Lava

Pahoehoe near the coast of Kilauea. Photo by Steve Mattox, 1989.


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Background:

There are three types of lava and lava flows: pillow, pahoehoe, and aa. Pillow lavas are volumetrically the most abundant type because they are erupted at mid-ocean ridges and because they make up the submarine portion of seamounts and large intraplate volcanoes, like the Hawaii-Emperor seamount chain. Pahoehoe is the second most abundant type of lava flow.

Eruptions under water or ice produce pillow lava. James Moore of the U.S. Geological Survey made the first underwater observations of the formation of pillow lavas (Moore and others, 1973). Moore and his coworkers studied lava from the Mauna Ulu eruption They described pillow lavas as elongate, interconnected flow lobes that are elliptical or circular in cross-section (Moore, 1975). This photo shows pillow lava forming off the south coast of Kilauea volcano, Hawaii. Photograph by Gordon Tribble and courtesy of U.S. Geological Survey.

Pillow lava. Photograph courtesy of U.S. Geological Survey.

Lava flows near the coast tend to spread out laterally and enter the ocean over a large front. Several lava tubes may be active across the flow front. Larger lava tubes feed down slope and maintain pressure in the growing lobes that extend into the water. Flow lobes develop most readily on steep slopes and result in stacks of pillow "tongues" that parallel the offshore slope. This offshore stack of lava produces beds of rubble parallel to the offshore slope. Geologists call these steeply dipping beds foreset beds.

On November 8, 1992, lava entered the ocean just east of the archeological sites at Kamoamoa on the south flank of Kilauea volcano. By the end of the month, nearly all of Kamoamoa was buried under lava. By May 1993, a lava delta had extended the coastline about 1,600 feet (500 m) to the south. This aerial view is to the west. Photograph by Christina Heliker, U.S. Geological Survey, May 12, 1993.

The foreset beds are part of a lava delta that grows towards the ocean and provides a platform on which subaerial lavas can extend, thus adding new land to the island. However, some lobes are not continuous and break apart to form pillow sacks, thus adding rubble to the flank of the volcano. The active front of a lava delta is called a bench. Like the delta, the bench is built on rubble. However, the bench is relatively unstable and can collapse, falling into the ocean (Mattox, 1993).

A bench collapsed at the Lae Apuki entry in April 1993, removing a block 690 feet (210 m) long, 46 feet (14 m) wide, and 26 feet (8 m) thick from the front of the delta. A visitor, standing on the bench at the time of the collapse, disappeared into the ocean. This aerial view shows an active bench adding new land out from the old sea cliff (bottom of photo). Dark areas are black sand. Photograph by Christina Heliker, U.S. Geological Survey, August 25, 1994.

Pillow lavas are also found near the summit of Mauna Kea These pillow lavas were produced by a subglacial eruption that occurred 10,000 years ago. The pillow is about 3 feet (1 m) in diameter and has a glassy rim. Figure 21.11 from Porter, 1987.

Pillow lavas can also form when flows enter a river or lake. The pillows in the photo formed in the Wailuku River above Hilo, Hawaii about 3,500 years ago. The round cobbles were transported by the river. Photograph by Jack Lockwood, U.S. Geological Survey, June 14, 1982.

Aa lava on pahoehoe, Hilina Pali, Kilauea. Photo by Steve Mattox, 1988.

"Pahoehoe" and "aa" are Hawaiian words first introduced into the geologic literature by Dutton (1884).

Pahoehoe lava, Kilauea, Hawaii. Photograph by J.D. Griggs, U.S. Geological Survey, June 15, 1989.

Pahoehoe lava is characterized by a smooth, billowy, or ropy surface. A ropy surface develops when a thin skin of cooler lava at the surface of the flow is pushed into folds by the faster moving, fluid lava just below the surface.

Pahoehoe flows can evolve into lava tubes. One way that tubes form is by the crusting over of channelized lava flows. As the crust on a flow becomes thicker, it insulates the lava in the interior of the flow. The lava drains down slope and feeds the advancing front or flows into the ocean. When the eruption stops or the vent is abandoned, the lava drains from the tube. Thurston lava tube is an excellent example of a lava tube. The rock surrounding a lava tube serves as a insulator to keep the lava hot and fluid. Because the lava remains hot, it can travel great distances from the vent. For example, tube-fed pahoehoe lava traveled 7.3 miles (11.7 km) from the Kupaianaha vent to the village of Kalapana (Mattox and others, 1993). Photo of volcanologist looking through a skylight to see inside of a lava tube. Photograph by J.D. Griggs, U.S. Geological Survey, February 2, 1989.

Pahoehoe flows tend to be relatively thin, from a few inches to a few feet thick. Road cuts and craters expose stacks of lava flows that make the volcano.

Aa is characterized by a rough, jagged, spinose, and generally clinkery surface. Aa flows advance much like the tread of a bulldozer. This photo is looking across an aa channel. Note the character of the aa that makes the wall of the channel. Photo by Steve Mattox, October 12, 1990.

Front of an advancing aa flow from the 1984 eruption of Mauna Loa. The molten core of an aa flow is visible. Photo by Peter Lipman, U.S. Geological Survey, March, 1984.

Interior of an aa lava flow. Note dense interior and rubble at base and top. Photo by Steve Mattox, July 25, 1995.

The interior of the flow is molten and several feet thick. The molten core grades upwards and downwards into rough clinkers. As the flow advances, clinker on the surface is carried forward relative to the molten interior. The clinkers continue to move forward until they roll down the steep front. The clinkers are then overridden by the molten core. Aa lava flows tend to be relatively thick compared to pahoehoe flows. Aa flows from the 1984 Mauna Loa eruption ranged from 6 to 23 feet (2-7 m) in thickness.

Unlike the advancing front of a pahoehoe flow, which is fed by a lava tube, an advancing aa flow is fed by a channel (Lipman and Banks, 1987).

Channel feeding aa lava flow, 1984 eruption of Mauna Loa. Photograph courtesy of U.S. Geological Survey, March 25, 1984.

During the early episodes of the current eruption, aa flows up to 36 feet (11 m) thick surged through the Royal Gardens subdivision at rates as great as 108 ft/min (33 m/min)(Neal and Decker, 1983). The molten core of the flow is exposed. Note vehicles for scale. Photograph by R.W. Decker, U.S. Geological Survey, July 2, 1983.

Since the mid-1800s, geologists have tried to explain the causes of pahoehoe and aa lava. Several factors were offered to explain the transition: impediment of flow by obstacles, flowage during cooling, quantity of lava, conditions under the flow, and deep cooling. In the last few decades, with careful observations of numerous lava flows, geologists have reached a better understanding of the transition of pahoehoe to aa. One important influence is the viscosity of the lava. Viscosity is the resistance of a fluid to flow. For example, molasses has a higher viscosity that water. The following paragraphs outline some of the more important observations on the transition of pahoehoe to aa.

Pahoehoe lava crossing Chain of Craters Road. Photograph by R.B. Moore, U.S. Geological Survey, November 16, 1979.

A study by Macdonald (1953) noted several generalizations concerning the transition from pahoehoe to aa:

  1. Pahoehoe and aa can be found as parts of the same lava flow, with no significant difference in chemical composition between the two forms.

  2. Pahoehoe may change to aa, but never the reverse.

  3. Most active pahoehoe flows are less viscous and have higher temperatures than aa flows.

  4. Vesicles in pahoehoe tend to be spheroids, whereas those in aa tend to be irregularly shaped, suggesting deformation caused by continued movement during final stages of solidification.

  5. Even though aa tends to be more viscous, molten lava of approximately the same initial viscosity may form either pahoehoe or aa.

  6. In addition to the effects of increasing viscosity, lava tends to change to aa when subjected to flow turbulence and internal shearing, such as during fountaining, flowing down steep slopes or over precipices, or during prolonged flowage for great distances. Macdonald concluded that a critical relationship between viscosity and the amount of internal disturbance due to flowage determines whether pahoehoe or aa is formed.

Pahoehoe lava at night. Steve Mattox, October, 1990.

A more recent paper by Peterson and Tilling (1980) included the rate of shear strain as a measure of Macdonald's internal shearing. An example of rate of shear strain is how quickly or slowly force is applied across a deck of cards. Factors influencing viscosity or rate of shear strain are listed below:

viscosity rate of shear strain
temperature flow velocity and duration
gas content flow dimensions
lava vesicularity ground slope
crystallinity channel configuration

Peterson and Tilling (1980, p. 273) suggested two general conditions that determine whether pahoehoe or aa forms:

  1. If lava slows, cools, and stops in direct response to the corresponding increase in viscosity only, it retains its pahoehoe form.

  2. If lava is forced to continue flowing after a certain critical relationship between viscosity and rate of shear strain is achieved, the lava changes to aa.

Pahoehoe on black sand, Kamoamoa. Photo by Steve Mattox, November, 1992.

Peterson and Tilling called this critical relationship the "transition threshold." They found that if the rate of shear strain is high, the transition threshold is reached at a lower viscosity than if the shear strain rate is low. The converse is also true. If the viscosity of the lava is high, a relatively low rate of shear strain may achieve the transition threshold, and the lava changes to aa.


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