11.3 Volcanic eruptions

Volcanic eruptions are complex multiphase flow phenomena that shape Earth's surface and pose significant hazards. These events involve interactions between magma, gases, and solid particles, creating diverse eruption styles from gentle lava flows to explosive ash plumes.

Understanding the mechanisms behind volcanic eruptions is crucial for predicting their behavior and mitigating impacts. This topic explores the types of eruptions, their underlying physics, and the multiphase flow models used to simulate these powerful natural events.

Types of volcanic eruptions

• Volcanic eruptions can be broadly classified into two main categories: explosive and effusive, based on the style and intensity of the eruption
• The type of eruption is influenced by factors such as magma composition, gas content, and magma chamber conditions
• Different types of eruptions are named after famous volcanoes that exemplify their characteristics (Plinian, Strombolian, Hawaiian, Vulcanian, Phreatic)

Explosive vs effusive eruptions

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• Explosive eruptions are characterized by violent fragmentation of magma, producing ash, pumice, and pyroclastic density currents
• Effusive eruptions involve the gentle outpouring of lava onto the surface, forming lava flows and domes
• The explosivity of an eruption depends on the magma's silica content and gas content, with higher silica and gas leading to more explosive behavior

Plinian eruptions

• Plinian eruptions are the most powerful and destructive type of volcanic eruption, named after Pliny the Younger's description of the 79 AD eruption of Mount Vesuvius
• Characterized by tall, sustained eruption columns reaching heights of several kilometers (Mount Pinatubo, 1991)
• Produce extensive ash fallout, pyroclastic density currents, and can cause significant damage to surrounding areas

Strombolian eruptions

• Strombolian eruptions are characterized by intermittent, mild to moderate explosive activity, named after the Italian volcano Stromboli
• Eruptions involve the ejection of incandescent lava fragments, bombs, and ash from the volcanic vent
• Strombolian eruptions are driven by the bursting of large gas bubbles within the magma (Parícutin, Mexico, 1943-1952)

Hawaiian eruptions

• Hawaiian eruptions are named after the volcanoes of Hawaii and are characterized by the effusive outpouring of fluid, basaltic lava
• Lava is erupted from fissures or central vents, forming lava fountains, lava flows, and lava lakes (Kilauea, ongoing)
• Hawaiian eruptions are generally less explosive due to the low silica content and low gas content of the magma

Vulcanian eruptions

• Vulcanian eruptions are characterized by short-lived, violent explosions that produce dense ash clouds and ballistic projectiles
• These eruptions are caused by the sudden release of pressurized gas trapped beneath a solidified lava plug in the volcano's conduit (Sakurajima, Japan, frequent eruptions)
• Vulcanian eruptions can generate small to moderate pyroclastic density currents and ash fallout

Phreatic eruptions

• Phreatic eruptions, also known as steam-blast eruptions, occur when magma or hot rocks interact with groundwater or surface water
• The rapid heating and expansion of water cause an explosion, ejecting steam, ash, and rock fragments from the volcano (Ontake, Japan, 2014)
• Phreatic eruptions can occur with little warning and pose significant hazards to nearby populations

Mechanisms of volcanic eruptions

• Volcanic eruptions are driven by complex interactions between magma properties, dissolved gases, and magma chamber conditions
• Understanding the mechanisms behind eruptions is crucial for predicting their behavior and mitigating their impacts
• Multiphase flow, involving the interplay of gas, liquid, and solid phases, plays a significant role in the dynamics of volcanic eruptions

Magma properties influencing eruptions

• Magma composition, particularly silica content, affects its viscosity and ability to trap gases, influencing eruption style (basaltic magmas: effusive, rhyolitic magmas: explosive)
• Magma temperature and crystallinity also impact its rheology and flow behavior
• Dissolved volatile content, such as water and carbon dioxide, contributes to the explosivity of eruptions

Role of dissolved gases in eruptions

• Dissolved gases in magma, primarily water vapor and carbon dioxide, exsolve and expand as magma ascends and decompresses
• The rapid expansion of gases drives magma fragmentation and explosive eruptions
• The solubility of gases in magma depends on pressure, with lower pressures leading to greater gas exsolution and more explosive potential

Magma chamber pressure and eruptions

• Magma chambers are reservoirs of molten rock beneath volcanoes, where magma is stored and evolves over time
• Overpressure in magma chambers, caused by magma injection or gas exsolution, can trigger eruptions by fracturing the overlying rock
• Changes in magma chamber pressure can be monitored through ground deformation and seismic activity

Magma ascent and eruption dynamics

• As magma ascends through the volcanic conduit, it undergoes decompression, leading to gas exsolution and expansion
• The rate of magma ascent influences the style of eruption, with rapid ascent favoring explosive eruptions and slower ascent promoting effusive activity
• The geometry of the volcanic conduit, such as constrictions or obstacles, can affect magma flow and fragmentation

Multiphase flow in volcanic eruptions

• Multiphase flow involves the simultaneous presence and interaction of gas, liquid, and solid phases in a fluid system
• In volcanic eruptions, multiphase flow is a critical aspect of magma ascent, fragmentation, and the generation of pyroclastic density currents and ash plumes
• Understanding multiphase flow processes is essential for developing accurate models of eruption dynamics and hazard assessment

Gas-liquid-solid interactions in magma

• Magma is a multiphase fluid consisting of a liquid melt, dissolved gases, and solid crystals
• The interactions between these phases govern the rheology and flow behavior of magma
• Gas bubbles can nucleate and grow within the liquid melt, leading to magma expansion and potential fragmentation

Bubble nucleation and growth in magma

• As magma ascends and decompresses, dissolved gases exsolve and form bubbles within the melt
• Bubble nucleation occurs when the gas pressure exceeds the ambient pressure and the surface tension of the melt
• Bubble growth is driven by gas diffusion from the melt into the bubbles and by bubble coalescence and expansion

Magma fragmentation and ash formation

• Magma fragmentation is the process by which a continuous magma body is broken into smaller pieces, such as ash and pumice
• Fragmentation can occur due to rapid bubble growth and coalescence, leading to the rupture of the magma
• The intensity of fragmentation determines the size distribution of the resulting ash particles (fine ash: <2 mm, coarse ash: 2-64 mm)

Pyroclastic density currents

• Pyroclastic density currents (PDCs) are ground-hugging, gravity-driven flows of hot ash, pumice, and gas generated during explosive eruptions
• PDCs can travel at high velocities (up to 700 km/h) and cover large areas, posing significant hazards to life and infrastructure
• The dynamics of PDCs involve complex multiphase flow processes, including particle sedimentation, entrainment, and turbulent mixing

Lava flow rheology and emplacement

• Lava flows are effusive eruptions of molten rock that move downslope under the influence of gravity
• The rheology of lava flows depends on factors such as composition, temperature, and crystallinity, which affect its viscosity and flow behavior
• Lava flow emplacement is influenced by topography, effusion rate, and cooling rate, which determine the final morphology of the flow (aa, pahoehoe, blocky)

Modeling volcanic eruptions

• Numerical modeling is a powerful tool for understanding and predicting the behavior of volcanic eruptions
• Models can simulate various aspects of eruptions, such as magma ascent, eruption column dynamics, pyroclastic density currents, and lava flows
• Integrating multiphase flow physics into these models is crucial for capturing the complexity of volcanic processes

Numerical models of magma ascent

• Magma ascent models simulate the flow of magma through the volcanic conduit, considering factors such as magma rheology, gas exsolution, and conduit geometry
• These models can predict the onset of magma fragmentation and the transition from effusive to explosive eruptions
• Examples of magma ascent models include the Conduit4 code and the DIKES code

Multiphase flow models of eruption columns

• Eruption column models simulate the rise and dispersion of ash and gas plumes generated during explosive eruptions
• Multiphase flow models account for the interactions between ash particles, gas, and atmospheric conditions
• Examples of multiphase flow models for eruption columns include the ATHAM model and the ASHEE model

Modeling pyroclastic density currents

• Pyroclastic density current (PDC) models simulate the flow and deposition of these hazardous volcanic phenomena
• PDC models must account for the complex multiphase flow dynamics, including particle sedimentation, entrainment, and turbulence
• Examples of PDC models include the Titan2D model and the VolcFlow model

Lava flow simulation techniques

• Lava flow models simulate the emplacement and evolution of lava flows, considering factors such as lava rheology, effusion rate, and topography
• Techniques for modeling lava flows include cellular automata, smoothed particle hydrodynamics (SPH), and finite element methods
• Examples of lava flow simulation software include FLOWGO and MAGFLOW

Coupling magma chamber and eruption models

• Integrated models that couple magma chamber dynamics with eruption processes provide a more comprehensive understanding of volcanic systems
• These models consider the feedback between magma chamber pressure, magma ascent, and surface eruption phenomena
• Coupled models can help investigate the triggering mechanisms of eruptions and improve eruption forecasting

Hazards and impacts of volcanic eruptions

• Volcanic eruptions pose significant hazards to human life, infrastructure, and the environment
• The main hazards associated with eruptions include volcanic ash, pyroclastic density currents, lava flows, and volcanic gases
• Monitoring and forecasting volcanic activity is crucial for mitigating the impacts of eruptions and ensuring public safety

Volcanic ash dispersion and fallout

• Volcanic ash is a fine-grained material ejected during explosive eruptions, which can be dispersed over large areas by atmospheric winds
• Ash fallout can cause respiratory issues, damage to infrastructure, and disruption of air travel (Eyjafjallajökull, Iceland, 2010)
• Modeling ash dispersion is important for predicting the areas affected by ash fallout and issuing timely warnings

Pyroclastic density current hazards

• Pyroclastic density currents (PDCs) are one of the deadliest hazards associated with explosive eruptions
• PDCs can travel at high speeds, destroying everything in their path and causing widespread devastation (Mount Vesuvius, 79 AD; Mount St. Helens, 1980)
• Hazard maps and evacuation plans are essential for mitigating the risks posed by PDCs

Lava flow hazards and infrastructure damage

• Lava flows can cause significant damage to infrastructure, such as roads, buildings, and pipelines
• The slow advance of lava flows allows for evacuation, but the destruction of property can be extensive (Kilauea, Hawaii, 2018)
• Diverting or cooling lava flows can help protect critical infrastructure in some cases

Volcanic gas emissions and environmental impacts

• Volcanoes emit a variety of gases, including water vapor, carbon dioxide, sulfur dioxide, and hydrogen sulfide
• Volcanic gas emissions can have significant environmental impacts, such as acid rain, atmospheric cooling, and respiratory health issues (Laki, Iceland, 1783-1784)
• Monitoring gas emissions can provide insights into the state of volcanic activity and potential eruption precursors

Volcano monitoring for eruption forecasting

• Volcano monitoring involves the use of various techniques to detect changes in volcanic activity and anticipate eruptions
• Monitoring methods include seismic monitoring, ground deformation measurements, gas emission analysis, and remote sensing
• Integrating monitoring data with numerical models can improve eruption forecasting and early warning systems for volcanic hazards