Geology 309 - Lecture 10

Pyroclastic eruptions

Three stages
• fragmentation by bubble growth
• blasting out of vent
• ascent of eruption column - sometimes > 40 km high

Important variables
• Magma viscosity
• Dissolved volatile content
• Mass eruption rate = intensity = rate at which magma is discharged (kg/s)
• Ejection velocity (km/s)
  
  Gases need to expand into growing vesicles
  
  
  • If magmas are low viscosity - no problem
    
  • If magmas are high viscosity - big problem (magmas have high yield strength and can't "move over" as bubbles grow)
    
  • If magmas have low volatile (gas) content - no problem
    
  • If magmas have high volatile content - bigger problem
    
    
Three types of pyroclastic eruptions
• Plinian - sustained jets with ejection velocities 100-400 m/s. Magma and bubbles moving upward together at same speed, so no relative difference. Eruption columns: 20-40 km See Fig 8.1
  
  
• Strombolian - Individual, closely spaced explosions feed rising jet of magma. Maximum ejection velocity 200 m/s. Magma solidifies and slows down in air, then falls to form parabolic path. Magmas rise slowly and smoothly, bubbles can coalesce, swell, and move upwards faster than rising magma. "Strombolian burps". Eruption columns: hundreds of meters. See Fig 8.2
  
  
• Vulcanian - Single event or discrete bursts taking place at intervals of minutes to hours. Ballistic ejection of blocks and bombs. Ejection velocities ~200 m/s calculated, sometimes 400 m/s measured (so groundwater probably involved). Ejected material is commonly shattered fragments of solid lava plug which chilled in volcanic vent (champagne cork idea). Some juvenile magmatic material, not very vesiculated. Eruption columns 5-10 km high. Associated shock waves. See Fig 8.4
  
Explanations of how eruption columns work are complicated equations. We won't work with them here.

A. Dynamics of Plinian eruptions

Plinian column (Fig. 8.7)
• Gas thrust region - just above the vent (1-2 km).
• Convective region - 2-40 km. Whole mass is less dense than air and rises
• Umbrella region - eruption column reaches height of neutral buoyancy and spreads sideways
  
  - Width of eruption columns increases as they get higher
  Width = initial width + (constant * height)
  Constant is ~0.125
  
  

B. Heights of eruption columns - limited

Depends on temperature of atmosphere & intensity of eruption

Temperature of atmosphere. Why important? Think of why hot air balloon rises.


• Troposphere - T decreases with height
• Stratosphere - T increases with height


Intensity of eruption. Most important factor. Measure of thermal energy being pumped into eruption column
• Intensity = mass eruption rate (kg/s)
• Can calculate expected height of column (complicated algebraic equation on p. 173 of text) by knowing mass eruption rate modified by temperature of atmosphere
• Simply, height of eruption column varies as 4th root of eruption rate
• Results summarized in Figure 8.9 of text
  
  
• Can also work backwards and calculate the intensity of a Plinian eruption knowing its column height. This has been done for Glacier Peak for an ancient eruption, using other methods to determine column height
  
  
  

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