🌋Volcanology Unit 3 – Volcanic Eruption Styles and Products

Key Concepts and Terminology

• Magma composition refers to the chemical makeup and physical properties of molten rock beneath Earth's surface
  • Includes silica content, viscosity, gas content, and temperature
  • Plays a crucial role in determining the style and intensity of volcanic eruptions
• Volcanic explosivity index (VEI) quantifies the magnitude and destructive power of volcanic eruptions
  • Ranges from 0 (non-explosive) to 8 (catastrophic) based on the volume of ejected material and eruption column height
• Pyroclastic flows are fast-moving, ground-hugging avalanches of hot ash, pumice, rock fragments, and volcanic gas
  • Can travel at speeds up to 700 km/h and reach temperatures of 1,000°C
• Lahar is an Indonesian term for a volcanic mudflow or debris flow
  • Occurs when volcanic ash and debris mix with water from rainfall, melting snow, or crater lakes
  • Can travel long distances and cause significant damage to infrastructure and loss of life
• Tephra is a general term for the fragmented material ejected from a volcano during an explosive eruption
  • Includes ash (particles <2 mm), lapilli (2-64 mm), and bombs or blocks (>64 mm)
• Lava flows are streams of molten rock that pour or ooze from an erupting vent
  • Can be classified as pahoehoe (smooth, ropy) or a'a (rough, jagged) based on their surface texture
• Volcanic gases are released during eruptions and can have significant environmental and health impacts
  • Common gases include water vapor, carbon dioxide, sulfur dioxide, and hydrogen chloride

Types of Volcanic Eruptions

• Hawaiian eruptions are characterized by effusive, non-explosive outpourings of fluid basaltic lava
  • Produce gentle lava fountains and extensive lava flows (Kilauea, Hawaii)
• Strombolian eruptions involve moderate-energy explosions of gas-rich magma
  • Eject incandescent cinder, lapilli, and lava bombs in rhythmic or episodic bursts (Stromboli, Italy)
• Vulcanian eruptions are short-lived, violent explosions that generate dense ash clouds and pyroclastic density currents
  • Often associated with intermediate to silicic magmas and dome-building activity (Sakurajima, Japan)
• Plinian eruptions are the most powerful and destructive type, named after Pliny the Younger's description of the 79 CE eruption of Mount Vesuvius
  • Produce sustained, towering eruption columns (>20 km), extensive ash fall, and devastating pyroclastic flows (Mount Pinatubo, Philippines, 1991)
• Phreatomagmatic eruptions occur when magma interacts with water, resulting in explosive fragmentation and the formation of ash and steam
  • Can generate ash fall, base surges, and maar volcanoes (Ukinrek Maars, Alaska, 1977)
• Submarine eruptions take place underwater and are often difficult to observe directly
  • Can produce pillow lava, hyaloclastite, and hydrothermal vents (Loihi Seamount, Hawaii)
• Subglacial eruptions occur beneath ice sheets or glaciers, leading to unique landforms and hazards
  • Can generate jökulhlaups (glacial outburst floods) and hyaloclastite ridges (Eyjafjallajökull, Iceland, 2010)

Factors Influencing Eruption Styles

• Magma composition, particularly silica content, affects magma viscosity and gas retention
  • Higher silica content leads to more viscous magmas and explosive eruptions
• Magma temperature influences viscosity and the ability of gases to escape
  • Higher temperatures generally result in more fluid magmas and effusive eruptions
• Gas content and solubility determine the potential for explosive fragmentation
  • Magmas with higher gas content and lower solubility are more likely to erupt explosively
• Magma ascent rate affects the time available for gas exsolution and bubble growth
  • Rapid ascent favors explosive eruptions, while slower ascent allows for degassing and effusive activity
• Conduit geometry and vent obstruction can influence the style and intensity of eruptions
  • Narrow or blocked conduits can lead to pressure buildup and explosive behavior
• External water (groundwater, surface water, or ice) can interact with magma, resulting in phreatomagmatic or subglacial eruptions
  • The ratio of water to magma and the depth of interaction are important factors
• Regional tectonic setting and stress regime can affect magma generation, storage, and ascent
  • Extensional settings (rift zones) often produce basaltic volcanism, while subduction zones are associated with more silicic and explosive eruptions

Volcanic Products and Deposits

• Lava flows are coherent streams of molten rock that originate from volcanic vents or fissures
  • Can form a variety of surface features, such as pahoehoe ropes, a'a clinker, and lava tubes
  • Thickness, extent, and morphology depend on factors like lava viscosity, effusion rate, and topography
• Pyroclastic density currents (PDCs) are ground-hugging mixtures of hot gases, ash, and rock fragments
  • Classified as pyroclastic flows (pumice flows and ash flows) or pyroclastic surges (ash clouds) based on their particle concentration and flow dynamics
  • Can travel at high speeds (up to 700 km/h) and cover extensive areas, posing significant hazards
• Tephra fall deposits result from the settling of ejected volcanic material from eruption columns or plumes
  • Classified by size as ash (<2 mm), lapilli (2-64 mm), or bombs and blocks (>64 mm)
  • Thickness and grain size distribution vary with distance from the vent and prevailing wind direction
• Volcanic gases can condense to form mineral deposits or contribute to acid rain and air pollution
  • Common gas species include water vapor (H2O), carbon dioxide (CO2), sulfur dioxide (SO2), and hydrogen sulfide (H2S)
  • Volcanic gas monitoring is crucial for assessing eruption hazards and environmental impacts
• Lahar deposits are the products of volcanic mudflows or debris flows
  • Can be primary (syn-eruptive) or secondary (post-eruptive) depending on their triggering mechanism
  • Often fill and modify river valleys, leading to complex stratigraphic relationships
• Hydrothermal alteration occurs when volcanic gases and fluids interact with host rocks, leading to mineral replacement and the formation of alteration zones
  • Can produce economically significant mineral deposits (gold, silver, copper) and geothermal resources
• Volcanic ash can have far-reaching impacts on the environment, human health, and infrastructure
  • Fine ash particles (<0.1 mm) can cause respiratory issues, damage crops, and disrupt air travel
  • Ash fall deposits can accumulate to significant thicknesses and pose challenges for cleanup and disposal

Eruption Classification Systems

• The Volcanic Explosivity Index (VEI) is a widely used scale that quantifies the magnitude and intensity of explosive eruptions
  • Ranges from 0 (non-explosive) to 8 (mega-colossal) based on the volume of ejected tephra and the height of the eruption column
  • Provides a standardized way to compare and communicate the size of explosive eruptions
• The Smithsonian Institution's Global Volcanism Program (GVP) maintains a comprehensive database of Holocene volcanoes and their eruptive histories
  • Classifies volcanoes based on their morphology, tectonic setting, and dominant eruption style (e.g., shield, stratovolcano, caldera)
  • Assigns unique volcano numbers (VNUMs) to facilitate identification and data management
• The Mercalli Intensity Scale is used to describe the observed effects of volcanic eruptions on people, structures, and the environment
  • Ranges from I (not felt) to XII (total destruction) based on qualitative criteria
  • Useful for historical eruptions and assessing the impact of volcanic activity on local communities
• Lava flow morphology classification schemes distinguish between different types of lava flows based on their surface features and emplacement characteristics
  • Common categories include pahoehoe (smooth, ropy), a'a (rough, clinkery), block lava, and pillow lava
  • Provides insights into the rheological properties and emplacement dynamics of lava flows
• Tephra fall deposit classification systems categorize tephra layers based on their grain size distribution, composition, and depositional characteristics
  • Examples include the Wentworth grain size scale (ash, lapilli, bombs/blocks) and the Walker classification scheme (plinian, phreatoplinian, subplinian)
  • Helps to reconstruct eruption dynamics and assess the hazards associated with tephra fall
• Hydrothermal alteration mineral assemblages can be used to classify hydrothermal systems and infer the conditions of alteration
  • Common alteration types include propylitic, argillic, sericitic, and potassic alteration
  • Provides valuable information for geothermal exploration and mineral prospecting

Case Studies of Notable Eruptions

• Mount Vesuvius, Italy (79 CE): A classic example of a plinian eruption that buried the Roman cities of Pompeii and Herculaneum
  • Produced a towering eruption column, extensive tephra fall, and devastating pyroclastic flows
  • Preserved a detailed record of Roman life and architecture beneath the volcanic deposits
• Krakatoa, Indonesia (1883): A catastrophic eruption that generated massive tsunamis and global atmospheric effects
  • Explosive activity culminated in a series of caldera-forming eruptions and the destruction of the pre-existing volcanic island
  • The eruption's acoustic waves were detected around the world, and the resulting ash veil caused vivid sunsets and temporary global cooling
• Mount St. Helens, USA (1980): A well-studied eruption that showcased the complex interplay of magmatic and phreatic processes
  • Began with a massive landslide that triggered a lateral blast, devastating the surrounding landscape
  • Subsequent plinian eruptions produced ash fall, pyroclastic flows, and lahars, causing extensive damage and loss of life
• Pinatubo, Philippines (1991): The second-largest eruption of the 20th century, with significant global climate impacts
  • Preceded by months of precursory seismic activity and dome-building, allowing for successful evacuation efforts
  • Explosive eruptions generated voluminous ash fall, giant pyroclastic flows, and secondary lahars, affecting populated areas and infrastructure
• Eyjafjallajökull, Iceland (2010): A moderate-sized eruption that had disproportionate impacts on global air travel
  • Explosive phreatomagmatic activity produced fine ash that drifted over Europe, leading to widespread flight cancellations and economic disruption
  • Highlighted the vulnerability of modern transportation networks to volcanic ash hazards
• Kilauea, Hawaii (2018): An ongoing example of effusive basaltic volcanism and associated hazards
  • Prolonged eruption from the Lower East Rift Zone resulted in the destruction of hundreds of homes and the creation of new land along the coast
  • Lava flows, fountaining, and gas emissions posed challenges for local communities and infrastructure, requiring adaptive risk management strategies

Hazards and Risk Assessment

• Pyroclastic density currents (PDCs) are one of the deadliest volcanic hazards due to their high speeds, temperatures, and destructive power
  • Can cause asphyxiation, incineration, and crush injuries, as well as damage to buildings and infrastructure
  • Risk assessment involves mapping potential PDC paths, establishing exclusion zones, and developing evacuation plans
• Lava flows can destroy property, infrastructure, and agricultural land, but generally pose less risk to human life due to their slower advance rates
  • Hazard assessment includes lava flow modeling, monitoring of flow fronts, and identification of at-risk areas
  • Mitigation measures may include diversion barriers, cooling with water, and evacuation of threatened communities
• Tephra fall can disrupt transportation, damage crops, contaminate water supplies, and cause respiratory issues
  • Hazard assessment involves modeling ash dispersal patterns, estimating fall thicknesses, and assessing impacts on critical infrastructure
  • Risk reduction strategies include ash collection and disposal, respiratory protection, and temporary closures of airports and schools
• Lahars can travel long distances, inundate populated areas, and damage bridges, roads, and buildings
  • Hazard assessment requires mapping of lahar-prone river valleys, modeling of potential flow paths and volumes, and installation of early warning systems
  • Mitigation measures include evacuation planning, construction of retention basins, and reinforcement of critical infrastructure
• Volcanic gases can cause respiratory irritation, acid rain, and contribute to global climate change
  • Risk assessment involves monitoring of gas emissions, dispersion modeling, and assessment of potential health and environmental impacts
  • Mitigation strategies may include evacuation of affected areas, distribution of gas masks, and installation of gas monitoring networks
• Volcanic earthquakes and ground deformation can damage buildings, trigger landslides, and provide warning signs of imminent eruptions
  • Hazard assessment involves seismic monitoring, geodetic surveys, and analysis of deformation patterns
  • Risk reduction measures include building codes for seismic resistance, slope stabilization, and establishment of volcano observatories
• Secondary hazards, such as fires, tsunamis, and flooding, can compound the impacts of volcanic eruptions
  • Comprehensive risk assessment requires consideration of cascading hazards and their potential interactions
  • Multi-hazard mitigation strategies involve coordination among different agencies, land-use planning, and public education and preparedness

Monitoring and Prediction Techniques

• Seismic monitoring is a fundamental tool for detecting and locating volcanic earthquakes, which can indicate magma movement and potential eruptions
  • Involves the installation of seismometers around the volcano to record ground vibrations
  • Different types of seismic signals (e.g., long-period events, tremor, hybrid events) can provide insights into magmatic processes and eruption likelihood
• Ground deformation monitoring tracks changes in the shape and volume of the volcanic edifice, which can result from magma intrusion or withdrawal
  • Techniques include GPS, InSAR, tiltmeters, and strain meters
  • Uplift, subsidence, or lateral displacement patterns can help to identify the location and depth of magma storage and migration
• Gas monitoring measures the composition, flux, and temporal variations of volcanic gas emissions, which can provide clues about the state of the magmatic system
  • Common methods include COSPEC, DOAS, and MultiGAS for SO2 and other gas species
  • Changes in gas ratios (e.g., CO2/SO2, H2S/SO2) can indicate magma ascent, degassing, or hydrothermal activity
• Remote sensing techniques allow for the monitoring of volcanic activity from a safe distance, using satellites, aircraft, or drones
  • Thermal infrared imaging can detect heat signatures associated with lava flows, domes, or fumarolic activity
  • Radar and lidar can map topographic changes, deformation, and ash plume heights
• Geophysical surveys provide information about the subsurface structure and properties of the volcanic system
  • Gravity and magnetic surveys can identify magma chambers, intrusions, and hydrothermal alteration zones
  • Electrical resistivity and magnetotelluric methods can image fluid pathways and conductive anomalies
• Geochemical analysis of volcanic products (lava, tephra, gases) can yield insights into the composition, origin, and evolution of the magmatic system
  • Petrological and geochemical studies can reconstruct magma storage conditions, mixing processes, and eruption triggers
  • Isotopic analyses can trace the source and age of magmas, as well as the influence of crustal contamination or hydrothermal interactions
• Eruption forecasting combines monitoring data, historical records, and statistical models to estimate the probability and timing of future eruptions
  • Short-term forecasting relies on the recognition of precursory signals and the identification of threshold levels for specific parameters
  • Long-term forecasting considers the volcano's eruptive history, magma supply rate, and tectonic setting to assess the likelihood of future activity over years to decades
• Hazard mapping integrates monitoring data, geological mapping, and numerical simulations to create spatial representations of potential volcanic hazards
  • Hazard maps delineate zones of