Lava domes and cryptodomes are unique volcanic structures formed by viscous magma. These features shape volcano morphology, creating bulbous mounds or hidden intrusions that can lead to explosive eruptions and hazardous collapses.

Understanding lava domes and cryptodomes is crucial for assessing volcanic risks. Their formation, growth mechanisms, and potential for catastrophic failure highlight the complex interplay between magma properties and volcanic landforms, influencing eruption styles and hazard potential.

Lava Domes and Cryptodomes: Growth and Structure

Lava Dome Formation and Characteristics

• Lava domes develop when highly viscous, silica-rich magma extrudes from a volcanic vent and accumulates around the opening rather than flowing away as a lava flow (rhyolitic or dacitic magmas)
• Cryptodomes are lava domes that form beneath the Earth's surface, causing the ground above to swell and bulge upwards (Usu volcano, Japan)
• Lava domes and cryptodomes often display a combination of endogenous and exogenous growth throughout their development leading to complex internal structures
• The outer surface of lava domes and cryptodomes usually consists of a carapace of brittle, fractured lava ranging from blocky and massive to highly brecciated and unconsolidated

Endogenous and Exogenous Growth Mechanisms

• Endogenous growth happens when magma is injected into the interior of the dome causing it to expand from within
  • Results in various internal structures such as shear lobe boundaries, magma tubes, and radial fractures
• Exogenous growth occurs when magma extrudes onto the outer surface of the dome frequently forming short, thick lava flows called coulees
  • Leads to a layered or "onion-skin" internal structure
• As lava domes and cryptodomes grow, they can become oversteepened and unstable resulting in partial or complete collapse
  • Collapses generate hazards like hot rockfalls, pyroclastic flows, and debris avalanches (Unzen volcano, Japan)

Hazards of Lava Dome Collapse and Cryptodome Eruptions

Pyroclastic Flows, Surges, and Debris Avalanches

• Lava dome collapses can produce devastating pyroclastic flows and surges which are hot, ground-hugging mixtures of ash, gas, and rock fragments traveling at high velocities and covering extensive areas (Merapi volcano, Indonesia)
• Larger lava dome collapses can also generate debris avalanches which are rapidly moving masses of rock debris and ash capable of traveling tens of kilometers from the source
• The hazards associated with lava domes and cryptodomes can persist for long periods as these features can remain active and unstable for months to years after their initial formation

Explosive Cryptodome Eruptions and Lateral Blasts

• Cryptodome eruptions can be especially hazardous due to their sudden and explosive nature
  • As magma intrudes beneath the surface, it can cause the overlying rock to fracture and bulge upwards potentially leading to a sudden decompression and explosive fragmentation of the magma (Mount St. Helens, USA)
• Cryptodome eruptions can generate powerful lateral blasts which are highly destructive, ground-hugging currents of hot ash, gas, and rock fragments capable of leveling forests and destroying infrastructure over a wide area
• Both lava dome collapses and cryptodome eruptions can be accompanied by the release of volcanic gases, particularly sulfur dioxide leading to respiratory hazards and acid rain formation

Magma Viscosity and Lava Dome Formation

Role of Magma Viscosity in Dome Formation

• Magma viscosity plays a crucial role in controlling the formation of lava domes and cryptodomes
  • High-viscosity magmas, typically rich in silica and dissolved gases, are more likely to form these features than low-viscosity, mafic magmas (andesitic or basaltic magmas)
• High-viscosity magmas resist flow and tend to accumulate around the vent forming a mound or dome-shaped extrusion
  • The magma's ability to trap gases also contributes to the formation of a cohesive, spiny or blocky dome
• Lower-viscosity magmas within the lava dome or cryptodome can sometimes extrude through fractures in the cooler, more viscous outer shell forming short lava flows or spines

Factors Influencing Magma Viscosity and Dome Formation

• The relationship between magma viscosity and dome formation can be influenced by factors such as:
  • Magma composition (silica content)
  • Temperature
  • Gas content (volatile content)
  • Rate of magma supply
  • Configuration of the volcanic conduit
• The high viscosity of the magma can also lead to the buildup of gas pressure within the dome or cryptodome increasing the likelihood of explosive decompression and collapse

Notable Examples of Lava Domes and Cryptodomes

Lava Dome Examples

• The Soufrière Hills volcano on the island of Montserrat has been producing a series of lava domes since 1995 with numerous collapses generating pyroclastic flows and ash plumes
• Santiaguito, Guatemala, is a complex of four lava domes that have been actively growing since 1922 often producing small to moderate explosive eruptions and pyroclastic flows
• Lava domes are a common feature in the crater of Mount Merapi, Indonesia, one of the most active and hazardous volcanoes in the world
  • Frequent dome collapses at Merapi generate deadly pyroclastic flows and lahars
• The Novarupta-Katmai eruption of 1912 in Alaska, USA, formed a lava dome within the vent area following the cataclysmic eruption that produced the Valley of Ten Thousand Smokes ash flow deposit

Cryptodome Examples

• The 1980 eruption of Mount St. Helens, USA, involved the catastrophic failure of a cryptodome resulting in a massive lateral blast, debris avalanche, and pyroclastic flows
• The 1951 eruption of Mount Lamington, Papua New Guinea, involved the explosive destruction of a cryptodome generating devastating pyroclastic flows that claimed over 3,000 lives