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VOLCANOLOGY. Eruption Phenomena and their Potential Hazards:. Volcanic phenomena directly associated with eruption • Lava flow, dome growth • Pyroclastic flow, pyroclastic surge, lateral blast • Tephra fall - ash fall, volcanic bomb • Volcanic gas.
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Eruption Phenomena and their Potential Hazards: Volcanic phenomena directly associated with eruption • Lava flow, dome growth• Pyroclastic flow, pyroclastic surge, lateral blast• Tephra fall - ash fall, volcanic bomb• Volcanic gas Volcanic phenomena indirectly associated with eruption • Lahar, flooding• Debris avalanche, landslide• Tsunami, seiche• Subsidence, fissuring• Secondary/hydrothermal explosion• Secondary pyroclastic flow
Types of Hazards Posed by an Active Volcano • LAVA FLOW • Lava flow is a highly elongated mass of molten rock materials cascading down slope 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
DOME GROWTH Lava dome is a pile or mound of lava that grew on the floor of an active crater, on the side slopes via a feeder vent that breached through the surface of the edifice, or inside the volcanic edifice. Types: Exodomes - lava domes that were formed on the surface of the volcanic edifice Cryptodomes - lava domes that grew anywhere inside the edifice
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. • Types: • Ground surge • Ash-cloud surge • Base surge
HOT BLASTS • Hot blasts arise when pent-up gases facilitate their way out through the impermeable overlying materials and cause a very rapid escape into the atmosphere. Blasts that are directed obliquely often do much damage and could exact a high toll in human lives. • Lateral blasts are combination of pyroclastic flows and pyroclastic surges with an especially strong initial laterally-directed thrust. They have an initial velocity of 600 kph and slow down to about 100 kph near its margin 25 km from the volcano.
TEPHRA FALLS Tephra falls 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) more than 64 mm diameter (blocks and bombs)
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: • hydrogen sulfide • water vapor • sulfur dioxide • carbon monoxide • hydrogen chloride • hydrogen fluoride
LAHAR • Lahar (an Indonesian term), sometimes called mudflows or volcanic debris flows, are flowing mixtures of volcanic debris and water. • Lahars are classfied into: • Primary or hot lahar - associated directly with volcanic eruption • Secondary or cold lahar - caused by heavy rainfall • • Lahar distribute and redistribute volcanic ash and debris deposited around the volcano after the materials has cooled and has become water logged.
Lahar in tropical areas can be produced by: • Sudden draining of a crater lake, caused by either an explosive eruption or collapse of a crater fall (e.g. Agua, Kelut, Ruapehu) • Movement of a pyroclastic flow into a river or lake, displacing and mixing with water • Avalanche of water-sustained rock debris, where water can be from heavy rain, hydrothermal activity or other sources • Torrential rainfall on unconsolidated deposits on slope of a volcano (e.g. Pinatubo) • Collapse of a temporary dam, where recent volcanic deposits have blocked a steam channel (e.g. Asama, Pinatubo)
OTHER ERUPTION PHENOMENA Debris avalanche - fast downhill movement of soil and rock speed: 70 km/hr (due to high water content and steep slopes) caused by slope failure on the cones of stratovolcanoes. Hydrothermal explosions - explosions from instantaneous flashing of steam upon contact with hot rocks Secondary explosions - are caused by the contact of water with hot pyroclastic flow deposits. Subsidence - is a ground deformation resulting from the downward adjustment of surface materials to the voids caused by volcanic activity. This may result also from mine workings or geothermal water or oil extraction.
PRECURSORS OF AN IMPENDING VOLCANIC ERUPTION • The following are commonly observed signs that a volcano is about to erupt. These precursors may vary from volcano to volcano: • Increase in the frequency of volcanic quakes with rumbling sounds; occurrence of volcanic tremors. • Increased steaming activity; change in color of steam emission from white to gray due to entrained ash. • Crater glow due to presence of magma at or near the crater. • Ground swells (or inflation), ground tilt and ground fissuring due to magma intrusion • Localized landslides, rockfalls and landslides from the summit area not attributable to heavy rains. • Noticeable increase in the extent of drying up of vegetation around the volcano's upper slopes. • Increase in the temperature of hot springs, wells (e.g. Bulusan and Canlaon) and crater lake (e.g. Taal) near the volcano. • Noticeable variation in the chemical content of springs, crater lakes within the vicinity of the volcano. • Drying up of springs/wells around the volcano. • Development of new thermal areas and/or reactivation of old ones;appearance of solfataras.
TYPES OF VOLCANIC ERUPTION Phreatic - explosion driven by steam produced by heating and expansion of groundwater due to an underlying hot source. This type involves only water, steam and ash with other rock fragments derived from pre-existing rocks, without ejection of fresh magmatic materials. Examples: 1993 & 1996 eruptions of Canlaon Volcano 1988 eruption of Bulusan Volcano
Phreatomagmatic - eruption resulting from the ejection of magmatic gases and steam produced by the conversion of groundwater to steam by ascending magma, mixed with water, fine ash with or without accretionary lapilli and variably-sized volcanic bombs fragmented from the pre-existing rock formations, and fresh magmatic ejecta. The eruption forms a high eruption column with a radially spreading ring-shaped horizontal cloud at the base due to peeling of the crater lip or deflection in the rise of later ejections caused by the pressure of falling pyroclastic materials. Example: 1965 & 1967 eruptions of Taal Volcano
Strombolian - weak to violent eruption characterized by lava fountaining and effusion of molten lava. Typical ejecta are ovoid and fusiform bombs and scoria (cinders). Ash is relatively minor in amount and eruption cloud is generally yellowish to white in color. Examples: 1968 & 1969 eruptions of Taal Volcano 1978 eruption of Mayon Volcano First phase of 2001 eruption of Mayon Volcano
Vulcanian - eruption resulting from the release of large quantities of accumulated magmatic gas which lefts fine ashes and blocks coming from the magma with great force high in the air forming voluminous eruption clouds. Examples: First phase of 1993 eruption of Mayon Volcano 2nd phase of 2000 & 2001 eruptions of Mayon Volcano
Peleean - eruption caused by the release of large quantities of gas from an extremely viscous magma that hurls out ash and other pyroclastic materials and is characterized by the presence of nuee ardente or glowing avalanche consisting of hot gases made dense by a suspended load of pyroclastic material. Example: 1948-1953 eruption of Hibok-Hibok Volcano
Plinian - eruption of great violence characterized by voluminous explosive ejections of pumice and pyroclastic flows. The copious extrusion of gas-rich silicious magma is commonly accompanied by collapse of the top of the volcanic cone forming a caldera. Example: Pinatubo Volcano 1991 eruption. Example: 1991 eruption of Pinatubo Volcano
Protocol in Releasing Volcano Information
Philippine Volcanoes are classified as Active, Inactive and Potentially active. Active Volcanoes - Eruption in historic times- Historical record - 500 years- C14 dating - 10,000 years- Local seismic activity- Oral / folkloric history • Potentially Active • Solfataras / Fumaroles- Geologically young (possibly erupted < 10,000 years and for calderas and large systems • possibly < 25,000 years).- Young-looking geomorphology (thin soil cover/sparse vegetation; low degree of erosion • and dissection; young vent featuresl; +/- vegetation cover).- Suspected seismic activity.- Documented local ground deformation- Geochemical indicators of magmatic involvement.- Geophysical proof of magma bodies.- Strong connection with subduction zones and external tectonic settings. • Inactive • No record of eruption and its form is beginning to change by the agents of • weathering and erosion via formation of deep and long gullies.
Volcanic Plumes • Eruptions place ash and magmatic gases into atmosphere. • Major gases are CO2 (10%), H2O (80%), with lesser SO2, H2S, CO, HCl, HBr, HF. • SO2 is the gas that causes major atmospheric impact. • Most others removed through interactions in troposphere.
Effects of the 1991 Pinatubo Eruption • Large scale experiment that measured albedo was conducted in association with the 1991 Pinatubo eruption. • In August, 1991, albedo between 30°N and 30°S increased from the 5 year mean. • Associated with an increased optical depth that was also associated with an order of magnitude increase in aerosol mass. • Called the “direct effect” of volcanic aerosol.
Stratospheric Effects and Observations • For the Agung (1963), El Chichon, and Pinatubo eruptions, research indicates that lower stratospheric T increased by about 2° in the tropics. • This represents 3 to 4 standard deviations from the mean of temperature data over time period 1958 to 1996. • Effects lasted for up to 2 years.
Additional Complications! • Larger eruptions do not necessarily lead to larger T anomalies. • Reason is increased output of sulfuric acid, etc. may lead the larger aerosol particles. • These larger particles may have shorter residence times in atmosphere. • Larger particles also have different radiative properties (e.g., optical depth). Thus, may not be associated with larger T variations.
Why Monitor Volcanoes? • Provides scientific data to assist our understanding of structure and dynamics of volcanoes • Necessary for hazard assessment, eruption forecasting, risk mitigation at times of volcanic unrest
It’s Not Just About the Science • Monitoring must be placed in context of the needs of the community. • Information provided must answer questions that communities have, such as, “is my house in danger?” “When will the volcano erupt?” “When will it stop?”
Data Collection: Instrumentation • What are some techniques/data used to monitor volcanoes? • Seismic data • Ground deformation • Gas monitoring • Gravity and Magnetics
Seismic Monitoring • A seismometer is an instrument that measures ground vibrations caused by a variety of processes, primarily earthquakes. • To keep track of a volcano's changing earthquake activity, we typically must install between 4 and 8 seismometers within about 20 km of its vent, with several located on the volcano itself. • This is especially important for detecting earthquakes smaller than magnitude 1 or 2; sometimes, these tiny earthquakes represent the only indication that a volcano is becoming restless.
Seismic Monitoring • Dramatic improvements in computer technology and increased scientific experience with volcano seismicity have improved our ability to provide eruption warnings and to characterize eruptions in progress. • Computers have enabled us to locate earthquakes beneath a volcano faster and with greater accuracy than was possible just 5 years ago, and now we can determine in real time the changing character of a volcano's earthquake activity. • They've also helped us to "map" subsurface structures like fault zones and magma reservoirs.
Mt. St. Helens-Seismic Activity as of May 26 • Red dots, if present, represent events occurring in the last hour. • Green dots, if present, represent events occurring in the last day. • Circles represent older events occurring this month. • Black triangles represent PNSN seismic stations. • Black star represents Mount St. Helens summit.
Mt. St. Helens-Seismic Activity as of May 26 • The above figure plots earthquake depths (vertical axis) as a function of increasing time (horizontal axis) for Mt. St Helens earthquakes from the past month. • Red dots, if present, represent events occurring in the last hour. • Green dots, if present, represent events occurring in the last day. • Circles represent older events occurring this month.
Gas Monitoring • Scientists have long recognized that gases dissolved in magma provide the driving force of volcanic eruptions, but only recently have new techniques permitted routine measurement of different types of volcanic gases released into the atmosphere. • A primary objective in gas monitoring is to determine changes in the release of certain gases from a volcano, chiefly carbon dioxide and sulfur dioxide. • Such changes can be used with other monitoring information to provide eruption warnings and to improve our understanding of how volcanoes work. In recent years, volcanologists have directed increased attention toward volcanic gas emissions because of the newly appreciated hazards they sometimes pose and their effects on the Earth's atmosphere and climate.
Curtain of lava Phase 1: Pu’u O’o
Phase 1: Pu’u O’o Fire Fountain
Surtsey, Iceland A new volcanic island formed in 1966
