Mid-ocean islands such as Hawaii, the Galapagos, Reunion, or Bermuda are formed by basalt, the most basic and most fluid, less viscous of all types of common lava. Geochemically, various types of basalts (tholeiite, alkaline etc.) can be distinguished but basalt in general has a very different chemical composition from the lavas that erupt from continental volcanoes and is much more fluid; therefore, gases escape readily from theses lavas. Consequently, as lava flows build up to produce a volcanic cone, basaltic edifices typically have low-angle slopes. They are then called shield volcanoes. Profiles of shield volcanoes resemble that of a Roman warrior's shield having a gently sloping, convex-upward landform.
Shield volcanoes are the largest known volcano type in the solar system. Olympus Mons on Mars is the largest mountain in the Solar System, rising 24 km above the surrounding plain, having a base which is more than 500 km in diameter, and rimmed by a cliff 6 km high. The biggest shield volcano on Earth is Mauna Loa, situated on the Big Island of the Hawaiian Islands. Mauna Loa stands 4169 m above sea level and a total of 8 km above the ocean floor, plus an additional km due to the loading of the oceanic lithosphere by the huge load. The 1710 m high Volcán Wolf at Isabela Island is the highest volcano of the Galápagos Islands. Volcán Wolf, like all other volcanoes at the Galapagos, appears only partially above the sea level while the lowermost three kilometers above its base are covered by the ocean.
Shield volcanoes consist largely of thin lava flows, with minor pyroclastic (mainly ash) layers. Their subaerial (above sea-level) slopes are mostly 4-8 degrees, having steep-walled summit calderas and also pit craters (sinkholes) that are similar to calderas in form but much smaller. The gentle slopes are the result of the low lava viscosity, allowing lavas to flow fast and far. The lava flows (pahoehoe and aa) commonly initiate their path from flank vents and fissures rather than from the summit. These flank vents are the result of the widening and / or subsidence of the volcano. Eruptions and lava flows occur also along collinear rift zones, which can extend very far from the summit. Eruptions are concentrated at the active rift zones. Rift zones are linear, elongated morphological features, which represent fractures in a volcano that radiate from the summit crater. They are zones of weakness within a volcano that magma can easily move into. This causes frequent eruptions along these zones that continue to widen over time because of magma flowing into them. Dike complexes (the subsurface manifestation of eruptive fissures) often underlie the caldera and rift zones. In some shield volcanoes of the Galapagos occurs a circumferential rift zone around the caldera rim, which has the consequence that their shields culminate in steep domes, deep summit calderas and radial dike swarms.