What are the different types of basaltic lava flows and how do they form?
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. Image Credit: Gordon Tribble/USGS
Eruptions under water or ice make pillow lava.
Pillow lavas have elongate, interconnected flow lobes that are elliptical or circular in cross-section.
Pillow lavas are often considered important when trying to decipher old rock sequences because they indicate the presence of water. However, you have to be very careful to make sure that you are not looking at regular old pahoehoe toes, which of course, indicate dry land. Many of the features that supposedly can be used to tell the difference between the two don't always work. The only absolute way to know that you are looking at true pillow lavas is to find water-lain sediments between the individual pillows. You might think "wait a minute, sediments are lain down really slowly, how are they going to get between the pillows while they're active?" Actually, when lava is flowing under water, there is a lot of sediment generated as pieces of the lava fall off during the rapid collapsing of the pillows as the quickly chill. Pillow lavas are essentially the underwater equivalent of pahoehoe. They form from low effusion-rate eruptions of fluid basalt lava. They have a rounder form than pahoehoe toes, mainly because of the ability of water to help buoy them up (gravity doesn't flatten them out so much). Also in contrast to pahoehoe toes, pillow lavas tend to have thicker skins of glass (because they are quenched more quickly), less vesicular skins (because even shallow water pressure is able to prevent bubbles from expanding very much), and generally radial fractures (in contrast to the generally concentric flow banding seen in pahoehoe toes). Nevertheless, it is often difficult to tell the two types of lava apart in exposures. The only way to absolutely know that the flows you're looking at are pillow lavas rather than pahoehoe toes, is to find submarine sediments (such as hyaloclastite debris formed from the violent reaction of lava and water) between the pillows.
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.
Pahoehoe is the second most abundant type of lava flow.
Pahoehoe lava is characterized by a smooth, billowy, or ropy surface.
Pahoehoe flows tend to be relatively thin, from a few inches to a few feet thick. In map-view the flows tend to be narrow and elongate.
Image Credit: Steve Mattox, 1989. (Kilauea)
A'a is characterized by a rough, jagged, spinose, and generally clinkery surface. Aa lava flows tend to be relatively thick compared to pahoehoe flows. During the early episodes of the current eruption of Kilauea volcano, 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).
Image Credit: R. W. Decker/USGS July 02, 1983.
The A'a / Pahoehoe difference:
If lava cools slowly and does not move too fast it forms smooth ropy lava called pahoehoe.
However, if it cools quickly and moves fast it can tear into clinkery pieces called a'a.
Temperature and gases certainly influence whether the lava becomes aa or pahoehoe. Probably the two biggest factors are viscosity and rate of shear strain. Viscosity is just how sticky something is (how much it resists flowing). An example of rate of shear strain is how quickly or slowly force is applied across a deck of cards.
Some factors influencing viscosity or rate of shear strain are listed below:
- flow velocity and duration
- gas content
- flow dimensions
- lava vesicularity
- ground slope
- 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.
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 a'a.
People often ask if there is a compositional difference between aa and pahoehoe lava. There is no systematic chemical difference between aa and pahoehoe lava. Lavas with the identical compositions can form both aa and pahoehoe. Lavas that have slight chemical differences tend to have different temperatures and viscosity's but the critical factor influencing the transition from pahoehoe to aa is the viscosity of the lava.
Other types of lavas include block lava, which has a surface of large angular blocks, and rhyolite lava. These two types are associated with lava chemistries other than basalt. They tend to be very thick (10-200 meters) and slow moving.
Source of Information:
Peterson, D.W., and Tilling, R.I., 1980, Transition of basaltic lava from pahoehoe to aa, Kilauea Volcano, Hawaii: field observations and key factors: Journal of Volcanology and Geothermal Research, v. 7, p. 271-293.