🌋Volcanology Unit 6 – Volcano Morphology and Landforms

Key Concepts and Terminology

• Magma composition influences volcano shape, eruptive style, and hazards associated with an eruption
• Viscosity measures a fluid's resistance to flow, with higher viscosity magmas leading to more explosive eruptions
• Effusive eruptions involve low-viscosity magma that flows easily, forming lava flows and shallow-sloped shield volcanoes (Kilauea)
• Explosive eruptions involve high-viscosity magma that fragments violently, forming steep-sided stratovolcanoes and calderas (Mount St. Helens)
• Pyroclastic material includes ash, lapilli, and bombs ejected during explosive eruptions, posing significant hazards to surrounding areas
• Lava domes form when viscous magma accumulates and solidifies near the vent, often leading to collapse and pyroclastic density currents (Soufrière Hills)
• Volcanic gases, such as water vapor, carbon dioxide, and sulfur dioxide, play a crucial role in driving eruptions and can be monitored to assess volcanic activity

Types of Volcanoes

• Shield volcanoes have broad, gently sloping flanks built by successive layers of low-viscosity lava flows (Mauna Loa)
  • Characterized by effusive eruptions and fluid basaltic lava that can travel great distances
  • Summit calderas often form due to magma chamber emptying and subsequent collapse
• Stratovolcanoes, also known as composite volcanoes, have steep, conical shapes built by alternating layers of lava, ash, and pyroclastic material (Mount Fuji)
  • Formed by a combination of effusive and explosive eruptions, with intermediate to felsic magma compositions
  • Prone to hazardous phenomena such as lava domes, pyroclastic density currents, and lahars
• Cinder cones are small, steep-sided volcanoes built primarily from accumulations of ejected ash, lapilli, and scoria (Parícutin)
  • Typically monogenetic, meaning they form during a single eruptive episode
• Lava domes are rounded, steep-sided mounds that form when viscous lava accumulates and solidifies near the vent (Chaos Crags)
  • Often associated with stratovolcanoes and can lead to explosive collapse events
• Calderas are large, circular depressions formed by the collapse of a volcano's summit or the emptying of its magma chamber (Yellowstone)
  • Can result from powerful explosive eruptions or gradual subsidence
  • Often associated with extensive ash fall and pyroclastic density currents

Volcanic Landforms and Features

• Craters are circular depressions at the summit of a volcano, formed by explosive excavation or collapse (Crater Lake)
  • Can be filled with water, forming crater lakes, or contain active vents and fumaroles
• Vents are openings through which magma and volcanic gases escape to the surface, including central, flank, and fissure vents
• Lava flows are outpourings of molten rock that move downslope under the influence of gravity, creating various surface textures and morphologies
  • Pahoehoe lava forms smooth, ropy surfaces due to low viscosity and steady flow rates (Kilauea)
  • A'a lava creates rough, jagged surfaces due to higher viscosity and variable flow rates (Mauna Loa)
• Lava tubes are underground conduits formed by the crusting over of lava channels, allowing lava to be transported efficiently over long distances (Thurston Lava Tube)
• Fumaroles are openings that emit steam and volcanic gases, indicating the presence of magma or hot rock beneath the surface (Valley of Ten Thousand Smokes)
• Geothermal features, such as hot springs and geysers, are manifestations of volcanic heat interacting with groundwater (Old Faithful)
• Volcanic necks are erosion-resistant remnants of solidified magma that once filled the conduit of a volcano, often standing as prominent landforms (Shiprock)

Eruption Styles and Their Effects

• Hawaiian eruptions are characterized by effusive outpourings of fluid, basaltic lava with low explosivity (Kilauea)
  • Lava fountains and lava flows are common, with limited ash production
  • Pose localized hazards to infrastructure and property, but rarely cause fatalities
• Strombolian eruptions involve moderate explosivity and rhythmic ejection of incandescent lava fragments, driven by the bursting of large gas bubbles (Stromboli)
  • Scoria and spatter accumulate around the vent, forming cinder cones
  • Hazards include ballistic projectiles and small-scale pyroclastic density currents
• Vulcanian eruptions are characterized by short-lived, violent explosions that generate dense ash plumes and pyroclastic material (Sakurajima)
  • Caused by the fragmentation of viscous magma in the upper conduit
  • Hazards include ash fall, ballistic projectiles, and localized pyroclastic density currents
• Plinian eruptions are the most explosive and destructive, involving sustained ash column formation and widespread tephra dispersal (Mount Vesuvius, 79 CE)
  • Driven by the rapid ascent and fragmentation of gas-rich, viscous magma
  • Hazards include extensive ash fall, pyroclastic density currents, and lahars
• Phreatomagmatic eruptions occur when magma interacts with water, resulting in violent steam explosions and the production of fine ash (Taal Volcano)
  • Can generate base surges and pose hazards to nearby populations
• Effusive eruptions involve the quiet outpouring of lava with minimal explosivity, building shield volcanoes and lava flow fields (Mauna Loa)
  • Hazards are typically localized, but can impact infrastructure and property

Factors Influencing Volcano Shape

• Magma composition determines the viscosity and gas content of the melt, influencing eruptive style and volcano morphology
  • Mafic magmas (basalt) have low viscosity and form broad, gently sloping shield volcanoes (Kilauea)
  • Felsic magmas (rhyolite) have high viscosity and form steep-sided stratovolcanoes and lava domes (Mount St. Helens)
• Magma supply rate affects the growth and size of a volcano, with higher rates leading to more frequent eruptions and larger edifices
• Tectonic setting influences the type and location of volcanism, with different eruptive styles associated with various plate boundaries and hot spots
  • Subduction zones produce explosive stratovolcanoes due to volatile-rich magmas (Cascades)
  • Mid-ocean ridges generate effusive eruptions and shield volcanoes due to low-viscosity magmas (Iceland)
• Edifice strength and stability control the shape and potential for collapse, with weaker structures more prone to failure and caldera formation
• Interaction with water can lead to phreatomagmatic eruptions, altering the eruptive style and generating unique landforms (maars, tuff rings)
• Erosion and weathering processes modify volcano shape over time, with older volcanoes exhibiting more subdued topography and incised valleys

Volcanic Hazards and Risk Assessment

• Lava flows can inundate and destroy infrastructure, but their predictable nature allows for effective hazard mitigation and evacuation (Kilauea)
• Pyroclastic density currents (PDCs) are fast-moving, ground-hugging flows of hot ash, gas, and rock fragments that pose significant threats to life and property (Mount Pelee, 1902)
  • Can travel at speeds exceeding 100 km/h and surmount topographic barriers
  • Hazard zones are delineated based on the potential extent and direction of PDCs
• Lahars are destructive mudflows generated by the mixing of volcanic ash and debris with water, often triggered by heavy rainfall or snow/ice melt (Nevado del Ruiz, 1985)
  • Can travel great distances along river valleys and cause catastrophic damage to infrastructure
  • Hazard maps identify lahar-prone areas and inform evacuation planning
• Ash fall can disrupt transportation, damage crops, and cause respiratory issues, with the extent of impact determined by wind patterns and eruption magnitude (Mount Pinatubo, 1991)
  • Ash fall thickness and particle size are used to assess hazards and inform mitigation strategies
• Volcanic gases, such as sulfur dioxide and carbon dioxide, can accumulate in low-lying areas and pose health risks to nearby populations (Lake Nyos, 1986)
  • Monitoring gas emissions helps to detect changes in volcanic activity and assess potential hazards
• Volcano monitoring techniques, including seismicity, ground deformation, and gas measurements, are used to assess the likelihood and potential impacts of future eruptions
• Hazard maps and risk assessments inform land-use planning, emergency response, and public education efforts to mitigate the consequences of volcanic eruptions

Case Studies and Famous Examples

• Mount Vesuvius (Italy, 79 CE): A catastrophic Plinian eruption buried the Roman cities of Pompeii and Herculaneum under ash and pyroclastic flows, providing valuable insights into volcanic processes and hazards
• Krakatoa (Indonesia, 1883): A massive caldera-forming eruption generated devastating tsunamis and global climatic effects, showcasing the far-reaching impacts of large-scale volcanic events
• Mount Pelee (Martinique, 1902): A deadly pyroclastic density current destroyed the town of Saint-Pierre, highlighting the importance of understanding and mitigating volcanic hazards
• Laki (Iceland, 1783): An extensive fissure eruption released large quantities of sulfur dioxide, causing widespread crop failures and influencing global climate patterns
• Mount St. Helens (USA, 1980): A catastrophic flank collapse and subsequent explosive eruption demonstrated the complex interplay between magmatic and gravitational forces in shaping volcano behavior
• Pinatubo (Philippines, 1991): A large explosive eruption, preceded by effective monitoring and evacuation efforts, showcased the value of volcano monitoring and hazard mitigation strategies
• Eyjafjallajökull (Iceland, 2010): A moderate-sized eruption disrupted global air travel due to the widespread dispersal of fine ash, underscoring the vulnerability of modern infrastructure to volcanic hazards

Research Methods and Tools

• Seismic monitoring uses networks of seismometers to detect and locate earthquakes associated with magma movement and volcanic unrest
  • Volcano-tectonic (VT) earthquakes indicate rock fracturing due to magma intrusion
  • Long-period (LP) and tremor signals are linked to fluid movement and gas escape
• Ground deformation measurements, using GPS, InSAR, and tiltmeters, track changes in the shape of a volcano caused by magma accumulation or withdrawal
  • Inflation often precedes eruptions, while deflation occurs during or after eruptive events
• Gas monitoring techniques, such as COSPEC and DOAS, quantify the emission rates of volcanic gases (e.g., SO2, CO2) to assess magmatic activity and degassing processes
• Remote sensing platforms, including satellites and drones, provide valuable data on thermal anomalies, ash plumes, and surface morphology changes
  • Thermal infrared imaging helps to detect and map lava flows and identify active vents
  • Radar and lidar sensors generate high-resolution digital elevation models (DEMs) to study volcano topography and monitor surface deformation
• Petrological and geochemical analyses of erupted products offer insights into magma storage conditions, evolution, and triggering mechanisms
  • Mineral and melt inclusion studies reveal magma chamber depths, temperatures, and volatile contents
  • Isotopic ratios help to identify magma sources and track magma mixing and contamination processes
• Numerical modeling techniques simulate volcanic processes, such as magma ascent, edifice stability, and pyroclastic density current propagation, to better understand and predict volcanic behavior
• Hazard mapping integrates geological, historical, and monitoring data to create spatial representations of potential volcanic hazards (e.g., lava flow inundation, ash fall distribution)
  • Used to inform land-use planning, emergency response, and public education efforts
• Interdisciplinary approaches, combining geological, geophysical, geochemical, and social science methods, are crucial for comprehensive volcano monitoring, hazard assessment, and risk mitigation strategies