Shield Volcano - Geology

Geology

Diagram of a Hawaiian eruption. (key: 1. Ash plume 2. Lava fountain 3. Crater 4. Lava lake 5. Fumaroles 6. Lava flow 7. Layers of lava and ash 8. Stratum 9. Sill 10. Magma conduit 11. Magma chamber 12. Dike) Click for larger version.

Shield volcanoes are one of the four major types of volcanoes, distinguished from the three other major volcanic types, stratovolcanoes, lava domes, and cinder cones, by distinct differences in structure and composition. Stratovolcanoes are built up by the accumulation of thick, viscous lavas, cinder cones are constructed of tephra ejected in explosive eruptions, and lava domes are built from extremely thick lava which can't flow far from the vent. 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. Additionally, while stratovolcanoes tend to be large, and domes and cinder cones small, shield volcanoes can vary greatly in size.

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 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, a mass 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 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.

Features common in shield volcanism include 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 portion 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 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.