EIGHTEEN months after the Pinatubo erupted, much of the landscape in the Zambales, Pampanga and Tarlac provinces is blanketed by what looks like slurry, but is in fact volcanic sediment deposited by lahars -- thick, flowing mixtures of volcanic material and water.

Lahars carry much more sediment than the muddiest rivers. Lahars containing upto 60 per cent of volcanic sediment are known locally as lahar na malabnaw; when the sediment content is higher, they are known as lahar na malapot.

Nearly all lahars are caused by rain falling on pyroclast (rocks formed as a result of volcanic eruption) scattered on volcano slopes. During the Pinatubo eruption, the relatively weak Typhoon Diding swept by about 100 km to the northeast, drenching the volcano with heavy, sustained rains. This caused the first lahars to form.

Lahars are so thick they can easily drag automobiles and huge boulders along and bulldoze buildings and bridges in their path. Lahars frequently overflow their banks and sweep away houses or pile sediment high around them.

Lahars can flow at speeds of 25 km per hour and more on steep upper slopes. Lower down, they slow to about 10 km per hour and on the flat lowlands, they creep along at about 3 km per hour. The depth of a lahar varies from 1 m to 20 m, depending on whether it flows through lowlands or narrow valleys. All but the largest malabnaw lahars deposited most of their sediment within 40 km of Pinatubo.

Transformations
A malabnaw lahar can change back and forth into a malapot lahar as it flows, depending on rainfalls and water from tributaries. Fine-grain sand and silt in lahars are often carried further downstream to rivers. This reduces the capacity of lowland channels and there are many instances of river channels that were once upto 20 m deep now just about 5-10 m deep. This, of course, increases the possibility of flooding.

According to vulcanologists, there are five factors to consider in assessing the risk from lahars: proximity to lower reaches of lahar-prone rivers; elevation of settlements; size of lahars; carrying capacity of rivers, and the possibility of dikes collapsing.

The Philippine Institute of Volcanology and Seismology has identified places that are at risk and prepared a 10-year lahar outlook that it concedes is not fail-safe. The lahar problem, unlike volcano explosions and eruptions, will not disappear overnight. Volcano sediment will be washed down into the area surrounding Pinatubo through the coming decade. Rough estimates are that about 800 million cum of sediment were carried into the lowlands by lahars in 1991 alone. No wonder that many experts rate the devastation caused by the eruption itself to be not as significant as the damage that lahars could cause in the years ahead.