For example, shield volcanoes such as Kilauea on the island of Hawaii produce large amounts of lava that is less viscous (less sticky) because it has less silica, SiO2, in it. Lava from these eruptions may flow as a rough textured type called aa (a Hawaiian word, pronounced "ah, ah"), or another type called pahoehoe, which often has wrinkle folds on its surface. During an eruption, pahoehoe may change into aa as it becomes more viscous.
Composite volcanoes such as Mount St Helens generally have relatively small amounts of viscous lava. The sticky nature of the lava and the magma that forms it is caused by its relatively high silica content and makes these eruptions very explosive.
Image: A pahoehoe basalt lava flow from Kilauea volcano on the island of Hawaii (credit: Stephen & Donna O'Meara/SPL)
David Attenborough explains how lava erupts and cools.
David Attenborough explains how basalt lava erupts and cools - sometimes creating beautiful formations such as Giant's Causeway in Northern Ireland and Fingal's Cave in Scotland.
Lava flows from this spectacular volcano have built the island of Hawaii.
Professor Iain Stewart explains how Mount Kilauea's eruptions of lava have built up the island of Hawaii over millions of years as a magma plume known as a hotspot rises up through the Earth's crust.
BBC News reports on a 1992 Mount Etna eruption.
The BBC's Justin Webb reports on attempts to divert lava flows on Mount Etna during a 1992 eruption.
Liz Bonnin gets a close look at hot, molten lava flowing from Mount Kilauea.
Liz Bonnin gets a close look at hot, molten lava flowing from Mount Kilauea in Hawaii.
Lava is the molten rock expelled by a volcano during an eruption. The resulting rock after solidification and cooling is also called lava. The molten rock is formed in the interior of some planets, including Earth, and some of their satellites. The source of the heat that melts the rock within the earth is geothermal energy. When first erupted from a volcanic vent, lava is a liquid usually at temperatures from 700 to 1,200 °C (1,292 to 2,192 °F).
A lava flow is a moving outpouring of lava, which is created during a non-explosive effusive eruption. When it has stopped moving, lava solidifies to form igneous rock. The term lava flow is commonly shortened to lava. Although lava can be up to 100,000 times more viscous than water, lava can flow great distances before cooling and solidifying because of its thixotropic and shear thinning properties.
Explosive eruptions produce a mixture of volcanic ash and other fragments called tephra, rather than lava flows. The word "lava" comes from Italian, and is probably derived from the Latin word labes which means a fall or slide. The first use in connection with extruded magma (molten rock below the Earth's surface) was apparently in a short account written by Francesco Serao on the eruption of Vesuvius between May 14 and June 4, 1737. Serao described "a flow of fiery lava" as an analogy to the flow of water and mud down the flanks of the volcano following heavy rain.