Lava flow

The Hawaiian words “pahoehoe” and “aa” are used worldwide to designate two very common types of lava flow. The outward appearance of lava flows tells much about their eruption temperatures, chemical compositions, and viscosities. The surfaces of lava flows vary from smooth and glassy to rough and clinkery depending on the viscosity of the lava and whether it is emplaced on land or underwater.

Lava Flows

Aa and pahoehoe are Hawaiian words that describe the surface textures of lava flows. Aa is a rough, clinkery variety of lava, and pahoehoe has a smooth, ropy surface. Although aa and pahoehoe are the most common types of lava flow on land, other forms occur if lava has erupted underwater or is extremely viscous. An explanation of why lava flows assume different forms requires a basic understanding of the physical properties of magma.

Magmas

Magma is molten rock that originates from the partial melting of either the crust or the mantle of the Earth. “Lava” is a general term for magma that has erupted onto the Earth's surface. Lava flows are bodies of magma emplaced onto the surface as coherent, flowing masses of lava. The viscosity of lava—its resistance to flowage—largely controls the external appearance of the lava. In turn, the viscosity of lava is controlled mainly by its temperature and chemical composition. Most magmas are dominantly composed of silicon and oxygen, forming strong chemical bonds in the magma. Together, these elements are referred to as silica. The silica content of magma exerts a major influence on magma viscosity. Like adding flour to batter, increased silica content means higher magma viscosity.

Although geologists recognize hundreds of kinds of volcanic rocks, there are three fundamental magma types. Basalt magma is produced in the Earth's upper mantle and is poorer in silica (about 50 percent by weight) and dissolved gases than other types of magma. It erupts at the highest temperatures of all lava, usually about 1,100 to 1,200 degrees Celsius. Because of its low silica content and high temperature, basalt lava has relatively low viscosity, or resistance to flowage. With the consistency of honey or peanut butter (although considerably denser, at around 2.7 grams per cubic centimeter), basalt lava is relatively fluid and usually erupts as thin, sheetlike lava flows.

Andesite magma erupts at slightly lower temperatures than basalt (around 1,000 degrees Celsius) and is richer in dissolved gases and silica (about 60 percent by weight). Andesite magma has a viscosity much greater than that of basalt. Andesite lava viscosity is similar to cold putty or frozen caulking compound, although its density of about 2.6 grams per cubic centimeter is considerably higher. Because of their relatively high viscosity, andesite lava flows are thick and tonguelike in shape and do not travel as far from the vent as fluid basalt lava flows.

Rhyolite, the third fundamental type of magma, is formed by the melting of the continental crust. It is very rich in silica (greater than 70 percent by weight) and often contains high amounts of dissolved gases. Thus, it tends to erupt explosively and form voluminous deposits of frothy pumice. The viscosity of molten rhyolite lava is extremely high. Even at its white-hot eruption temperature of around 900 degrees Celsius, rhyolite behaves more like a solid material than a liquid. It fractures when struck with a hammer, and its viscosity is sometimes similar to that of glacial ice. Thus, rhyolite magma moves very slowly and oozes onto the Earth's surface as thick, pasty lava flows or steep-sided volcanic domes. The latter are steep piles of lava that grow directly on top of volcanic vents because the lava is too viscous to flow away from its source. Because the crystallization of rhyolite is very sluggish, its lava flows and domes are often composed of obsidian, a type of silica-rich volcanic glass.

Pahoehoe

Pahoehoe seldom forms on lava flows other than basalt, the hottest and most fluid type of magma. The smooth surface texture on pahoehoe lava flows is formed during quenching of the flow surface against the Earth's atmosphere. A several-centimeter-thick crust of brittle lava is formed, commonly with a thin skin of glass. The smooth, brittle crust forms a barrier to rising gas bubbles (vesicles), and there is often a frothy zone of round vesicles directly underneath the crust, which rides atop the hot, fluid interior of the lava flow. The lava crust is an insulating barrier that prevents the flow of heat from the interior of the lava flow, which can remain hot for weeks, months, or even years, depending on its thickness. During flowage of the molten interior, the smooth crust is commonly deformed into folds and ridges, as when a carpet is shoved against a wall. The folds and ridges assume many forms, as they are repeatedly stretched and refolded onto themselves. Several varieties of pahoehoe have been recognized. Entrail pahoehoe is commonly formed when lava tubes are breached, spilling their contents as entwined, elongate bulbs of glassy-skinned lava. Shelly pahoehoe forms only near volcanic vents, where centimeter-thick lava crusts are stacked atop one another and are separated by cavernous gas pockets.

Pahoehoe lava flows can travel tens of kilometers from their vents because rocks are poor conductors of heat. As the top and sides of the lava flow solidify, an insulating barrier is formed around the interior of the flow, which remains hot and fluid. Like blood within a system of blood vessels, fluid lava moves within circular channels known as lava tubes, and this network feeds the advancing nose of the lava flow. When the eruption of lava ceases, some tubes drain out, and their roofs may collapse, leaving accessible caves with flow marks on the walls and lava dripstones hanging from the ceilings.

Pillow Lava

Pillow lava is the subaqueous counterpart of pahoehoe and forms only when fluid basalt lava has erupted underwater. Pillow lava has been observed forming around the Hawaiian Islands, and beneath its blanket of sediment, most of the sea floor is composed of this type of basalt lava. When basalt lava erupts underwater, large quantities of steam are formed, sapping heat from the lava surface. As a result, a glassy skin quickly forms. Back-pressure from the still-fluid lava inside the lava flow eventually bursts the brittle skin, which is shattered into plates, chunks, and small shards of glass. New bulbs of lava ooze out of the cracks in a budding process. The process continues to build entwined masses of bulbous, glassy-skinned lava that look like toothpaste extrusions. The lava is mingled with layers of glassy fragments from the brittle skin of the advancing lava flow. Because it looks like a stack of pillows, the bulbous, glassy lava is appropriately called pillow lava.

Aa

Aa is a type of lava with a rough, clinkery surface. The term “clinkery' refers to hardened lava. It is typical of basalt and andesite lava flows that are somewhat fluid but are too viscous to have pahoehoe surfaces. Jagged, spiney blocks of lava are formed during the crumbling of the viscous mass. The spiney fragments continually rub against one another, eventually forming a thick outer envelope of debris that grades into the lava flow's hotter, more fluid interior. The clinkery rubble rides on the flow until it eventually tumbles down the nose of the moving flow front. Like an advancing tractor tread, the fluid interior of the lava flow overrides its own debris. Eventually, the interiors of aa flows may become so viscous that they can no longer flow as liquids and instead begin to shear along horizontal fractures (platy joints) that form near their bases. The shear planes are similar to those that develop near the bases of glaciers, and the process is much like spreading out a deck of cards across a tabletop.

Cooling of Lava

Single lava flows can change from pahoehoe into aa, but the reverse of this process has never been observed. When lava is first emitted, it is the hottest and most fluid. As the lava cools, loses its gases, and slowly crystallizes, its viscosity is irreversibly increased. The originally smooth lava crust becomes thicker, and slabs of it begin to grind against one another. Eventually, a smooth crust can no longer develop because the outer portion of the lava flow has become too brittle.

In addition to cooling and the resulting increase in viscosity, the pahoehoe-to-aa transition has been observed to occur when lava flows undergo high rates of internal shear. This can occur, for example, when a pahoehoe flow travels over steep terrain such as a cliff face, sometimes continuing as an aa flow at the base of the cliff. Under such circumstances, the lava must flow faster, thus increasing internal shearing. During the pahoehoe-to-aa transition, the brittle lava crust becomes thicker, and the interior of the flow becomes more sluggish, as shown by the generally slower rates of movement of active aa flows (meters to hundreds of meters per hour), as compared to pahoehoe flows (hundreds of meters to tens of kilometers per hour). Pahoehoe flows have been clocked at speeds up to about 60 kilometers per hour in open channels and lava tubes.

Block Lavas

A third type of surface is formed on lava flows with extremely high viscosities. In some andesite and most rhyolite lava flows, fragmentation of the lava is very thorough because of the high viscosity of the mass. Great volumes of large, angular blocks are formed, each with relatively smooth (not sharp, clinkery) faces and sharp edges between the faces. Called block lavas, the flows are thick, crumbling masses of fine-grained lava or obsidian that can barely move away from the vent. The noses of block-lava flows are steep (30- to 35-degree) embankments of coarse rubble, and the tops of the flows are pocked with irregular depressions, lava spines, and automobile-sized lava chunks. The jagged rubble of block and aa lava flows is called friction breccia, crumble breccia, or autoclastic (“self-fragmented”) breccia. “Breccia” is a general size term that refers to any deposit composed of coarse volcanic rock fragments (greater than 64 millimeters).

When the output of block lava is relatively low, and magma viscosity is high, steep-sided volcanic domes may form over the vent. Most domes grow from within by internal expansion rather than by adding surface flows of lava. As a result, the thick carapace (shield or shell) of blocky rubble and lava spines is continually shoved aside, and the debris tumbles down the margins of the growing dome. Volcanic domes are very unstable and tend to collapse unpredictably into piles of blocky rubble. Ground-hugging avalanches of hot debris are sometimes jetted outward from the dome during collapse. Hence, volcanic domes are among the most dangerous of volcanic phenomena. Lava domes have repeatedly formed in the crater of Mount St. Helens since its catastrophic eruption of 1980, and the explosion of a Mount Pelée lava dome destroyed the city of St. Pierre on the Caribbean island of Martinique in 1904.

Study of Lava Flows

Lava flows are a major component of most volcanoes. Together with chemical analyses of lava, the physical features of ancient lava flows have revealed much about how lava flows are emplaced. The viscosity of virtually any type of magma can be derived from laboratory experiments and theoretical calculations based on the chemistry of lava.

Observations on active lava flows have revealed much about how lava moves. The temperatures of active lava flows can be estimated in several ways. The incandescent color of the lava gives a general indication of temperature in much the same way that blacksmiths judge the temperature of forged steel: dull red is about 600 degrees Celsius, orange is about 900 degrees Celsius, and golden yellow is about 1,100 degrees Celsius.

A more precise method uses an optical pyrometer to measure the wavelength of visible and near-infrared radiation, which varies uniformly as a function of temperature. The instrument is essentially a telescope in which a wire filament is mounted. The filament is heated by increasing an electric current until its color temperature matches the object being viewed. The temperature of the filament and, hence, the lava can then be calculated.

Both the visual method and the optical pyrometer can give inaccurate results because of atmospheric effects and because only the surfaces of objects are measured (the interiors of lava flows are much hotter than their surfaces). Another method less subject to error but considerably more hazardous in use is the thermocouple, a pair of wires of different compositions welded together at both ends. When one end of the circuit is immersed in hot material (a difficult procedure in viscous lava), an electrical current is generated, its strength depending on the temperature difference between both ends of the circuit. An ammeter near the cold end of the circuit reads the electrical current, from which the temperature at the hot end of the circuit can be calculated.

The use of penetrometers can estimate the viscosities of active lava flows. When a known amount of force is applied to a steel rod of a known diameter, the penetration of the rod into the lava will depend on the viscosity of the lava. Again, this is a superficial measurement that may not give an accurate estimate of the viscosity of the fluid interior of the lava. The chemical compositions of lavas are routinely analyzed, and theoretical viscosities can be calculated from the chemical data. Similarly, if the chemical composition of a lava is known, its crystallization temperature can be accurately estimated, and such a value is usually close to the eruption temperature of the lava. Field measurements are often used with theoretical calculations to better understand lava flow behavior and its physical properties.

In the twenty-first century, scientists continue to make discoveries regarding lava flow and devise increasingly accurate and efficient methods to measure its properties. Researchers at the University of Buffalo have developed a portable penetrometer to measure the viscosity of lava in the field. Scientists have also begun measuring the distance between large standing waves that develop in lava flows under certain conditions to allow them to predict flow velocity and depth. Both of these innovations were used during the 2022 eruptions of Mauna Loa on the Island of Hawaii.

Principal Terms

aa: a Hawaiian term (pronounced “ah-ah”) that has been adopted for lava flows with rough, clinkery surfaces

autoclastic breccia: the clinkery or blocky rubble that forms on some lava flows

block lava: lava flows whose surfaces are composed of large, angular blocks; these blocks are generally larger than those of aa flows and have smooth, not jagged, faces

breccia: a general term for any deposit composed mainly of coarse volcanic rock fragments

pahoehoe: a Hawaiian term (pronounced “pa-hoy-hoy”) that is used in reference to lava flows with smooth, ropy surfaces

pillow lava: a type of bulbous, glassy-skinned lava that forms only when basaltic lava flows erupt under water

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