Download VOLCANIC HAZARDS

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 preexisting 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
1776
Mayon
Eruption
Dead: 46
Lahar
Dead: >1,000
All lakeshore towns were buried by
airfall tephra
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
Dead: 1
Thousands evacuated; damage at
Camalig and Guinobatan
Lahar
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
19911992
Pinatubo
Eruption/Lahar
Dead: 938
Injured: 184
1,180,132 persons Affected; families
resettled; damaged houses and
Missing: 23
farmlands
People withun 10 km radius evacuated
1993
Mayon
Eruption
Dead: 77
Injured: 5
1996
Kanlaon
Eruption
Dead: 3
DESTRUCTIVE VOLCANIC ACTIVITIES IN THE PHILIPPINES
YEAR
VOLCANO
ACTIVITY
CASUALTIES
2000
Mayon
Eruption
Thousands evacuated
2001
Mayon
Eruption
Thousands evacuated
2006
Mayon
Eruption
Thousands evacuated
2006
Mayon
Lahar
~1200 dead
PROPERTY DAMAGES/IMPACTS
Thousands advised to evacuate
Mayon Volcano Alert Levels
Alert
Level
0
No Alert
1
Abnormal
Main Criteria
Interpretation/ Recommendations
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.
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
3
Increased
Tendency
Towards
Eruption
4
Hazardous
Eruption
Imminent
5
Hazardous
Eruption
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.
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.
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.
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.