January 13, 2000

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

Mapping new lava as easy or difficult as finding the flow margin

Geologists everywhere like to study rocks in the field, but few of them get to map rocks that are still molten and on the move. Here in Hawai`i, we map the new lava flows erupted from Kilauea Volcano to determine their extent and volume. Mapping also helps us to keep track of how fast flows are advancing toward inhabited areas.

In the past three years, the way we map has undergone a revolution. Instead of trying to figure out a flow's location and then mark it on paper maps or aerial photographs, we now map chiefly with portable GPS receivers. These devices receive the radio waves from global positioning system satellites to determine the position of the receiver on the Earth's surface. Our portable receivers record positions to within 5 m (15 feet), which is well suited for our lava-flow maps.

To map flows, we walk around them as near their edges as possible, noting their characteristics and position. With GPS receivers, the position is recorded by pushing a button, and the data are recorded in the computerized memory of the device. Back in the office we download the coordinates and connect the dots, so to speak. The result is an outline of the lava flow.

It's easy to distinguish the margin of a new flow where it laps against substantially older flows. New pahoehoe is a shiny, silvery gray color, because none of its glass, which forms most of the rock, has altered chemically or physically. If the flow is less than a week old, we may feel substantial heat as we approach it or step onto it. Shimmering heat waves in the air above a recent lava flow may give it away before we even get close.

Sometimes we don't get around to mapping a flow in detail until after it has cooled. If the new flow overlies another flow that is only a few days older, the heat contrast between the flows is difficult to recognize. In these instances, we rely on other clues to determine the boundary of the new flow.

Here are some useful tricks, if you want to try it. If the flow has burned trees or brush along its margin, ash from the burned vegetation will still be lying on a recently emplaced flow. It takes a few weeks of wind and rain to wash the wood ash away.

Where a new flow overlaps another recent flow, subtle differences in color may persist for a few weeks. Fresh pahoehoe loses its brilliant shine fairly slowly, so the contrast between flows that differ in age by only a few weeks may be visible even if the youngest flow has cooled completely. This technique works when the light is low, but the color differences can be impossible to see in the glare of the noon-day sun.

`A`a flows also change color slowly, from grayish red to orange or reddish brown. This change results from the oxidation of iron-bearing minerals as the flow cools. The process is analogous to the rusting of a cast-iron skillet, although the latter occurs at much lower temperatures. The color contrasts help to distinguish flows that differ in age by several weeks.

Lava flows begin to revegetate surprisingly quickly on Kilauea's east rift zone, where rainfall is high. Flows only a few months old may have sparse tiny ferns sprouting in overhangs and clefts where moisture collects. Sometimes we'll think we're mapping the margin of a flow only three weeks old when suddenly a small fern catches our eye, and we realize we've wandered onto a flow that is five or six months old. A few choice words are spoken, as we retrace our steps to where we went astray.

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 tube to the seacoast. The ocean entry at Lae`apuki has been active, but diminishing all week. Surface breakouts of this flow on Pulama pali and the coastal flats have provided good viewing at night from the end of the Chain of Craters Road to Highcastle. The public is reminded that the ocean-entry area is extremely hazardous, with explosions accompanying unpredictable collapses of the new land. The active lava flow is hot and has places with very thin crust. The steam clouds are highly acidic and laced with glass particles.

Another flow has crossed the eastern boundary of the National Park and cut the Royal Gardens access road in two places. A third is descending Pulama pali just inside the Park.

There were no earthquakes reported felt during the week ending on January 13.