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 Icelandic volcanoes follow a pattern of
central eruptions dominated by summit calderas, causing the lava to be
more evenly distributed or symmetrical.
Diagram of a Hawaiian eruption. (key: 1.
Ash plume 2.
Lava fountain 3.
Lava lake 5. Fumaroles 6.
Lava flow 7. Layers of lava and
Stratum 9. Sill 10.
Magma conduit 11.
Magma chamber 12. Dike)
Click for larger version.
Most of what is currently known about shield volcanic eruptive
character has been gleaned from studies done on the volcanoes of
Hawaiʻi island, by far the most intensively studied of all shields
due to their scientific accessibility; the island lends its name
to the slow-moving, effusive eruptions typical of shield volcanism,
known as Hawaiian eruptions. These eruptions, the calmest of
volcanic events, are characterized by the effusive emission of highly
fluid basaltic lavas with low gaseous content. These lavas travel a
far greater distance than those of other eruptive types before
solidifying, forming extremely wide but relatively thin magmatic
sheets often less than 1 m (3 ft) thick. Low
volumes of such lavas layered over long periods of time are what
slowly constructs the characteristically low, broad profile of a
mature shield volcano.
Also unlike other eruptive types, Hawaiian eruptions often occur at
decentralized fissure vents, beginning with large "curtains of fire"
that quickly die down and concentrate at specific locations on the
volcano's rift zones. Central-vent eruptions, meanwhile, often take
the form of large lava fountains (both continuous and sporadic), which
can reach heights of hundreds of meters or more. The particles from
lava fountains usually cool in the air before hitting the ground,
resulting in the accumulation of cindery scoria fragments; however,
when the air is especially thick with clasts, they cannot cool off
fast enough due to the surrounding heat, and hit the ground still hot,
accumulating into spatter cones. If eruptive rates are high enough,
they may even form splatter-fed lava flows. Hawaiian eruptions are
often extremely long lived; Puʻu ʻŌʻō, a cinder cone of Kīlauea,
has been erupting continuously since 1983.
Flows from Hawaiian eruptions can be divided into two types by their
structural characteristics: pāhoehoe lava which is relatively smooth
and flows with a ropey texture, and ʻaʻa flows which are denser,
more viscous (and thus slower moving) and blockier (see lava
§ Pāhoehoe and lava § ʻAʻā). These lava flows can be
anywhere between 2 and 20 m (10 and 70 ft) thick. ʻAʻa
lava flows move through pressure—the partially solidified front of
the flow steepens due to the mass of flowing lava behind it until it
breaks off, after which the general mass behind it moves forward.
Though the top of the flow quickly cools down, the molten underbelly
of the flow is buffered by the solidifying rock above it, and by this
mechanism ʻaʻa flows can sustain movement for long periods of time.
Pāhoehoe flows, in contrast, move in more conventional sheets, or by
the advancement of lava "toes" in snaking lava columns. Increasing
viscosity on the part of the lava or shear stress on the part of local
topography can morph a pāhoehoe flow into an a'a one, but the reverse
Although most shield volcanoes are by volume almost entirely Hawaiian
and basaltic in origin, they are rarely exclusively so. Some
Mount Wrangell in Alaska and
Cofre de Perote
Cofre de Perote in
Mexico, exhibit large enough swings in their historical magmatic
eruptive characteristics to cast strict categorical assignment in
doubt; one geological study of de Perote went so far as to suggest the
term "compound shield-like volcano" instead. Most mature shield
volcanoes have multiple cinder cones on their flanks, the results of
tephra ejections common during incessant activity and markers of
currently and formerly active sites on the volcano. One
prominent such parasitic cones is
Puʻu ʻŌʻō on
Kīlauea—continuous activity ongoing since 1983 has built up a
2,290 ft (698 m) tall cone at the site of one of the
longest-lasting rift eruptions in known history.
The Hawaiian shield volcanoes and the Galápagos islands are
unique[clarification needed] 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 aluminium.
Features common in shield volcanism include 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.
Lava tubes can
account for a large portion of shield volcano activity; for example,
an estimated 58% of the lava forming
Kīlauea comes from lava
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 broad plateau.
Plateaus of this type exist in Iceland, Washington, Oregon, and Idaho;
the most prominent ones are situated along the
Snake River in Idaho
Columbia River in Washington and Oregon, where they have been
measured to be over 1 mi (2 km) in thickness.
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
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
ʻAʻa advances over solidified pāhoehoe on Kīlauea, Hawaiʻi.
A pāhoehoe lava fountain on
A lava lake in the caldera of Erta Ale, an active shield volcano in
Pāhoehoe flows enter the Pacific Ocean on Hawaiʻi island.
Puʻu ʻŌʻō, a parasitic cinder cone on Kīlauea, lava fountaining
at dusk in June 1983, near the start of its current eruptive cycle.
The Thurston lava tube on Hawaiʻi island, now a tourist attraction in
the Hawaiʻi Volcanoes National Park
Main article: List of shield volcanoes
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
Hawaiian–Emperor seamount chain and the Galápagos Islands, or over
more conventional rift zones, such as the Icelandic shields and the
shield volcanoes of East Africa. Many shield volcanoes are found in
ocean basins, such as Tamu Massif, the world's largest, although they
can be found inland as well—East Africa being one example of
The largest and most prominent shield volcano chain in the world is
the Hawaiian Islands, a chain of hotspot volcanoes in the 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. They are fueled by the movement of the Pacific Plate
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
Kuril–Kamchatka Trench is 85 million years old. The
volcanoes follow a distinct evolutionary pattern of growth and
The chain includes the second 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. Kīlauea, meanwhile, is one of the most active volcanoes on
Earth, with the current ongoing eruption having begun in January
Galápagos Islands are an isolated set of volcanoes, consisting of
shield volcanoes and lava plateaus, located 1,200 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
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. 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.
Blue Hill is a shield volcano on the south western part of Isabela
Island in the
Galápagos Islands and is one of the most active in the
Galapagos, with the last eruption between May and June 2008. The
Geophysics Institute at the National Polytechnic School in Quito
houses an international team of seismologists and volcanologists
whose responsibility is to monitor Ecuadors numerous active volcanoes
in the Andean Volcanic Belt and the Galapagos Islands. La Cumbre is an
active shield volcano on
Fernandina Island in the Galapagos that has
been erupting since April 11, 2009.
The Galápagos islands are geologically young for such a big 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
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
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. 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 mostly 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
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
nine percent 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
Lava flows from the most recent century extend down the flanks
more than 30 km (19 mi) from the summit, reaching as far as
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.
See also: Category:Lists of extraterrestrial mountains
Scaled image showing Olympus Mons, top, and the Hawaiian island chain,
bottom. Martian volcanoes are far larger than those found on Earth.
Volcanoes are not limited to Earth; they can exist on any rocky planet
or moon 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; cryovolcanoes 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
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 range in size up to 14 mi (23 km) high and
370 mi (595 km) in diameter, far larger than the 6 mi
(10 km) high, 74 mi (119 km) wide Hawaiian
shields. The highest of these, Olympus Mons, is the
tallest known mountain on any planet 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 (430 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
Pyroclastic shield volcano
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Media related to
Shield volcanoes at Wikimedia Commons
List and volcanoes groups
Lists of volcanoes