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:
- Pahoehoe and aa can be found as parts of the same lava flow, with no
significant difference in chemical composition between the two forms.
- Pahoehoe may change to aa, but never the reverse.
- Most active pahoehoe flows are less viscous and have higher
temperatures than aa flows.
- 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.
- Even though aa tends to be more viscous, molten lava of approximately
the same initial viscosity may form either pahoehoe or aa.
- 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:
- If lava slows, cools, and stops in direct response to the
corresponding increase in viscosity only, it retains its pahoehoe form.
- 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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