Volcanism. Washington, D.C.: American Geophysical Union, 1997.

Robock, Alan, and Clive Oppenheimer, eds. Volcanism and the Earth's Atmosphere. Washington, D.C.: American Geophysical Union, 2003.

lava Molten rock or magma that flows on the surface of the Earth is known as lava. Lavas have a wide range in composition, texture, temperature, viscosity, and other physical properties, based on the composition of the melt and the amount of volatiles present. They tend to be very viscous (sticky, resistant to flow) when they are rich in silica and form slow-moving and steep-sided flows. The addition of a large amount of volatiles to silicic magma can cause explosive eruptions. Mafic, or low-silica, lavas are less viscous and tend to flow more easily, forming planar flows with gently sloping surfaces. Some basaltic flood lavas have flowed over hundreds or thousands of square miles (square kilometers), forming flat-lying layers of crystallized lava. Other mafic and intermediate lavas form shield volcanoes such as the Hawaiian Islands, with gently sloping sides built by numerous eruptions. If mafic lavas are rich in volatiles, they tend to become abundant in empty gas bubbles known as vesicles, forming pumice. Mafic lavas that flow on the surface often form ropey lava flows known as pahoehoes, or blocky flows known as aa lavas.

some types of lava flows are extremely hazardous whereas others may be relatively harmless if treated with caution. In the most passive types of volcanic eruptions lava bubbles up or effuses from volcanic vents and cracks and flows like thick water across the land surface. During other eruptions lava oozes out more slowly, producing different types of flows with different hazards. Variations in magma composition, temperature, dissolved gas content, surface slope, and other factors lead to the formation of three main different types of lava flows. These include aa, pahoehoe, and block lava. Aa are characterized by a rough surface of spiny and angular fragments, whereas pahoehoe have smooth, ropeylike or billowing surfaces. Block lavas have larger fragments than aa flows and are typically formed by stickier, more silicic (quartz rich) lavas than aa and pahoe-hoe flows. some flows are transitional between these main types, or may change from one type to another as surface slopes and flow rates change. Pahoehoe

Lava flow crossing Chain of Craters road from the west toward Hiiaka crater during eruption of Kilauea Volcano, Hawaii, May 5, 1973 (R.L. Christiansen/USGS)

Lawson, Andrew Cowper

flows commonly change into aa flows with increasing distance from the volcanic source.

Lava flows are most common around volcanoes that are characterized by eruptions of basalt with low contents of dissolved gasses. About 90 percent of all lava flows worldwide are made of magma with basaltic composition, followed by andesitic (8 percent) and rhyolitic (2 percent). Places with abundant basaltic flows include Hawaii, Iceland, and other exposures of oceanic islands and midoceanic ridges, all characterized by nonexplosive eruptions. Virtually the entire volume of all the islands of the Hawaiian chain are made of a series of lava flows piled high one on top of the other.

Lava flows generally follow topography, flowing from the volcanic vents downslope in valleys, much as streams or water from a flood would travel. Some lava flows move as fast as water, up to almost 40 miles per hour (65 km/hour) on steep slopes, but most lava flows move considerably more slowly. More typical rates of movement range from about 10 feet per hour (several meters per hour) to 10 feet (3 m) per day for slower flows. These rates of lava movement allow most people to move out of danger to higher ground, but lava flows are responsible for significant amounts of property damage in places like Hawaii. Lava flows have buried roads, farmlands, and other low-lying areas. One must keep in mind, however, that the entire Hawaiian Island chain was built by lava flows, and the real estate that is being damaged would not even exist if it were not for the lava flows. In general pahoehoe flows are the fastest, aa are intermediate, and blocky flows are the slowest.

Basaltic lava is extremely hot (typically about 1,830°-2,100°F, or 1,000°-1,150°C) when it flows across the surface, so when it encounters buildings, trees, and other flammable objects, they typically burst into flame and are destroyed. More silicic lavas are slightly cooler, in the range of 1,560°-1,920°F (850°-1050°C). Most lavas will become semisolid and stop flowing at temperatures approaching 1,380°F (750°C). Lavas cool quickly at first, until a crust or hard skin forms on the flow, then they cool more slowly. This property of cooling creates one of the greatest hazards of lava flows. A lava flow that appears hard, cool, and safe to walk on can hide an underlying thick layer of molten lava at temperatures of about 1,380°F (750°C) just below the thin surface. Many people have mistakenly thought it was safe to walk across a recent crusty lava flow, only to plunge through the crust to a fiery death. Thick flows take years to crystallize and cool, and residents of some volcanic areas have learned to use the heat from flows for heating water and piping it to nearby towns.

See also igneous rocks; volcano.

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