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 Devastated area: 360 sq. km. Injured: 199 1951 Hibok -Hibok Eruption Dea d: 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 Mali nao Hyrothermal Damage to buildings; damage to Tiwi eruption National Park; families resettled 1981 Mayon Lahar Dead: 40 Damaged riceland: 18 sq. km. Injured: 7 Missing: 9 1984 Mayon Eruption Injured: 4 70,000 Evacuated Lahar Damaged farmlands: 2 sq. km. Damaged properties and infrastructuredL < $3 million 1991 - Pinatubo Eruption/Lahar Dead: 938 1,180,132 persons Affected; families 1992 Injured: 184 resettled; damaged houses and Missing: 23 farmlands 1993 Mayon Eruption Dead: 77 People withun 10 km radius evacuated Injured: 5 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 Main Criteria Interpretation/ Recommendations Level Quiet. No eruption in foreseeable future. 0 All monitored parameters within Entry in the 6-km radius Permanent Danger No Alert background levels. Zone (PDZ) is not advised because phreatic explosions and ash puffs may occur without precursors. Low level unrest. No eruption imminent. 1 Slight increase in seismicity. Activity may be hydrothermal, magmatic or Abnormal Slight increase in SO 2 gas output above the background tectonic in origin. level. No entry in the 6-km radius PDZ. Very faint glow of the crater may occur but no conclusive evidence of magma ascent. Phreatic explosion or ash puffs may occur. Moderate unrest. Unrest probably of magmatic origin; could 2 Low to moderate level of seismic activity. Episodes of eventually lead to eruption. Increasing harmonic tremor. 6-km radius Danger Zone may be extended to Unrest Increasing SO 2 flux. 7 km in the sector where the crater rim is low. Faint / intermittent crater glow. Swelling of edifice may be detected. Confirmed reports of decrease in flow of wells and springs during rainy season. Relatively high unrest . Magma is close to the crater . 3 Volcanic quakes and tremor may become more frequent. If trend is one of increasing unrest, eruption is Increased Further increase in SO 2 flux. possible within weeks . Tendency Occurrence of rockfalls in summit area. Extension of Danger Zone in the sector where Towards Vigorous steaming / sustained crater glow. the crater rim is low will be considered.