Volcanology: Shield Volcanos

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.

Etymology
Shield volcanoes are built up 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 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, 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. 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 eruptions, 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 a 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. 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, and is over 60 mi (97 km) wide.

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. 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 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.

Another hallmark of shield volcanism are lava tubes. 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 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 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 2,290 ft (698 m) by over 25 years of activity.

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. 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 Isles.

Distribution
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.

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.

They 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. The volcanoes follow a distinct evolutionary pattern of growth and death.

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. Kilauea, meanwhile, is one of the most active volcanoes on Earth, with the current ongoing eruption having begun in January 1983.

Galápagos islands
An animated tour of the Galápagos Islands.The 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 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 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.

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.

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.

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.

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.

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.

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 700km (435 miles). 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.

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
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.