Geology 309 - Lecture 12

I. Pyroclastic fall deposits related to eruption type

A. Hawaiian eruptions

Gas and liquid separate easily, magma not explosively fragmented. Little ash accumulation
Remember, scoria cone vs. spatter cone determined by eruption rate.

Deposits are:
• Bombs
  
• Pele's hair
  
• Pele's tears
  
• Reticulite



B. Strombolian eruptions

• Small eruptions, <0.01 cubic kilometers, heights of eruption column rarely more than few hundred meters
  
• No Pele's hair or tears since magma is so viscous
  
• No reagglutination of lava after eruption
  
• Basaltic scoria most common
  
• Maximum clast sizes decrease rapidly away from vent (bc is small)
  
• Deposits thin rapidly away from vent (bt is small)
  
• Thus, cone forming eruptions are typical (Fig 9.8 of text) with beautiful layering from successive pulses (Fig 9.9 of text).



C. Vulcanian eruptions

• Eruption volumes <1 cubic kilometer, short blasts with eruption columns <10 km high
• Between Strombolian and Plinian
• Typically, material ejected is already solidified andesite-dacite
• Bombs, especially breadcrust bombs, and Blocks are common
• Fine grained, and moderate dispersal, so plot high on Pyle diagram
  


D. Sub-plinian eruptions

• Same tephra as Plinian, but typically 5-20 km high column height


E. Plinian eruptions

• 20-40 km eruption column heights
• Extensive sheets of tephra that mantle topography, so bt large (See Fig. 9.2)
• Read about Vesuvius (Chapters 9 and 4) in 79 AD
  Discussion of white and grey pumice, the different sizes, and what they tell us about the progression of the eruption column. Look at isopach and isopleth maps.
  See Volcanic Phenomena at Pompeii for another discussion of this eruption with some photographs.
  
  
• Dense rock equivalent - calculate volume of tephra erupted using isopach maps, then convert to magma volume (dense rock equivalent) by removing volume of vesicles


F. Ultra-Plinian eruptions

• 40 km eruption column height
• Very thick deposits far away from vent (bt very large)



G. Surtseyan eruptions

• Basaltic to basaltic andesite magma in contact with seawater
• Overall fine grain size because of high degree of fragmentation, so at top of Pyle diagram (Fig. 9.2)
• Don't extend far from vent (so bt small)
• Average grain size is small, btu range in clast size is great. So more poorly sorted than Strombolian.
• Finely laminated

H. Phreatoplinian eruptions

• large volumes of silicic magmas contact water
• Only distinction is ultra-fine grain size
• Accretionary lapilli common (ash around raindrop?)

II. Putting it all together - Pyle diagrams

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