Effusive eruptions occur when low-viscosity, low-gas magma rises to the surface. These gentle events produce lava flows, domes, and fountains. Understanding the mechanisms behind effusive eruptions helps us predict their behavior and assess potential hazards.

Key factors influencing effusive eruptions include magma properties, gas content, and supply rate. By studying these elements, volcanologists can better forecast eruption duration, intensity, and potential impacts on surrounding areas.

Physical Processes in Effusive Eruptions

Magma Ascent and Exsolution

• Effusive eruptions occur when magma with low viscosity and low gas content rises to the surface
• Magma ascends through the crust via buoyancy due to its lower density compared to the surrounding rock
• As magma approaches the surface, decreasing pressure allows dissolved gases to exsolve and form bubbles
  • If the magma ascent rate is slow and steady, gases can escape without building up excessive pressure, resulting in an effusive eruption

Eruption Characteristics and Features

• Effusive eruptions are characterized by the relatively gentle effusion of lava onto the Earth's surface
• Lava flows, lava domes, and lava fountains are common features of effusive eruptions
  • Lava flows involve the outpouring of molten rock onto the surface (pahoehoe and a'a flows)
  • Lava domes are mounds of viscous lava that pile up around the vent (Soufrière Hills, Montserrat)
  • Lava fountains are jets of molten lava propelled into the air by rapid gas release (Kilauea, Hawaii)

Magma Properties and Effusive Eruptions

Magma Viscosity

• Magma viscosity is a measure of its resistance to flow, which is influenced by factors such as composition, temperature, and dissolved gas content
• Low-viscosity magmas, typically basaltic in composition, have a lower silica content and higher temperature, allowing them to flow more easily
  • Examples of low-viscosity magmas include those erupted at Kilauea, Hawaii and Piton de la Fournaise, Réunion
• High-viscosity magmas, such as rhyolitic compositions, have a higher silica content and lower temperature, making them more resistant to flow
  • Examples of high-viscosity magmas include those erupted at Chaitén, Chile and Cordón Caulle, Chile

Gas Content and Bubble Formation

• Gas content in magma affects the explosivity of an eruption. Low gas content promotes effusive behavior, while high gas content can lead to explosive eruptions
• As magma rises and decompresses, dissolved gases exsolve and form bubbles. In low-viscosity magmas, these bubbles can easily escape, allowing for effusive eruptions
  • Gases commonly exsolved during magma ascent include water vapor, carbon dioxide, and sulfur dioxide
• The balance between magma viscosity and gas content determines the style of the eruption, with low-viscosity, low-gas magmas favoring effusive behavior

Types of Effusive Eruptions

Lava Flows

• Lava flows are the most common manifestation of effusive eruptions, involving the outpouring of molten rock onto the Earth's surface
  • Pahoehoe lava flows have a smooth, ropy, or wrinkled surface and advance slowly, forming as the lava cools and develops a skin
  • A'a lava flows have a rough, jagged, or clinkery surface and move as a thick, viscous mass, often forming levees and channels

Lava Domes and Fountains

• Lava domes are mounds of viscous lava that pile up around the vent, growing by the extrusion of pasty lava with a high crystal content
  • Lava domes can be associated with hazards such as dome collapse, pyroclastic density currents, and explosive eruptions
  • Examples of lava domes include those formed during the eruptions of Mount St. Helens, USA and Santiaguito, Guatemala
• Lava fountains are jets of molten lava propelled into the air by the rapid release of dissolved gases in low-viscosity magma
  • Lava fountains can range in height from a few meters to several hundred meters and often feed lava flows or build spatter cones
  • Examples of lava fountains can be observed at Kilauea, Hawaii and Etna, Italy

Factors Influencing Effusive Eruptions

Magma Supply and Volume

• The duration of an effusive eruption depends on the magma supply rate and the volume of available magma in the reservoir
  • Effusive eruptions can last from a few hours to several decades, depending on the balance between magma supply and drainage
  • Long-duration effusive eruptions, such as Kilauea, Hawaii, are sustained by a steady magma supply
• The intensity of an effusive eruption is influenced by the magma ascent rate, which controls the volume of lava erupted over time
  • Higher magma ascent rates lead to more intense effusive eruptions with greater lava output, while lower ascent rates result in less intense eruptions

Structural Controls and Magma Properties

• The geometry and size of the magma reservoir, as well as the presence of structural controls like faults or fissures, can affect the duration and intensity of effusive eruptions
  • Large, interconnected magma reservoirs can sustain longer-duration effusive eruptions (Kilauea, Hawaii)
  • Fissure eruptions, such as those in Iceland, are controlled by the presence of extensional tectonic structures
• Regional stress fields and tectonic settings can influence the location, frequency, and characteristics of effusive eruptions
  • Effusive eruptions are common in rift zones (East African Rift) and hot spot settings (Hawaii)
• Changes in magma composition, temperature, or gas content during an eruption can cause variations in the duration and intensity of effusive activity
  • Magma differentiation and mixing processes can lead to changes in eruption style over time (Stromboli, Italy)