Lava (World of Earth Science)
Lava is molten rock that has been extruded onto Earth's surface. Before it reaches the surface, lava is called magma. Magma contains crystals, unmelted rock, and dissolved gasses, but it is primarily a liquid. Oxygen, silica, aluminum, iron, magnesium, calcium, sodium, potassium, titanium, and manganese are the primary elements found in magma, but other trace elements may be present in small amounts.
Viscosity of lava, or its resistance to flow, is determined by its temperature and chemical composition. In general, hotter lava that is low in silica flows much more readily than
cooler, high-silica lava. The composition and viscosity of magma determines both its eruptive style and the rock type that will be formed when it cools. The three main types of lava, named for the rock types that they form, are basaltic (for basalt), rhyolitic (for rhyolite), and andesitic (for andesite).
Basaltic flows are the hottest, erupting at temperatures of 1,832,192°F (1,000,200°C). Basaltic lavas are high in calcium, magnesium, and iron, and low in silica, sodium, and potassium. Based on its composition, basalt is known as a mafic rock. The high temperature and low silica content allow basaltic lavas to flow readily and travel far. The Hawaiian Islands were built up from the seafloor from successive basaltic lava flows, forming large shield volcanoes. When basaltic lava erupts onto relatively flat land, flows known as flood basalts may spread out, with successive flows being piled on top of one another. The Columbia Plateau in Oregon and Washington and the Deccan Traps in northwestern India were formed from such eruptions. Viscosity differences within basaltic lavas result in two distinct flow types, characterized by their surface forms. Pahoehoe (pah-hoy-hoy), formed from less viscous lava, is named for the Hawaiian word for ropy. When solidified, the surface of a pahoehoe flow has a smooth texture that looks like coiled rope, which forms as the outer layer of the lava cools, then is dragged and folded as the flow continues to move beneath the surface. More viscous basalt flows give rise to aa (ah-ah) lava, characterized by blocky clumps. Aa flows move more slowly than do pahoehoe flows, allowing a thick surface layer to cool as the flow creeps forward. As the flow continues to move, the surface layer is broken into jagged pieces. Pahoehoe is common near the source of a basaltic flow, where the lava is hottest, and aa is normally found farther from the source, where the lava has cooled off significantly. Another unique feature of basaltic lava is the formation of pillow lavas. Mounds of ellipse shaped pillows form when basaltic lava is erupted under water or ice. As lava is extruded, the water (or ice) quickly chills the outer layer. Molten lava beneath the chilled surface eventually breaks through the skin and the process is repeated, resulting in a pile of lava pillows. Pillow lava deposits found on land indicate that the region was once under water.
Rhyolitic lavas are high in potassium, sodium, and silica, and low in calcium, magnesium, and iron. Rhyolite is a classified as a felsic rock. Its felsic composition, in addition to its low eruptive temperature (1,472,832°F, or 800,000°C), results in highly viscous lava that can just barely flow. Such lavas usually produce a volcanic dome that eventually is destroyed in a massive explosion as the viscous lava tries to escape. Lassen Peak in northeastern California, which last erupted between 1914 and 1917, is an example of a lava dome.
Andesitic lava has a composition that falls in between that of basaltic lava and that of rhyolitic lava. Andesite is hence classified as an intermediate rock. Andesitic lava flows more readily than rhyolitic lava, but not as easily as basaltic lava. Eruptions are characterized by a mixture of explosive activity and lava flows. Such eruptions form composite volcanoes, built up of alternating lava flows and pyroclastic deposits (deposits of debris ejected from the volcano). Composite volcanoes such as Mt. Fuji, in Japan, and Mt. Rainier, in Washington, are cone shaped.
See also Volcanic eruptions