🌋Volcanology Unit 4 – Effusive Eruptions and Lava Flows

Key Concepts and Definitions

• Effusive eruptions characterized by the outpouring of lava onto the Earth's surface
• Lava composition primarily consists of mafic magma (basaltic) with low silica content and low viscosity
• Lava flows classified based on their surface morphology and emplacement style (pahoehoe, aa, block lava)
• Effusion rate measures the volume of lava erupted per unit time, influencing flow behavior and morphology
  • Typically ranges from 1-100 m³/s for basaltic eruptions
• Lava tubes form when the outer surface of a lava flow cools and solidifies while the interior remains molten, insulating the flow and allowing it to travel greater distances
• Inflation occurs when lava is injected beneath a solidified crust, causing the surface to uplift and form distinctive features (tumuli, pressure ridges)

Types of Effusive Eruptions

• Hawaiian eruptions characterized by the effusion of fluid, basaltic lava from central vents or fissures with low explosivity (Kilauea, Hawaii)
  • Produces lava fountains, lava lakes, and extensive lava flows
• Strombolian eruptions involve intermittent explosions of gas and incandescent lava fragments, forming scoria cones and short lava flows (Stromboli, Italy)
• Vulcanian eruptions characterized by short-lived, violent explosions followed by the effusion of viscous, blocky lava flows (Sakurajima, Japan)
• Fissure eruptions occur when lava is erupted from elongated fractures or cracks in the Earth's surface, often associated with rift zones and shield volcanoes (Laki, Iceland)
• Flood basalt eruptions involve the prolonged and voluminous effusion of basaltic lava, covering extensive areas and forming thick lava plateaus (Deccan Traps, India)

Lava Composition and Properties

• Basaltic lava (45-52% SiO₂) has low viscosity, allowing for fluid and fast-moving lava flows
  • Eruption temperatures typically range from 1000-1200°C
• Andesitic lava (52-63% SiO₂) has intermediate viscosity and forms thicker, slower-moving lava flows
• Rhyolitic lava (>63% SiO₂) has high viscosity, often resulting in thick, blocky lava flows or domes
• Lava viscosity influenced by factors such as temperature, composition, and dissolved gas content
  • Higher temperatures and lower silica content lead to lower viscosity
• Lava rheology describes the flow behavior of lava, ranging from Newtonian (basaltic) to non-Newtonian (rhyolitic) fluids
• Lava density typically ranges from 2.6-3.0 g/cm³, depending on composition and vesicularity

Lava Flow Dynamics

• Lava flow morphology influenced by effusion rate, viscosity, topography, and cooling rate
• Pahoehoe lava characterized by smooth, ropy, or wrinkled surface texture, formed by low-viscosity lava at low effusion rates
  • Often associated with lava tubes and inflation features
• Aa lava characterized by rough, jagged, and clinkery surface texture, formed by higher-viscosity lava at higher effusion rates
  • Tends to form channelized flows with levees and flow fronts
• Block lava composed of angular, fractured blocks, typically associated with more viscous, silicic lava flows
• Lava flow velocity depends on factors such as slope, lava viscosity, and channel geometry
  • Basaltic lava flows can reach speeds of several kilometers per hour on steep slopes
• Lava flow thickness varies from a few centimeters to several meters, depending on the effusion rate, viscosity, and topography
  • Thicker flows often develop a layered structure with a dense interior and vesicular crust

Volcanic Landforms from Effusive Eruptions

• Shield volcanoes formed by the accumulation of numerous, fluid lava flows, creating a broad, gently-sloping edifice (Mauna Loa, Hawaii)
  • Characterized by a central caldera and rift zones radiating from the summit
• Lava domes created by the extrusion of viscous, silicic lava, forming steep-sided, dome-shaped structures (Mount St. Helens, USA)
• Lava plateaus formed by the extensive and voluminous effusion of basaltic lava, covering large areas with thick, horizontal lava flows (Columbia River Basalt Group, USA)
• Lava fields consist of multiple, overlapping lava flows, often covering vast areas and displaying a variety of surface morphologies (Krafla, Iceland)
• Lava deltas formed when lava flows enter a body of water, creating new land through the accumulation of fragmented lava and hyaloclastites (Pu'u 'Ō'ō, Hawaii)
• Lava tubes and caves formed by the drainage of molten lava from beneath a solidified crust, leaving behind a network of subsurface passages (Thurston Lava Tube, Hawaii)

Monitoring and Predicting Effusive Eruptions

• Seismic monitoring detects and locates earthquakes associated with magma movement and fracturing of rock
  • Increased seismicity often precedes effusive eruptions
• Ground deformation measurements (GPS, InSAR) track changes in the shape of the volcano, indicating magma intrusion or accumulation
  • Inflation of the edifice suggests pressurization of the magmatic system
• Gas monitoring measures the composition and flux of volcanic gases (SO₂, CO₂), providing insights into magma degassing and ascent
  • Increased gas emissions often signal impending eruptive activity
• Remote sensing techniques (satellite imagery, thermal infrared) used to detect and map lava flows, measure effusion rates, and monitor volcanic activity
• Lava flow modeling predicts the path, extent, and potential hazards of lava flows based on topography, effusion rate, and lava properties
  • Used for risk assessment and hazard mitigation planning

Case Studies and Notable Examples

• Kilauea, Hawaii: Ongoing effusive eruptions since 1983, with the formation of lava lakes, lava tubes, and extensive lava fields (Pu'u 'Ō'ō, Halema'uma'u)
  • 2018 Lower East Rift Zone eruption destroyed over 700 homes and created new land along the coast
• Mount Etna, Italy: Persistent effusive activity with frequent lava flows, lava fountains, and Strombolian explosions
  • 1669 eruption produced a 17-km-long lava flow that reached the city of Catania
• Nyiragongo, Democratic Republic of Congo: Hosts one of the world's most active lava lakes, with periodic drainage and catastrophic lava flows
  • 1977 and 2002 eruptions led to rapid lava flows that caused significant damage and fatalities in the city of Goma
• Laki, Iceland: 1783-1784 fissure eruption produced a 27-km-long lava flow field, covering an area of ~600 km²
  • Released large quantities of sulfur dioxide, causing widespread environmental and health impacts across Europe

Environmental and Societal Impacts

• Lava flows can destroy infrastructure, buildings, and agricultural land, leading to significant economic losses and displacement of communities
  • Slow-moving flows allow for evacuation, while fast-moving flows pose a greater risk to life
• Lava flows can alter local ecosystems by creating new land, modifying drainage patterns, and burying vegetation
  • Long-term ecological succession can lead to the establishment of unique lava flow communities
• Volcanic gases released during effusive eruptions can impact air quality and contribute to acid rain, affecting human health and vegetation
  • Sulfur dioxide can cause respiratory issues and exacerbate pre-existing conditions
• Lava entering the ocean can create laze (lava haze), a corrosive mixture of steam, hydrochloric acid, and volcanic glass particles
  • Inhalation of laze can cause respiratory irritation and damage to the eyes and skin
• Effusive eruptions can attract tourists and boost local economies through volcano tourism and geothermal energy development
  • Requires careful management to ensure visitor safety and minimize environmental impacts
• Lava flows can preserve archaeological sites and artifacts, providing insights into past human activities and environmental conditions (Pompeii, Italy)