Geology 309 - Lecture 11

C. Tephra Fallout and Ash Cloud Mechanics

1. Tephra Fallout - What is Tephra?

A. Mechanics - General case

To know when a particle will fall out of eruption column, will need to terminal fall velocity of particle and the velocity distribution within the column

Terminal fall velocity: Maximum downward velocity attainable by a particle


• Defined by equation defined in class and in text: depends on drag coefficient, clast diameter, clast density, and atmospheric density.
  
  
• Basically:
  • If terminal fall velocity is less than upwards velocity of column, will be carried upwards to umbrella region
    
  • If terminal velocity greater than upwards velocity of column, then will fall out (but must fall outside the plume, otherwise it will get carried up again)
    
    Generally, most eruptions can carry fist size particles to top of eruption column.
    
    
Velocity distribution in column: assume that upwards vertical velocity within the column decreases from center of column outwards. From this, can construct clast support envelopes.

Fig 8.11 Clast support envelope
Fig 8.12 - larger eruptions disperse more widely

Summary: Larger the particle, closer to the vent it will fall out. Smaller the particle, further out in the "umbrella" they can get carried.

B. Effect of Wind

Plume (eruption column) is obviously deflected downwind. Fig. 8.14
• Stagnation point = point at which upwind spreading velocity balances downwind drift velocity
• d = displacement of column axis
  
  
Most important for small eruptions, where strongest winds are in troposphere (See Fig. 8.17)



C. Why do we care about predicting shape and size of eruption column?

• Important for determining conditions during past eruptions. Can infer column height and wind speed using equations mentioned in text.
  
  
• Incredibly important for aircraft. Must be able to predict where the ash clouds will go. Travels 100 km/hr, so must be able to make rapid predictions
  
  Hazards of volcanic ash to aircraft
  Volcanic Ash and Aircraft
  
  

C. Pyroclastic fall deposits

1. Things to measure

• Thickness of deposits Isopach maps. Used to determine volume of eruption.
  Example: Rio Blanco tephra isopach map
  
  Thickness half distance (bt) - distance over which measured thickness of airfall deposit decreases by half from maximum value.
  
  
• Grain size - maximum clast in any given area. Isopleth maps. Used to determine height of eruption column.
  Example: Rio Blanco tephra isopleth map
  Example of isopleth map affected by wind
  
  Clast size half distance (bc) - distance over which maximum clast size decreases by half from maximum value.
  
  
• Degree of sorting
  
  
• Vesicularity
  
  

2. Classifying eruptions and their deposits

• Half distance ratio - ratio of bc/bt
  

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