How Volcanoes Work



The single most important factor on the form of a lava flow is probably the effusion rate, or rate of discharge, measured in cubic meters per second. The higher the effusion rate, the greater the distance traveled before cooling lowers the viscosity and impedes flow. The effusion rates for andesite to dacite lavas (10 to 0.05 cubic meters per second) are a few orders of magnitude lower than those for basalt (0.5 to 5000 cubic meters per second). Thus, basalt lavas are generally much more extensive than the more felsic rock types. Basaltic lavas extruded at relatively low effusion rates generally produce a number of small flow units extruded closely in time so that they overrun each other in anastomising channels. Upon cooling, these flow units produce compound flows. Basalt lavas with higher effusion rates, however, produce extensive flows, often extruded as large-volume flowsheets. These flows are composed of a single cooling unit and are called simple flows. The high effusion rates associated with flood basalt provinces suggest that they contain large-volume simple flows.

Compound flow composed of numerous, thin flow units

 Simple flows

 Compound flow composed of numerous, thin flow units

 Simple flows


As a lava flow cools and crystallizes into a coherent rock, it begins to contract. Shrinkage of the rock mass results in the development of numerous cracks, called joints. Although lava flows can display a variety of jointing patterns, many of the thicker simple flows exhibit a three-tiered character: from bottom to top, a lower colonade, a middle entablature, and an upper colonade. The lower colonade is composed of columnar joints, which develop perpendicular to the cooling surface, at the base of the flow, where they intersect to form a series of columns. The columns may vary in length from one to five meters, with diameters that are generally less than one meter. Each column is polygonal in cross-section (typically hexagonal) and bounded by 4-to-8 joints. If present, the entablature is composed of an array of closely spaced, subvertical joints, which forms a more convoluted pattern of cracks known as hackly jointing. The upper colonade is generally poorly develped or non-existent. If present, the crude columnar joints are vesicular and much shorter than those displayed in the lower colonade.

 Jointing in basalt
Jointing in basalt -- This lava flow from the Staffa, Scotland exhibits a well-developed lower colonade and a hackly jointed entablature.


Lava ChannelsAlthough highly effusive eruptions may advance downslope as massive sheets of basaltic lava, such flows are rare in the historic record. Fluid basalt will more commonly move down slope by creating its own channelways above gently sloping terrains, or by flowing down in pre-existing stream channels. A well-developed lava channel is shown here near a series of eruptive vents on the northeast rift zone of the Mauna Loa Volcano, Hawaii. Lava channels can develop in both pahoehoe and a'a basalt. Once established, lava channels will often develop lava levees along their lengths. In pahoehoe flows, natural levees will be constructed as the channel periodically overflows, thus allowing the lava to congeal and cool along the banks of the channel. In a'a flows, levee buildup can also occur by the bulldozing effect of the moving lava, which will push a'a blocks onto the lava banks. Although lava channels may initially be restricted to a topographic valley, as levees build up over time, they may eventually rise above the surrounding ground surface. Sometimes several small lava channels will flow around subdued hills or high ground to produce a kipuka, an Hawaiian term for "island." In Hawaii, these features are generally noted by their mature vegetation, which stands in contrast to the stark lack of vegetation in the younger, surrounding basalt flows.

One characteristic of a'a channels is the occurrence of nearly spherical masses of lava with diameters of a few inches to over ten feet. These features are called accretionary lava balls. When broken apart, they reveal a spiral structure which appears to form when a small fragment of solidified lava rolls along the surface of an a'a flow. The lava then adheres to the rolling mass, much like snow sticks to a rolling snowball.

Lava Channel and Levee
Accretionary Lava Ball

 Lava Channel and Levee


 Accretionary Lava Ball


Lava Lakes and Lava FallsIt is not unusual for lava to accumulate in volcanic craters, filling the craters to a high level to generate lava lakes. If the effusion rate is constant and high, the lava may rise above the crater rim, breaching the crater and spilling out onto the adjacent surface. One example of this is the Kupaianaha lava lake in Hawii, which overtopped it's crater in 1986. A much older example shown in the image to the left. This is the Jabal Hil scoria cone, located on the Harrat Kishb lava field, about 150 kilometers northeast of Mecca in western Saudi Arabia. The Jabal Hil flow field is Post-Neolithic in age and composed of anastomosing pahoehoe and a'a lava flows that have cascaded down from the crater rim due to overspilling of a crater lake.

 Lava Cascades As lava descends down an irregular topographic surface it will occasionally flow over an abrupt escarpment to produce lava cascades, or lava falls, such as those shown here. 

Lava Drapery 
As the effusive rate decreases, and the lava begins to cool, the basalt may solidify in place to cover the escarpment in an apron of crystalline rock, called a lava drapery.


On flat or gentle slopes, the surfaces of pahoehoe flows are sometimes marked by elliptical domed structures, from 2 to 10 meters high, called tumuli. These are best developed pressure ridgeson the surface of flows that are ponded in depressions, like craters or calderas. A tumulus develops when slow-moving lava beneath a solidified crust wells upward. The brittle crust usually buckles to accommodate the inflating core of the flow, thus creating a central crack along the length of the tumulus. These structures sometimes grade into elongate forms called pressure ridges, which commonly develop subparallel to the flow direction at flow margins, but perpendicular to the flow direction in the central portions of the flow. Localized upheaval of the underlying lava to produce tumuli can result from a variety of mechanisms, including (a) obstruction of lateral flow downstream behind a slow-moving lava front, (b) diversion of flow upward against an irregular topographic surface, (c) local upheaval from the volatilization of groundwater, and (d) deflation of an inflated pahoehoe sheet flow leading to differential sagging and crustal buckling.


Rootless eruptions are not connected at depth to a magma chamber, but rather result from surface eruptions on pahoehoe surfaces. When the pahoehoe crust thickens and the underlying lava becomes cool, viscous, and gas-depleted, pasty squeeze-uplava can squeeze up through the axial fracture of the tumulus like cold toothpaste. This process creates a protrusion of cheesy basalt commonly called a squeeze-up. Squeeze-ups can occur as bulbous mounds on pahoehoe surfaces, or as linear wedges in the axial core of tumuli. It the lava remains hot and liquid, local extrusions through the cracked pahoehoe surfaces can produce rootless cones of spatter, called hornitos, or driblet cones. Such features can erupt through the axial cracks of tumuli as well as through cracks and openings overlying active lava tubes.


The chilling and crystallization of basaltic lava around the sides, bottom, and top of lava channels produces a rock-encased conduit called a lava tube. The surrounding crystalline basalt remains relatively hot and insulates the interior lava in the lava tube from further crystallization. Although lava tubes are generally restricted to pahoehoe lava flows, they can form in some thick a'a flows, like those that occur on Mt. Etna. The tube morphology provides a very efficient mechanism for basaltic lava flows to travel great distances away from their source without significant heat loss. Their lateral extents are highly variable. Although lava tubes are typically < 1 km long, they may exceed several kilometers, as demonstrated, for example, by a lava tube in Queensland, Australia that is > 100 km long. The diameters of lava tubes are also highly variable, from < 1m to as much as 15m.

Lava TubeHigh lava temperatures of about 1150 degrees Centigrade (~ 2100 degrees Fahrenheit) are maintained throughout the tube system. These high temperatures allow the lava to remain very fluid. The high fluidity of lava in lava tubes and lava channels leads to high transport rates. During the 1984 eruption of Mauna Loa, for example, channel flow was measured by USGS volcanologists at 35 mph. Lava speeds within the current tube system associated with the Pu'u O'o eruption have been measured up to 23 mph. This fast moving, turbulent lava, combined with the high lava temperatures, can result in thermal erosion of the tube interior. Once established, well-insulated lava tubes can thus erode both downward and laterally over time. The turbulence from fluid motion and from constantly escaping gases generates spatter fragments that coat the walls of many lava tubes. Eventually, the eruption will cease and the remaining lava in the tube will drain downslope to expose a hollow, inactive lava tube. Such tubes typically contain a flat floor and exhibit high-lava marks on the conduit walls. A thin coating of agglutinated spatter is often found on the tube walls, together with icicle-shaped lava stalactites hanging from the roof. Mound-like lava stalagmites derived from spatter dripping onto the floor of tubes are less common. The example shown here is the well-known Thurston lava tube near the Kilauea summit caldera in Hawaii Volcanoes National Park (Photo courtesy of the USGS).

SkylightBecause lava tubes remain buried, they often go unrecognized on the surface. However, many lava tubes are delineated on the surface by a linear or curvilinear series of collapse depressions, called skylights along the axis of the tube. On active lava tubes, skylights provide a unique view of actively flowing incandescent lava in the lava tube system. The example shown here from a skylight developed above an active lava tube associated with the Kupaianaha eruption in 1989 on the east rift system of the larger Kilauea volcano, Hawaii.

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