VOLCANIC HAZARDS

Source: Department of Science and Technology PHILIPPINE INSTITUTE OF VOLCANOLOGY AND SEISMOLOGY

FORMATION OF A VOLCANO

The term VOLCANO signifies a vent, hill or mountain from which molten or hot rocks with gaseous

materials are ejected. The term also applies to craters, hills or mountains formed by removal of pre-

existing materials or by accumulation of ejected materials.

Subduction Zone Volcanism (Convergent)

Subduction zone volcanism occurs where two plates are converging on one another. One plate

containing oceanic lithosphere descends beneath the adjacent plate, thus consuming the oceanic

lithosphere into the earth's mantle. This on-going process is called subduction.

Classification of Philippine Volcanoes

In the Philippines, volcanoes are classified as active, potentially or inactive. An ACTIVE volcano has

documented records of eruption or has erupted recently (within 10,000 years). Although there are no

records of eruption, a POTENTIALLY ACTIVE volcano has evidences of recent activities and has a

young-looking geomorphology. An INACTIVE volcano has not erupted within historic times and its

form is beginning to be changed by agents of weathering and erosion via formation of deep and long

gullies.

Mayon (active)

Malinao (Potentially active)

Cabalian (inactive)

VOLCANIC HAZARDS

Volcanic hazard refers to any potentially dangerous volcanic process (e.g. lava flows, pyroclastic flows,

ash).

A volcanic risk is any potential loss or damage as a result of the volcanic hazard that might be incurred

by persons, property, etc. or which negatively impacts the productive capacity/sustainability of a

population. Risk not only includes the potential monetary and human losses, but also includes a

population's vulnerability.

Primary Volcanic Hazards

Directly associated with eruption

Lava flow, dome growth

Pyroclastic flow, pyroclastic surge, lateral blast

Tephra fall - ash fall, volcanic bomb

Volcanic gas

Lava Flow

Lava flow is a highly elongated mass of molten rock materials cascading downslope from an

erupting vent. The lava flow being extruded has low silica and low water contents.

Rate of flow: 3 km/day (slightly high viscosity) or 45 km/hour (low viscosity)

Speed and geometry of lava flows depend on local topography. Steep slopes encourage

faster and longer flows than gentle slopes or terrain.

Lava flow during the 1968 eruption of Taal Volcano

Pyroclastic Flow

Pyroclastic flow refers to hot dry masses of fragmented volcanic materials that move along the

slope and in contact with ground surface. This includes:

pumice flow

ash flow

block-and-ash flow

nuee ardente

glowing avalanche

Pyroclastic Surge

Pyroclastic surges are turbulent low-concentration density currents of gases, rock debris and in

some cases, water, that move above the ground surface at high velocities. They overtop high

topographic features and are not confined to valleys. However, this type of flow usually does

not travel as far as a pyroclastic flow. (Scott, 1989).

A base surge, a type of pyroclastic surge, is usually formed when the volcano initially starts to

erupt from the base of the eruption column as it collapses. It usually does not travel greater

than 10 kilometers from its source.

Tephra Falls

Tephra falls are showers of fine- to coarse-grained volcanic materials and other airborne

products of a volcanic eruption

They may consist of pumice, scoria, dense lithic materials or crystals or combination of the four.

Particle size:

� less than 2 mm diameter (ash)

� 2-64 mm diameter (lapilli)

� 64 mm diameter (blocks and bombs)

ASHFALL

-showers of fine- to coarse-grained volcanic materials and other airborne products of a volcanic

eruption. Ashfall distribution/dispersal is dependent on prevailing wind direction.

Volcanic Gas

Volcanic gas is one of the basic components of a magma or lava.

Active and inactive volcanoes may release to the atmosphere gases in the form of:

water vapor

hydrogen sulfide

sulfur dioxide

carbon monoxide

hydrogen chloride

hydrogen fluoride

Secondary Volcanic Hazards

Indirectly associated with eruption

Lahar, flooding

Debris avalanche, landslide

Tsunami, seiche

Lahar

Lahars (an Indonesian term), sometimes called mudflows or volcanic debris flows, are flowing

of volcanic debris and water.

Lahars are classified into:

– Primary or hot lahar - associated directly with volcanic eruption

– Secondary or cold lahar - caused by heavy rainfall

Lahars distribute and redistribute volcanic ash and debris deposited around the volcano after

the materials has cooled and has become water logged.

Mayon 1992 Lahar

Lahar Impacts

Lahars can:

destroy by direct impact

lead to increased deposition of sediments

block tributary streams

bury valleys and communities with debris

Debris Avalanche

Volcanic Landslides (Debris Avalanches): A landslide is a rapid downslope movement of rock, snow,

and ice. Landslides range in size from small movements of loose debris on the surface of a volcano to

massive failures of the entire summit or flanks of a volcano.

Volcanic landslides are not always associated with eruptions; heavy rainfall or a large regional

earthquake can trigger a landslide on steep slopes. Volcanoes are susceptible to landslides because

they are composed of layers of weak, fragmented, volcanic rocks that tower above the surrounding

terrane. Furthermore, some of these rocks have been altered to soft, slippery, clay minerals by hot,

acidic ground water inside the volcano.

Tsunami

Tsunamis are long-period sea waves or wave trains that are generated by the sudden displacement of

water.

Volcano-related tsunamis are caused by volcanic or volcano-tectonic earthquakes, explosions,

collapse or subsidence, landslides, lahars, pyroclastic flows or debris avalanches entering bodies of

water, and atmospheric waves that couple with the sea.

MITIGATING VOLCANIC HAZARDS

Volcanic Hazards Mapping and Vulnerability Assessment

• Identify deposits of past eruptions & related phenomena

• Determine nature of past eruptive activity

• Map generation (volcano-geologic map & hazards zonation map)

DESTRUCTIVE VOLCANIC ACTIVITIES IN THE PHILIPPINES

YEAR VOLCANO ACTIVITY CASUALTIES PROPERTY DAMAGES/IMPACTS

1628? Iriga Avalanche Old Buhi town buries

1730 Banahaw Eruption (?); lahar Old Sariaya town damaged

1754 Taal Eruption Dead: 12 All lakeshore towns were buried by

airfall tephra

1776 Mayon Eruption Dead: 46

Lahar Dead: >1,000 Farmlands damaged

1814 Mayon Eruption Dead: 1,200 Destructions of the towns of Budiao,

Cagsawa, Sto. Domingo, Ligao,

Guinobatan, Tabaco

Lahar

1853 Mayon Eruption Dead: >34

Lahar Farmlands damaged

1871 Hibok-Hibok Eruption Complete destruction within 3 km radius

from Mt. Vulcan summit

1874 Taal Eruption All livestocks in the Volcano Island killed

1875 Mayon Lahar Dead: 1,500 Heavy destruction

1897 Mayon Eruption Dead: 350 Destruction of 7 barrios; burial of

farmlands

1911 Taal Eruption Dead: 1,335

Injured: 199

Devastated area: 360 sq. km.

1951 Hibok-Hibok Eruption Dead: 500 House and trees burned

1965 Taal Eruption Dead: 235 Devastated area: 50 sq. km.

1968 Mayon Eruption

Lahar Dead: 1 Thousands evacuated; damage at

Camalig and Guinobatan

1978 Mayon Eruption 23,000 people evacuated

1980 Malinao Hyrothermal

eruption

Damage to buildings; damage to Tiwi

National Park; families resettled

1981 Mayon Lahar Dead: 40

Injured: 7

Missing: 9

Damaged riceland: 18 sq. km.

1984 Mayon Eruption

Lahar

Injured: 4 70,000 Evacuated

Damaged farmlands: 2 sq. km.

Damaged properties and

infrastructuredL < $3 million

1991-

1992

Pinatubo Eruption/Lahar Dead: 938

Injured: 184

1,180,132 persons Affected; families

resettled; damaged houses and

Missing: 23 farmlands

1993 Mayon Eruption Dead: 77

Injured: 5

People withun 10 km radius evacuated

1996 Kanlaon Eruption Dead: 3

DESTRUCTIVE VOLCANIC ACTIVITIES IN THE PHILIPPINES

YEAR VOLCANO ACTIVITY CASUALTIES PROPERTY DAMAGES/IMPACTS

2000 Mayon Eruption Thousands evacuated

2001 Mayon Eruption Thousands evacuated

2006 Mayon Eruption Thousands evacuated

2006 Mayon Lahar ~1200 dead Thousands advised to evacuate

Mayon Volcano Alert Levels

Alert Level

Main Criteria Interpretation/ Recommendations

0 No Alert

Quiet. All monitored parameters within background levels.

No eruption in foreseeable future. Entry in the 6-km radius Permanent Danger Zone (PDZ) is not advised because phreatic explosions and ash puffs may occur without precursors.

1 Abnormal

Low level unrest. Slight increase in seismicity. Slight increase in SO2 gas output above the background level. Very faint glow of the crater may occur but no conclusive evidence of magma ascent. Phreatic explosion or ash puffs may occur.

No eruption imminent. Activity may be hydrothermal, magmatic or tectonic in origin. No entry in the 6-km radius PDZ.

2 Increasing Unrest

Moderate unrest. Low to moderate level of seismic activity. Episodes of harmonic tremor. Increasing SO2 flux. Faint / intermittent crater glow. Swelling of edifice may be detected. Confirmed reports of decrease in flow of wells and springs during rainy season.

Unrest probably of magmatic origin; could eventually lead to eruption. 6-km radius Danger Zone may be extended to 7 km in the sector where the crater rim is low.

3 Increased Tendency Towards Eruption

Relatively high unrest. Volcanic quakes and tremor may become more frequent. Further increase in SO2 flux. Occurrence of rockfalls in summit area. Vigorous steaming / sustained crater glow. Persistent swelling of edifice.

Magma is close to the crater. If trend is one of increasing unrest, eruption is possible within weeks. Extension of Danger Zone in the sector where the crater rim is low will be considered.

4 Hazardous Eruption Imminent

Intense unrest. Persistent tremor, many “low frequency”-type earthquakes. SO2 emission level may show sustained increase or abrupt decrease. Intense crater glow. Incandescent lava fragments in the summit area.

Hazardous eruption is possible within days. Extension of Danger zone to 8 km or more in the sector where the crater rim is low will be recommended.

5 Hazardous Eruption

Hazardous eruption ongoing. Occurrence of pyroclastic flows, tall eruption columns and extensive ashfall.

Pyroclastic flows may sweep down along gullies and channels, especially along those fronting the low part(s) of the crater rim. Additional danger areas may be identified as eruption progresses. Danger to aircraft, by way of ash cloud encounter, depending on height of eruption column and/or wind drift.