Magma Viscosity, Volatiles, and Eruption Dynamics

Posted on Jan 25, 2025 in Geology

Magma Viscosity and Volatiles

Felsic magmas are more viscous than mafic magmas due to polymerization.

  • Crystallization increases magma viscosity.
  • Crystallization of anhydrous minerals increases volatile concentration.
  • Loss of water by degassing increases magma viscosity.

Subduction-related magmas are richer in volatiles (H2O, Cl) due to the recycling of subducted material.

Exsolution sequence: CO2 > He >> H2O > S > Cl > F

Volatile Content

Melt inclusions are small pockets of silicate melt trapped in phenocrysts during their growth. Different techniques are used to analyze H2O, CO2, S, Cl, and F.

Degassing path of a lava erupted at the surface:

  1. Bubble nucleation
  2. Bubble growth
  3. Outgassing (loss of gas)
  4. Fragmentation

Volatiles are incompatible with minerals, so crystallization increases the volatile content of silicate melt, leading to over-saturation.

Crystallization and Nucleation

Crystallization favors nucleation, also known as second boiling.

Bubbles can influence the ascent rate, leading to explosive eruptions.

Exsolution increases melt viscosity, making it more difficult for bubbles to rise, coalesce, and degas. Pressure can build up, leading to explosive eruptions.

Flow is a function of coalescence and magma viscosity, especially the mobility of bubbles with respect to the melt.

Viscous and Fluid Magmas

Viscous magmas: Bubbles move at the same velocity as the magma in bubbly flow, with low coalescence.

Fluid magmas: Bubbles ascend faster than the melt, with strong coalescence. Slug flow leads to Strombolian eruptions, and annular flow leads to Hawaiian eruptions.

Outgassing in Different Melts

Outgassing in basaltic melts: Low ascent rate – Bubbles rise buoyantly = separate flow – Interconnected network = permeable flow – Degassing through magma convection in the conduit. Quiescent degassing, effusive eruptions.

Outgassing in viscous melts: Relaxation of viscous stresses keeps pace with bubble growth – Permeable bubble network develops, allowing outgassing. Effusive eruptions as steep-sided lava flows and lava domes.

  • Low ΔP: slow magma ascent → degassing → effusive activity
  • High ΔP: fast magma ascent → no degassing → explosive activity

Fragmentation: Rupture due to overpressure inside the bubbles. High strain rates cause the melt to cross the glass transition.

Ductile fragmentation for fluid magmas: Magma fragments when large bubbles burst at the surface due to stretching as the gas decompresses and expands.

Brittle fragmentation for viscous magmas: Closed-system degassing leads to overpressure inside the bubbles. Rupture produces a fragmented mixture of gases and particles that explosively rushes out into the atmosphere.

Internal triggers: Overpressure in bubbles. External triggers: Rapid decompression caused by the collapse of an edifice or lava dome.

Internal triggers: Rapid acceleration due to overpressure or high strain rate. External triggers: Rapid decompression caused by the collapse of an edifice or lava dome.

Classification according to the size of pyroclasts:

  • Blocks, bombs (d > 64 mm)
  • Lapilli (64 mm > d > 2 mm)
  • Ash (d < 2 mm)

Classification according to the density:

  • Vesiculated particles
  • Dense particles = Lithic

Degassing: First (decompression-induced) and second (crystallization-induced) boiling.

Magma fragmentation occurs when the bubble overpressure is too high, producing a mixture of gas and particles exploding as a jet out of the conduit.

Magma ascent rate is controlled by:

  • Temperature
  • Composition
  • Volatile budget
  • Crystallinity

All factors affect viscosity, density, and permeability.