🌋Volcanology Unit 5 – Explosive Eruptions and Pyroclastic Deposits

Key Concepts and Terminology

• Explosive eruptions occur when magma is fragmented and violently ejected from a vent due to the rapid expansion of gases
• Pyroclastic materials include tephra (ash, lapilli, and bombs), pumice, and scoria formed during explosive eruptions
• Pyroclastic density currents (PDCs) are ground-hugging flows of hot gas and pyroclastic materials that move at high speeds
  • Pyroclastic flows are dense, ground-hugging currents that follow topography
  • Pyroclastic surges are more dilute and can surmount topographic barriers
• Tephra fall deposits form when pyroclastic materials are ejected into the atmosphere and settle back to the ground
• Volcanic explosivity index (VEI) is a scale used to quantify the magnitude and intensity of explosive eruptions
• Plinian eruptions are characterized by sustained, powerful eruption columns that can reach stratospheric heights (>11 km)
• Vulcanian eruptions are short-lived, violent eruptions that produce dense, ash-laden plumes and ballistic projectiles

Types of Explosive Eruptions

• Plinian eruptions are the most powerful and destructive type of explosive eruption
  • Characterized by sustained, high eruption columns (>20 km) and widespread tephra fall deposits
  • Examples include the 79 AD eruption of Vesuvius and the 1991 eruption of Mount Pinatubo
• Sub-Plinian eruptions are similar to Plinian but with lower eruption column heights (10-20 km) and shorter duration
• Vulcanian eruptions are short-lived, violent eruptions that produce dense, ash-laden plumes and ballistic projectiles
  • Characterized by the formation of volcanic plugs and domes that can collapse and generate pyroclastic flows
  • Examples include the 1902 eruption of Mount Pelée and the 1997 eruption of Soufrière Hills
• Strombolian eruptions are characterized by rhythmic, low-level explosions that eject incandescent lava fragments and ash
• Surtseyan eruptions occur when magma interacts with water, producing steam-driven explosions and the formation of tuff cones
• Phreatomagmatic eruptions result from the interaction of magma with groundwater or surface water, generating ash-rich plumes and pyroclastic surges

Factors Influencing Eruption Explosivity

• Magma composition plays a crucial role in determining eruption explosivity
  • Silica-rich magmas (rhyolitic) are more viscous and tend to produce more explosive eruptions
  • Mafic magmas (basaltic) are less viscous and often result in less explosive, effusive eruptions
• Magma temperature affects viscosity and gas solubility, with higher temperatures promoting more fluid magmas and greater gas escape
• Magma gas content is a key factor in driving explosive eruptions, as the rapid expansion of gases fragments the magma
  • Water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2) are common magmatic gases
• Magma ascent rate influences the ability of gases to escape, with faster ascent rates leading to more explosive eruptions
• Vent geometry and conduit shape can affect the dynamics of magma ascent and gas escape, impacting eruption explosivity
• External factors such as groundwater interaction or edifice collapse can trigger or enhance explosive eruptions

Pyroclastic Materials and Their Formation

• Tephra is a general term for pyroclastic materials ejected during an explosive eruption
  • Ash refers to particles <2 mm in diameter, often formed by the fragmentation of magma or rock
  • Lapilli are pyroclasts between 2-64 mm in size, typically formed from the solidification of molten droplets
  • Volcanic bombs are larger pyroclasts (>64 mm) that are ejected in a semi-molten state and can have aerodynamic shapes
• Pumice is a highly vesicular, low-density volcanic rock formed from the rapid cooling of felsic magma
  • Pumice can be light enough to float on water due to its high bubble content
• Scoria is a vesicular, basaltic to andesitic volcanic rock that forms from the cooling of mafic to intermediate magmas
  • Scoria has a higher density than pumice and typically does not float on water
• Pyroclastic materials are formed through various processes during explosive eruptions
  • Magma fragmentation occurs when the rapid expansion of gases shatters the magma into smaller particles
  • Comminution is the mechanical breakdown of particles through collisions and abrasion within the eruption column or pyroclastic flows
  • Accretionary lapilli form when ash particles aggregate around a nucleus, often in the presence of moisture

Pyroclastic Flows and Surges

• Pyroclastic density currents (PDCs) are ground-hugging flows of hot gas and pyroclastic materials that move at high speeds
• Pyroclastic flows are dense, ground-hugging currents that follow topography and are controlled by gravity
  • They can reach speeds of 100-700 km/h and temperatures of 200-700°C
  • Pyroclastic flows are often generated by column collapse, dome collapse, or the fallback of ejected materials
• Pyroclastic surges are more dilute, turbulent, and less confined by topography compared to pyroclastic flows
  • They can surmount topographic barriers and are often associated with phreatomagmatic eruptions or dome collapse events
• The destructive potential of PDCs is due to their high temperatures, high velocities, and the presence of toxic gases and ash
  • PDCs can cause extensive damage to infrastructure, vegetation, and loss of life in affected areas
• The behavior and runout distance of PDCs are influenced by factors such as eruption magnitude, topography, and particle concentration
• Deposits from pyroclastic flows and surges can provide valuable information about eruption dynamics and flow properties
  • Flow deposits are often massive, poorly-sorted, and can contain charred vegetation or other entrained materials
  • Surge deposits are typically finer-grained, better-sorted, and may exhibit cross-bedding or dune-like structures

Tephra Fall Deposits

• Tephra fall deposits form when pyroclastic materials are ejected into the atmosphere and settle back to the ground
• The distribution and thickness of tephra fall deposits are influenced by factors such as eruption column height, wind direction, and particle size
  • Larger particles (bombs and lapilli) are deposited closer to the vent, while finer particles (ash) can be transported over greater distances
• Isopach maps are used to represent the thickness and distribution of tephra fall deposits
  • Isopachs are lines of equal deposit thickness, which can help in estimating the volume of ejected material and eruption magnitude
• Tephra fall deposits can have significant impacts on the environment, human health, and infrastructure
  • Ash fall can cause respiratory issues, damage crops, and disrupt transportation and communication networks
  • The accumulation of thick tephra deposits can lead to roof collapse, burial of vegetation, and changes in landscape
• The study of tephra fall deposits can provide insights into eruption dynamics, plume behavior, and wind patterns
  • Grain size analysis can reveal information about fragmentation processes and transportation mechanisms
  • Chemical analysis of tephra can help in determining the magma composition and identifying the source volcano

Hazards and Risk Assessment

• Explosive eruptions pose significant hazards to human life, infrastructure, and the environment
• Pyroclastic density currents (PDCs) are one of the most dangerous hazards associated with explosive eruptions
  • PDCs can cause direct injury through heat, asphyxiation, and impact from entrained debris
  • The high velocities and long runout distances of PDCs make them difficult to escape or mitigate
• Tephra fall can have widespread impacts, affecting air quality, agriculture, and transportation
  • Fine ash particles can cause respiratory issues and contaminate water supplies
  • The accumulation of tephra can lead to roof collapse and damage to vegetation
• Volcanic gases released during explosive eruptions can pose health risks and contribute to environmental impacts
  • Sulfur dioxide (SO2) can cause acid rain and respiratory irritation
  • Carbon dioxide (CO2) can accumulate in low-lying areas, leading to asphyxiation
• Lahars (volcanic mudflows) can be triggered by the interaction of pyroclastic deposits with water, posing a significant hazard to downstream communities
• Risk assessment involves evaluating the likelihood and potential consequences of volcanic hazards
  • Hazard maps are used to delineate areas at risk from various volcanic processes
  • Vulnerability assessments consider the exposure and resilience of populations, infrastructure, and critical facilities
• Monitoring and early warning systems are crucial for mitigating the risks associated with explosive eruptions
  • Seismic monitoring can detect changes in volcanic activity and provide early warning of impending eruptions
  • Gas monitoring can help in identifying changes in magma chemistry and degassing patterns
• Effective communication and education are essential for promoting awareness and preparedness among at-risk communities

Case Studies and Historical Eruptions

• The 79 AD eruption of Mount Vesuvius is a well-known example of a Plinian eruption
  • The eruption buried the Roman cities of Pompeii and Herculaneum under thick layers of tephra and pyroclastic flows
  • The preserved remains provide valuable insights into Roman life and the impacts of explosive eruptions
• The 1883 eruption of Krakatoa in Indonesia was a catastrophic event that generated powerful pyroclastic flows and tsunamis
  • The eruption caused widespread destruction and resulted in the deaths of over 36,000 people
  • The eruption had global climatic impacts, with a significant reduction in average global temperatures due to the injection of ash and aerosols into the stratosphere
• The 1980 eruption of Mount St. Helens in Washington, USA, is an example of a Plinian eruption with a lateral blast component
  • The eruption was preceded by a massive landslide that exposed the cryptodome and triggered a lateral blast
  • The blast devastated an area of nearly 600 km², flattening forests and destroying infrastructure
• The 1991 eruption of Mount Pinatubo in the Philippines was the second-largest eruption of the 20th century
  • The eruption produced voluminous tephra fall deposits and extensive pyroclastic flows
  • The eruption had significant global climatic impacts, with a reduction in average global temperatures and ozone depletion
• The ongoing eruption of Soufrière Hills on the island of Montserrat, which began in 1995, has been characterized by dome growth and collapse events
  • Pyroclastic flows generated by dome collapses have caused destruction and loss of life in the southern part of the island
  • The eruption has led to the evacuation and relocation of a significant portion of the island's population
• The 2010 eruption of Eyjafjallajökull in Iceland disrupted air travel across Europe due to the widespread dispersal of fine ash
  • The eruption highlighted the vulnerability of modern transportation networks to volcanic ash hazards
  • The economic impacts of the eruption were significant, with losses estimated in the billions of dollars