DESCRIPTION:
Magma, Lava, Lava Flows, Lava Lakes, etc.

Molten rock below the surface of the Earth that rises in volcanic vents is known as magma, but after it erupts from a volcano it is called lava. Originating many tens of miles beneath the ground, the ascending magma commonly contains some crystals, fragments of surrounding (unmelted) rocks, and dissolved gases, but it is primarily a liquid composed principally of oxygen, silicon, aluminum, iron, magnesium, calcium, sodium, potassium, titanium, and manganese. Magmas also contain many other chemical elements in trace quantities. Upon cooling, the liquid magma may precipitate crystals of various minerals until solidification is complete to form an igneous or magmatic rock.

... heat concentrated in the Earth's upper mantle raises temperatures sufficiently to melt the rock locally by fusing the materials with the lowest melting temperatures, resulting in small, isolated blobs of magma. These blobs then collect, rise through conduits and fractures, and some ultimately may re-collect in larger pockets or reservoirs ("holding tanks") a few miles beneath the Earth's surface. Mounting pressure within the reservoir may drive the magma further upward through structurally weak zones to erupt as lava at the surface. In a continental environment magmas are generally in the Earth's crust as well as at varying depths in the upper mantle. The variety of molten rocks in the crust, plus the possibility of mixing with molten materials from the underlying mantle, leads to the production of magmas with widely different chemical compositions.

If magmas cool rapidly, as might be expected near or on the Earth's surface, they solidify to form igneous rocks that are finely crystalline or glassy with few crystals. Accordingly, lavas, which of course are very rapidly cooled, form volcanic rocks typically characterized by a small percentage of crystals or fragments set in a matrix of glass (quenched or super-cooled magma) or finer grained crystalline materials. If magmas never breach the surface to erupt and remain deep underground, they cool much more slowly and thus allow ample time to sustain crystal precipitation and growth, resulting in the formation of coarser grained, nearly completely crystalline, igneous rocks. Subsequent to final crystallization and solidification, such rocks can be exhumed by erosion many thousands or millions of years later and be exposed as large bodies of so-called granitic rocks, as, for example, those spectacularly displayed in Yosemite National Park and other part of the majestic Sierra Nevada mountains of California.

Lava is red hot when it pours or blasts out of a vent but soon changes to dark red, gray, black, or some other color as it cools and solidifies. Very hot, gas-rich lava containing abundant iron and magnesium is fluid and flows like hot tar, whereas cooler, gas-poor lava high in silicon, sodium, and potassium flows sluggishly, like thick honey in some cases or in others like pasty, blocky masses.

All magmas contain dissolved gases, and as they rise to the surface to erupt, the confining pressures are reduced and the dissolved gases are liberated either quietly or explosively. If the lava is a thin fluid (not viscous), the gases may escape easily. But if the lava is thick and pasty (highly viscous), the gases will not move freely but will build up tremendous pressure, and ultimately escape with explosive violence. Gases in lava may be compared with the gas in a bottle of a carbonated soft drink. If you put your thumb over the top of the bottle and shake it vigorously, the gas separates from the drink and forms bubbles. When you remove your thumb abruptly, there is a miniatures explosion of gas and liquid. The gases in lava behave in somewhat the same way. Their sudden expansion causes the terrible explosions that throw out great masses of solid rock as well as lava, dust, and ashes.

The violent separation of gas from lava may produce rock froth called pumice. Some of this froth is so light -- because of the many gas bubbles -- that it floats on water. In many eruptions, the froth is shattered explosively into small fragments that are hurled high into the air in the form of volcanic cinders (red or black), volcanic ash (commonly tan or gray), and volcanic dust. ...

Most composite volcanoes have a crater at the summit which contains a central vent or a clustered group of vents. Lavas either flow through breaks in the crater wall or issue from fissures on the flanks of the cone. Lava, solidified within the fissures, forms dikes that act as ribs which greatly strengthen the cone. ...

In some shield-volcano eruptions, basaltic lava pours out quietly from long fissures instead of central vents and floods the surrounding countryside with lava flow upon lava flow, forming broad plateaus. Lava plateaus of this type can be seen in Iceland, southeastern Washington, eastern Oregon, and southern Idaho. ...

Volcanic or lava domes are formed by relatively small, bulbous masses of lava too viscous to flow any great distance; consequently, on extrusion, the lava piles over and around its vent. ...

Congealed magma, along with fragmental volcanic and wallrock materials, can be preserved in the feeding conduits of a volcano upon cessation of activity. These preserved rocks form crudely cylindrical masses, from which project radiating dikes; they may be visualized as the fossil remains of the innards of a volcano (the so-called "volcanic plumbing system") and are referred to as volcanic plugs or necks. The igneous material in a plug may have a range of composition similar to that of associated lavas or ash, but may also include fragments and blocks of denser, coarser grained rocks -- higher in iron and magnesium, lower in silicon -- thought to be samples of the Earth's deep crust or upper mantle plucked and transported by ascending magma. ...

Lava, erupting onto a shallow sea floor or flowing into the sea from land, may cool so rapidly that it shatters into sand and rubble. The result is the production of huge amounts of fragmental volcanic debris. The famous "black sand" beaches of Hawaii were created virtually instantaneously by the violent interaction between hot lava and sea water. On the other hand, recent observations made from deep-diving submersibles have shown that some submarine eruptions produce flows and other volcanic structures remarkable similar to those formed on land. ...

Ash and Tephra and Pumice and Scoria Menu

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Molten rock (magma) that pours or oozes onto the Earth's surface is called lava. The higher a lava's silica content, the more viscous it becomes. For example, low-silica basalt lava can form fast-moving (10-30 miles per hour), narrow lava streams or spread out in broad sheets up to several miles wide. Between 1983 and 1993, basalt lava flows erupted at Kilauea Volcano in Hawaii destroyed nearly 200 houses and severed the coast highway along the volcano's south flank.

In contrast, higher-silica andesite and dacite lava flows tend to be thick, move slowly, and travel short distances from a vent. Dacite and rhyolite lava flows often form mound-shaped features called domes.

Streams of molten rock that either effuse quietly from a vent or are fed by lava fountains. Fluid basalt flows can move at velocities from 15 to as high as 50 kilometers per hour on steep slopes and travel up to tens of kilometers from their source. Viscous andesite flows move only a few kilometers per hour and rarely extend more than 8 kilometers from their vent. Lava flows destroy everything in their path, but most move slowly enough that people can escape.

Lava (usually dacite or rhyolite) that is too sticky to flow far from its vent forms steep-sided mounds called lava domes.

Lava flows are streams of molten rock that erupt relatively nonexplosively from a volcano and move slowly downslope. The distance traveled by a lava flow depends on such variables as the viscosity of the lava, volume erupted, steepness of the slope, and obstructions in the path of the flow. Basalt flows may reach 50 kilometers from their sources, but few andesite flows extend more than 10-15 kilometers. Because of their high viscosity, dacite and rhyolite lava extrusions typically form short, thick flows or domes.

Lava flows cause extensive damage or total destruction by burning, crushing, or burying everything in their paths. They need not directly threaten people, however, because they usually move a few meters to a few hundred meters per hour, and their paths of movement can be at least roughly predicted. Lava flows that move onto snow or ice can cause destructive debris flows and floods, and those that move into forests can cause fires. The flanks of lava flows typically are unstable during their growth and collapse repeatedly, occasionally producing explosive blasts and (or) small pyroclastic flows. ...

Scoria forms when blobs of gas-charged lava are thrown into the air during an eruption and cool in flight, falling as dark volcanic rock containing cavities created by trapped gas bubbles.

Lava flows are streams of molten rock that erupt relatively nonexplosively from a volcano and move downslope. The distance traveled by a lava flow depends on such variables as the effusion rate, fluidity of the lava, volume erupted, steepness of the slope, channel geometry, and obstructions in the flows path. Basalt flows are characterized by relatively low viscosity and may reach more than 50 kilometers from their sources; in fact, one Icelandic basalt flow reached 150 kilometers. Andesite flows have higher viscosity and few extend more than 15 kilometers; however, one andesite flow of Pleistocene age in the Cascades is 80 kilometers long. Because of their high viscosity, dacite and rhyolite lava extrusions typically form short, thick flows or domes.

Lava flows cause extensive damage or total destruction by burning, crushing, or burying everything in their paths. They seldom threaten human life, however, because of their typically slow rate of movement, which may be a few meters to a few hundred meters per hour. In addition, their paths of movement generally can be predicted. However, lava flows that move onto snow or ice can cause destructive lahars and floods, and those that move into forests can start fires. The flanks of moving lava flows typically are unstable and collapse repeatedly, occasionally producing small explosive blasts or small pyroclastic flows.

Lava flows have been erupted at many vents in the Cascade Range during Holocene time; their compositions range from basalt to rhyolite. The longest known basalt, andesite, and rhyolite lava flows erupted at Cascade volcanic centers during Holocene time are, respectively, the 45-km-long Giant Crater basalt flow at Medicine Lake volcano the 12-kilometer-long Schriebers Meadow andesite flow at Mount Baker, and the 2-kilometer-long Rock Mesa rhyolite flow at Three Sisters. Lava flows of varied composition are likely to erupt again in the Cascade Range and will endanger all non-movable objects in their paths.

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Scientists use the term lava for molten rock (and contained gases) that breaks through the Earth's surface, and the term magma for molten rock underground. ...

Lava flows form more than 99 percent of the above-sea parts of Hawaiian volcanoes. Pahoehoe (pronounced "pah-hoy-hoy") and aa (pronounced "ah-ah") are the two main types of Hawaiian lava flows, and these two Hawaiian names, introduced into the scientific literature in the late 19th century, are now used by volcanologists worldwide to describe similar lava-flow types. Pahoehoe is lava that in solidified form is characterized by a smooth, billowy, or ropy surface, while aa is lava that has a rough, jagged, spiny, and generally clinkery surface. In thick aa flows, the rubbly surface of loose clinkers and blocks hides a massive, relatively dense interior.

The contrast between the surfaces of pahoehoe and aa flows is immediately obvious to anyone hiking Hawaiian lava fields. Walking on dense pahoehoe can almost be as easy as strolling on a paved sidewalk. But walking across aa is like scrambling over a building-demolition site or battle zone, strewn with loose, unstable debris of all shapes and sizes. The jagged rubble of aa flows quickly destroys field boots and, should the hiker stumble or fall (not at all uncommon), it can tear clothing and flesh.

Many Hawaiian lava flows solidify as pahoehoe throughout their extent, and a few flows solidify completely as aa. Most flows, however, consist of both pahoehoe and aa in widely varying proportions. In a given flow, pahoehoe upstream commonly changes to aa downstream, but aa lava flows do not change into pahoehoe flows. ...

Hawaiian lava is fluid enough to travel great distances, especially if it is transported through lava tubes. Some historic flows are longer than 30 miles; in general, pahoehoe flows tend to be longer than aa. Lava tubes may be preserved when the eruption ends and the lava drains away to leave open tunnels. They may be as much as several tens of feet in diameter, and some have been followed by spelunkers (cave explorers) for nearly 10 miles. Ancient Hawaiians used lava tubes as places of shelter and as burial caves. ...

During long-lived eruptions, lava flows tend to become "channeled" into a few main streams. Overflows of lava from these streams solidify quickly and plaster on to the channel walls, building natural levees or ramparts that allow the level of the lava to be raised. Lava streams that flow steadily in a confined channel for many hours to days may develop a solid crust or roof and thus change gradually into streams within lava tubes. Because the walls and roofs of such tubes are good thermal insulators, lava flowing through them can remain hot and fluid much longer than surface flows. Tube-fed lava can be transported for great distances from the eruption sites. For example, during the 1969-74 Mauna Ulu eruptions at Kilauea, lava flows traveled underground through a lava-tube system for more than 7 miles long to enter the ocean on five occasions. ...

Link to: Hawaiian Volcano Observatory Website

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Aerial view, Novarupta Dome, Katmai Vicinity, Alaska.
-- USGS Photo by Gene Iwatsubo, July 29, 1987

Volcanic domes are mounds that form when viscous lava is erupted slowly and piles up over the vent, rather than moving away as a lava flow. The sides of most domes are very steep and typically are mantled with unstable rock debris formed during or shortly after dome emplacement. Most domes are composed of silica-rich lava which may contain enough pressurized gas to cause explosions during dome extrusion.

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Another common lava product is the ponded flow or lava lake ... The surface of lava that is ponded is smooth, broken only by polygonal cooling cracks, formed in much the same way as shrinkage cracks in mud that has been dried by the sun.

Erta Ale is an isolated basaltic shield volcano that is the most active volcano in Ethiopia. The broad, 50-kilometer-wide volcano rises more than 600 meters from below sea level in the barren Danakil depression. Erta Ale is the namesake and most prominent feature of the Erta Ale Range. The 613-meter-high volcano contains a 0.7 x 1.6 kilometers, elliptical summit crater housing steep-sided pit craters. Another larger 1.8 x 3.1 kilometer-wide depression elongated parallel to the trend of the Erta Ale range is located to the southeast of the summit and is bounded by curvilinear fault scarps on the southeast side. Fresh-looking basaltic lava flows from these fissures have poured into the caldera and locally oveflowed its rim. The summit caldera is renowned for one, or sometimes two longterm lava lakes that have been active since at least 1967, or possibly since 1906. Recent fissure eruptions have occurred on the northern flank of Erta Ale.

Lava Lakes Menu

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Ape Cave, Mount St. Helens National Volcanic Monument
-- U. S. Forest Service Image, courtesy Mount St. Helens National Volcanic Monument

During long-lived eruptions, lava flows tend to become "channeled" into a few main streams. Overflows of lava from these streams solidify quickly and plaster on to the channel walls, building natural levees or ramparts that allow the level of the lava to be raised. Lava streams that flow steadily in a confined channel for many hours to days may develop a solid crust or roof and thus change gradually into streams within lava tubes. Because the walls and roofs of such tubes are good thermal insulators, lava flowing through them can remain hot and fluid much longer than surface flows. Tube-fed lava can be transported for great distances from the eruption sites. For example, during the 1969-74 Mauna Ulu eruptions at Kilauea, lava flows traveled underground through a lava-tube system for more than 7 miles long to enter the ocean on five occasions.

Lava Tubes and Lava Tube Caves Menu

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Glass Mountain from Medicine Lake caldera rim
-- USGS Photo by J. Donnelly-Nolan

Glass Mountain consists of a spectacular, nearly treeless, steep-sided rhyolite and dacite obsidian flow that erupted just outside the eastern caldera rim and flowed down the steep eastern flank of Medicine Lake volcano. ...

Newberry volcano is a broad shield volcano located in central Oregon. It has been built by thousands of eruptions, beginning about 600,000 years ago. At least 25 vents on the flanks and summit have been active during several eruptive episodes of the past 10,000 years. The most recent eruption 1,300 years ago produced the Big Obsidian Flow. Thus, the volcano's long history and recent activity indicate that Newberry will erupt in the future.

Fluid lava erupted or flowing under water may form a special structure called pillow lava. Such structures form when molten lava breaks through the thin walls of underwater tubes, squeezes out like toothpaste, and quickly solidifies as irregular, tongue-like protrusions. This process is repeated countless times, and the resulting protrusions stack one upon another as the lava flow advances underwater. The term pillow comes from the observation that these stacked protrusions are sack-or pillow-shaped in cross section. Typically ranging from less than a foot to several feet in diameter, each pillow has a glassy outer skin formed by the rapid cooling of the lava by water. Much pillow lava is erupted under relatively high pressure created by the weight of the overlying water; there is little or no explosive interaction between hot lava and cold water. The bulk of the submarine part of a Hawaiian volcano is composed of pillow lavas. ...

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