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.

Diagram of a shield volcano, showing the gradual buildup
of lava in many layers that eventually constructs them

          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. 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. 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  just 1 m (3 ft) thick. The gradual buildup of thousands of these flows slowly constructs a low-lying, broad, and gently sloping form of a mature shield volcano.


Skjaldbreiður, eponymous for a shield volcano


          Continuous shield volcanic activity is very common, and will, over time, build up splatter cones at the eruptive sites, despite Hawaiian activity being 90% lava flows. An example of this is Puʻu ʻŌʻō, a product of Kīlauea's continuous activity.
A hallmark of shield volcanism are lava tubes, 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. They are an important eruptive element; for example, an estimated 58% of Kilauea is covered by lava tube lava.
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. These eruptions are especially prevelent at the waterbound volcanoes of the Hawaiian Isles.


Dark profile of Hualālai, showing typical shape of a shield volcano.


      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.
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.