REDUCING THE RISK FROM VOLCANO HAZARDS

O

Lahars of Mount Pinatubo, Philippines

n June 15, 1991, Mount

Pinatubo in the Philippines exploded in the second

largest volcanic eruption on

Earth this century. This

eruption deposited more than

1 cubic mile (5 cubic kilometers) of volcanic ash and rock

fragments on the volcano’s

slopes. Within hours, heavy

rains began to wash this

material down into the surrounding lowlands in giant,

fast-moving mudflows called

lahars. In the next four rainy

seasons, lahars carried about

half of the deposits off the

volcano, causing even more

destruction in the lowlands

than the eruption itself.

nology and Seismology (PHIVOLCS) intensified their monitoring of the volcano; they

were soon joined by scientists from the U.S.

Geological Survey (USGS) and other organizations. While evaluating the threat from

the volcano, these scientists discovered that

the gentle slopes surrounding Pinatubo had

largely been created by countless giant mudflows of volcanic debris (lahars) from the

powerful ancient eruptions that built the volcano. They also concluded that lahars would

again rush down Pinatubo’s slopes in its next

major eruption, threatening the communities

that were built while it slumbered.

Nature and Causes of Lahars

Explosive eruptions can deposit huge

amounts ash and other volcanic debris on a

volcano’s slopes. Lahars form when water

from intense rainfall, melting snow and ice,

or the sudden failure of a natural dam, mixes

with this loose volcanic material, creating

mudflows that can be particularly dangerous

and destructive. Although lahars contain at

least 40% (by weight) volcanic ash and rock

fragments—making them dense and viscous

like wet concrete—they actually flow faster

than clear-water streams. These mudflows

can rush down the flanks of a volcano at

speeds as great as 40 miles (65 kilometers)

per hour and can travel more than 50 miles

(80 kilometers). Lahars that contain the most

debris (as much as 90%) move the fastest and

are the most destructive.

Conditions that Create Lahars at Pinatubo

Annual rainfall at Mount Pinatubo ranges

from 80 inches (2,000 millimeters) on the

volcano’s northeast flank to more than 160

inches (4,000 millimeters) on its summit and

southwest flank. Nearly all of this rain falls

during the time of the southwest monsoon

(June to September) and the overlapping typhoon season (June to October). Rainfall during typhoons can be as intense as 2 inches

(50 millimeters) per hour. Since the volcano's

cataclysmic June 1991 eruption, the maximum rainfall at Pinatubo in one 24-hour period was more than 30 inches (780 millimeters)—as much rain as many parts of the

United States receive in a whole year!

Ash and other deposits from Mount

Pinatubo’s 1991 eruption destroyed or buried much of the lush vegetation that had covered the volcano. When rainfall on Pinatubo

exceeded about 0.5 inches (12 millimeters)

Background

Before April 1991, when steam explosions blasted out three small craters on the

north flank of Mount Pinatubo on the island

of Luzon in the Philippines, the volcano had

been dormant for 500 years, allowing lush

tropical vegetation to cover its slopes. Eventually, the population of the region grew to

nearly 1,000,000 people, as towns, cities, and

hundreds of villages, as well as the largest

U.S. military base in the Philippines (Clark

Air Base), were built on the broad gentle

slopes surrounding the volcano.

When Pinatubo reawakened in April 1991,

scientists from the Philippine Institute of VolcaU.S. Department of the Interior

U.S. Geological Survey

In August 1991, 2 months after Mount Pinatubo, Philippines, exploded in a cataclysmic volcanic eruption, this

house (see roof) along the Sacobia-Bamban River 15 miles from the volcano was buried by a fast-moving

mudflow of volcanic ash and debris (lahar) caused by heavy rains. Inset photo shows the house before it was

buried. (Photos by Raymundo S. Punongbayan, Philippine Institute of Volcanology and Seismology.)

The village of Lourdes, built on lowlands 15 miles

northeast of Mount Pinatubo, was inundated by

giant, fast-moving mudflows of volcanic debris

(lahars) in the months following the volcano’s

cataclysmic June 1991 eruption. Since that eruption,

lahars have destroyed the homes of more than

100,000 people in the area surrounding Pinatubo.
For more information contact:

U.S. Geological Survey

Cascades Volcano Observatory

5400 Mac Arthur Blvd., Vancouver, WA 98661

Tel: (360) 696-7693, Fax: (360) 696-7866

http://volcanoes.usgs.gov/

See also The Cataclysmic 1991 Eruption of Mount

Pinatubo, Philippines (USGS Fact Sheet 113-97)

U.S. Geological Survey Fact Sheet 114-97

1997

COOPERATING ORGANIZATIONS

Armed Forces of the Philippines

National Disaster Coordinating Council,

Philippines

Philippine Institute of Volcanology and Seismology

U.S. Agency for International Development

United States Air Force

United States Navy

Chris Newhall, Peter H. Stauffer, and

James W. Hendley II

Graphic design by

 Sara Boore and Susan Mayfield

in 30 minutes, rapid runoff down the still bare

slopes quickly grew into torrents that became

lahars by eroding these deposits. Fortunately,

vegetation is beginning to grow on the 1991

deposits, and new stream channels are becoming established, reducing the runoff and

erosion that create lahars.

Lahars at Pinatubo Since 1991

The volume of Pinatubo’s lahars staggers the mind. In the first few years following the cataclysmic 1991 eruption, they

have deposited more than 0.7 cubic miles

(3 cubic kilometers; equivalent to 300 million dump-truck loads) of debris on the

lowlands surrounding the volcano, burying hundreds of square miles of land. During heavy rains, lahars at Pinatubo can

transport and deposit tens of millions of

cubic yards of mud in a single day.

When the largest and fastest lahars reach

the lowlands surrounding Pinatubo, they have

speeds of more than 20 miles (32 kilometers) per hour, are as much as 30 feet (10

meters) thick and 300 feet (100 meters) wide,

and can transport more than 35,000 cubic feet

(1,000 cubic meters) of debris and mud per

second. Within minutes, such powerful lahars can erode many feet into the banks or

floor of a channel. They can carry large boulders, cars, or even bridges and can destroy or

bury almost anything in their paths. When

lahars reach low-lying areas, they spread out,

slow down, and deposit their vast loads of

debris over many square miles.

One particularly dangerous type of lahar

has occurred at Mount Pinatubo as a result

of the damming of stream channels and tributary valleys by the 1991 ash deposits or by

subsequent lahars, creating temporary lakes.

After a few weeks or months, the lakes overtop and quickly erode through their dams,

releasing large volumes of water that generate major lahars. Because these large and

powerful mudflows can occur without warning, even when there is no rain, they are

among the most dangerous of Pinatubo’s lahars. One lake on the Pasig-Potrero River,

on the volcano’s east flank, has formed and

broken out three times (1991, 1992, and

1994), creating giant lahars that have killed

dozens of people.

Mitigation Efforts and Continuing

Hazards

When Pinatubo erupted in 1991, stream

channels were small and were quickly

filled by lahars deposits. Civil engineers

raced to trap the sediment behind small

dams and to confine it between levees, but

most of these measures were too small to

contain the lahars. Near the base of the

volcano, lahars eroded whole city blocks,

and in the lowlands they inundated and buried entire towns.

Since the 1991 eruption, lahars from

Pinatubo have destroyed the homes of more

than 100,000 people. With most old stream

channels filled, lahars can now spread

widely on the gentle slopes and lowlands

surrounding Mount Pinatubo, threatening

successors to the early dams and levees.

Fortunately, by 1995, the volume of material carried by Pinatubo’s lahars had already decreased to less than a quarter of

that carried in 1991, and new towns had

been built on high ground for many of those

displaced or threatened by lahars. Nonetheless, 100,000 people remain at risk from

the volcano’s lahars.

Lahar Warnings

Once the potential for lahars at Mount

Pinatubo was recognized, scientists quickly

prepared a preliminary hazard map. Since

mid-June 1991, when violent eruptions

triggered the volcano’s first lahars in 500

years, a system to monitor and warn of lahars has been in operation. Radiotelemetered rain gauges provide data on

U.S. DEPARTMENT OF THE INTERIOR • U.S. GEOLOGICAL SURVEY

Within hours of Mount

Pinatubo’s explosive

June 15, 1991,

eruption, heavy rains

began to wash the ash

and debris deposited by

this explosion down into

the surrounding

lowlands in giant, fastmoving mudflows called

lahars. By 1993, lahars

had already caused

more devastation in the

lowlands than the

eruption itself.

Fortunately, a system

to monitor and warn of

lahars was rapidly

established in the days

following the eruption

by the Philippine

Institute of Volcanology and Seismology,

U.S. Geological Survey,

and other organizations.

This system has saved

hundreds of lives by

enabling warnings to

be sounded for most

but not all major lahars

at Pinatubo. (U.S.

bases have reverted to

Philippine control since

1991.)

rainfall in lahar source regions, acoustic

flow monitors on stream banks detect

ground vibration as lahars pass, and

manned watchpoints further confirm that

lahars are rushing down Pinatubo’s slopes.

This system has enabled warnings to be

sounded for most but not all major lahars at

Pinatubo, saving hundreds of lives.

yyyy

San Fernando

Cabanatuan

Manila

0 10 MILES

0 10 KILOMETERS

Angeles

City

Tarlac

Olongapo

CLARK

AIR BASE

 (U.S.)

MOUNT

PINATUBO

MOUNT

 NATIB

MARIVELES

MOUNTAIN

BASA

AIR BASE

SUBIC BAY

NAVAL STATION (U.S.)

CUBI POINT

NAVAL AIR STATION (U.S.)

MANILA BAY

SUBIC BAY

S O U T H

C H I N A S E A

Pasig- Potrero River O'Donnell River

Marella River

Bamban River

SacobiaPampanga River

Pampanga River

PHILIPPINES

MOUNT PINATUBO

Manila

Extent of 

ash cloud

from the

June 15, 

1991

eruption

Lahars