May 27, 1999

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Pahoehoe and `a`a lava flows

Lava flows are one of the most common hazards produced by active volcanoes. Here in Hawai`i, they may endanger property but seldom endanger people's lives. The threat they pose is largely dependent on how the flow moves, which is in turn determined by the properties of the lava and the landscape. The two main types of lava, pahoehoe and `a`a, differ in various properties, such as crystal and gas bubble content, as well as having slight differences in temperature. These properties, along with the steepness of the underlying slope, determine the ease with which a lava flow moves.

Once cooled, pahoehoe and `a`a are easily identifiable by their characteristic textures. Pahoehoe has a smooth, billowy, ropy surface and has at times been likened to the surface texture of a pan of chocolate brownies. On the other hand, `a`a has a rough, jagged and clinkery surface. Although individuals are often seen crossing pahoehoe wearing rubber slippers (definitely not recommended), it is difficult to imagine traversing `a`a in anything less than sturdy footwear. In an eruption with a high effusion rate - where a lot of lava is being discharged at once - `a`a flows tend to form. If the rate of effusion is low, a pahoehoe flow is more likely to develop.

In an `a`a flow, widespread fracturing of the exterior of the flow allows the steady advance of the whole front as a single unit. In a pahoehoe flow, more localized cracking of the crust occurs, allowing the flow to advance by the propagation of individual toes or lobes. These toes or lobes of a pahoehoe flow may advance by flowing around and slowly burning objects in their path. In contrast, an `a`a flow may move forward as a steep-sloping wall of lava chunks that can crash through an area, pushing down whatever is in its path, like the track of a bulldozer. An `a`a front often advances in surges, with the front gradually getting higher before a surge begins. In one hour it may only move a few meters (yards), but then suddenly surge forward 100 meters (yards) in a few minutes, with the front reduced to its original thickness in the process.

`A`a flows usually develop where lava is being transported through open channels on the ground surface. Open channels allow the lava to cool with increasing distance from the vent, causing it to become thicker and more pasty. As this thickened lava moves along it squeezes out many of the gas bubbles trapped inside it, so `a`a tends to lack the tiny voids, called vesicles, that are left behind when gases are trapped by solidifying lava. The vesicles that are present are usually irregular in shape due to the continued stretching motion of the lava right up to the point where it finally solidifies. The constant heat loss in an `a`a flow causes many more crystals to form than in a pahoehoe flow. The greater abundance of crystals further thickens the consistency of the flow, making it even more pasty and resistant to flow.

Pahoehoe flow fields are often fed by a well-insulated underground tube system that carries the lava long distances and prevents it from cooling appreciably. Fewer gas bubbles escape in pahoehoe flows than in `a`a flows so they typically have more vesicles. These vesicles are more nearly spherical in shape because they have not undergone as much deformation as those in `a`a flows.

Often what starts out as a pahoehoe flow may go through the transition to `a`a when there is a change in conditions, such as a sudden steepening in slope, or just by the continuous loss of heat and gas as distance from the vent increases. However, once the `a`a has undergone irreversible processes such as losing gas or forming crystals, it generally does not change back to pahoehoe. It is common to get both pahoehoe and `a`a flows from the same eruption with no difference in chemical composition, showing that what causes pahoehoe or `a`a to form is largely related to the physical conditions the lava undergoes once erupted.

Eruption Update

Eruptive activity of Kilauea Volcano continued unabated during the past week. Lava is erupting from Pu`u `O`o and flowing through a network of tubes from the vent to the sea. Surface flows from breakouts of the tube system were observed on the coastal flats. Lava is entering the ocean near Kamokuna and enlarging the bench. The public is reminded that the ocean-entry area is extremely hazardous, with explosions accompanying unpredictable collapses of the new land. The steam clouds are highly acidic and laced with glass particles.

There were six earthquakes reported felt for the week ending on May 27. One of these, a magnitude 2.4 earthquake, was an aftershock of the April 16 magnitude-5.6 earthquake and was felt in Pahala on the 24th. Four were felt at and near Kilauea's summit on May 26: at 6:01 a.m. (4.3, felt as far away as Hilo), at 6:04 a.m. (3.3), at 8:16 a.m. (3.5), and at 11:21 a.m. (3.7). Another earthquake, a magnitude 2.6, was felt on May 27 at 1:28 a.m. in Kealakekua.