🌋Volcanology Unit 3 – Volcanic Eruption Styles and Products
Volcanic eruption styles and products are key to understanding how volcanoes behave and impact their surroundings. This topic covers the various types of eruptions, from gentle Hawaiian flows to explosive Plinian events, and the materials they produce, like lava, ash, and pyroclastic flows.
Factors influencing eruption styles include magma composition, gas content, and external conditions. Understanding these elements helps scientists assess volcanic hazards, predict eruptions, and develop effective monitoring techniques. This knowledge is crucial for managing volcanic risks and protecting communities near active volcanoes.
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
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