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

Distribution
Main article: List of shield volcanoes
Shield volcanoes are distinctive products of hotspot volcanism, but can form at rift and subduction zones as well.[4]
The largest and most prominent shield volcano chain in the world are the Hawaiian Islands, a chain of hotspot volcanoes in Pacific Ocean. This chain includes the largest volcano on Earth, Mauna Loa. Mauna Kea stands 4,170 m (13,681 ft) above sea level. In addition, its submarine flanks reach a further 5 km (3 mi) below the waterline, and Mauna Loa's massive size depresses the sea floor on which it stands a further 8 km (5 mi), making the volcano's summit about 17 km (56,000 ft) above its base. The volcano is approximately 80,000 km3 (19,193 cu mi) in total volume.[6]
Kilauea is one of the most active volcanoes on Earth, with the current ongoing eruption having begun in January 1983.[3] The Hawaiian volcanoes are characterized by frequent rift eruptions, their large size (thousands of km3 in volume), and their rough, decentralized shape.[1]
Another major center of shield volcanic activity is Iceland. There, the volcanoes are small (~15 km3 (4 cu mi)), symmetrical, and are characterized by eruptions from summit calderas.[1]
Olympus Mons on Mars is also a shield volcano.[6]

Structure

Dark profile of Hualālai, showing typical shape of a shield volcano.
Because of their gradual buildup and near continuous eruptive characteristics, shield volcanoes are the largest volcanoes on Earth,[4][n 1] usually being at least 3 to 4 mi (5 to 6 km) across and surpassing 1,500 to 2,000 ft (457 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.[3]
Shield volcanoes are composed almost exclusively of basalt.[4] 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.[1]
Over the volcano's lifespan, collapse-driven calderas that form on shield volcanoes are often filled up, and new ones formed elsewhere, in an ongoing cycle of collapse and regeneration.[4]

Geology

Eruptive mechanics

Shield volcanoes are built almost entirely of highly fluid basaltic lava. They are distinct from the two other major volcanic types, stratovolcanoes, which are driven by the accumulation of more viscous lavas, and cinder cones, which are built up by the consolidation of tephra ejected by explosive eruptions.[1] The types of eruptions that occur at shield volcanoes have been named Hawaiian eruptions, after the Hawaiian chain where they are most prominent. Hawaiian eruptions are characterized by the effusive emission of fluid lavas.[2] The nature of these lavas allows them to travel a longer distance than flows from other volcanic types, resulting in a large, spread-out sheet of lava[3] just 1 m (3 ft) thick.[1] The gradual buildup of thousands of these flows slowly constructs a low-lying, broad, and gently sloping form of a mature shield volcano.[3]
Continuous shield volcanic activity is very common,[2] and will, over time, build up splatter cones at the eruptive sites, despite Hawaiian activity being 90% lava flows.[4] An example of this is Puʻu ʻŌʻō, a product of Kīlauea's continuous activity.[5]
A hallmark of shield volcanism are lava tubes,[6] cave-like volcanic straights that are formed by the hardening of overlaying lava. These structures further the propagation of lava, as the walls of the tube insulate the flows within.[7] They are an important eruptive element; for example, an estimated 58% of Kilauea is covered by lava tube lava.[6]
Interactions between water and lava at shield volcanoes can cause some eruptions to become hydrovolcanic, which are an explosive eruptive type drastically different from usual shield volcanic activity.[4] These eruptions are especially prevelent at the waterbound volcanoes of the Hawaiian Isles.[2]
Rift zones are another prevalent feature on shield volcanoes that is rare on other volcanic types. The large, decentralized shape of Hawaiian volcanoes versus their small, symmetrical Icelandian cousins can be attributed to these types of eruptions; fissure venting is common in Hawaiʻi, accounting for their asymettrical, non-centralized shapes, and rare in Iceland, where central eruptions from summit calderas dominate and thus the lava distribution is far more even.[1][3][5]
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.[3] 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.[2]

Shield volcano

A shield volcano is a type of volcano built almost entirely of fluid lava flows. They are named so because of their large size and low profile, resembling a warrior's shield. This is caused by the highly fluid lava they erupt, which travel farther than those erupted from more explosive volcanoes. This results in the steady accumulation of broad sheets of lava, building up the shield volcano's distinctive form.

Lava

Main article: lava
Lava flows from stratovolcanoes are generally not a significant threat to people because the highly viscous lava moves slowly enough for people to move out of the path of flow. The lava flows are more of a property threat.
However, not all stratovolcanoes have viscous lava. Mount Nyiragongo is dangerous because its magma has an unusually low silica content, making it quite fluid (even when comparing to Hawaiian lava) and having lower viscosity. Compounded by the very steep slope of Nyiragongo gives the lava the ability to flow at up to about 100 km/h (60 mph).

Volcanic bombs

Main article: volcanic bomb
Volcanic bombs are extrusive igneous rocks that range from the size of a book to small automobile, that are explosively ejected from Stratovolcanoes during their peak eruptive phases. These bombs can travel over fifteen miles (20 km) away from the volcano and present a risk to buildings and people while traveling at very high speeds (hundreds of miles per hour or km/h) through the air. The bombs do not themselves explode on impact, but rather carry enough force so as to have destructive effects as if they exploded