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 above sea level, and the volcano, over wide at its base, is estimated to contain about of basalt. The mass of the volcano is so great that it has slumped the crust beneath it a further . 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 . Mount Everest, by comparison, is in height. In September 2013, a team led by the University of Houston's William Sager announced the discovery of Tamu Massif, an enormous extinct submarine shield volcano of previously unknown origin which, approximately in area, dwarfs all previously known volcanoes on the planet. However, the extents of the volcano have not been confirmed. Shield volcanoes feature a gentle (usually 2° to 3°) slope that gradually steepens with elevation (reaching approximately 10°) before 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, there are regional differences in their size and morphological characteristics. Typical shield volcanoes found in California and Oregon measure in diameter and in height, while shield volcanoes in the central Mexican Michoacán–Guanajuato volcanic field average in height and in width, with an average slope angle of 9.4° and an average volume of . 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 Hawaii; 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. Eruptive characteristics Most of what is currently known about shield volcanic eruptive character has been gleaned from studies done on the volcanoes of Hawaii Island, by far the most intensively studied of all shields because of their scientific accessibility; the island lends its name to