Geology 309 - Lecture 8

I. Basaltic Lava Flows

Temperatures Mauna Loa 1200 C; Kilauea ~1150 C; Etna ~1080 C
The lava is effectively solid by <1000 degrees Celsius

Rheology Lavas can flow 60 miles/hr at 1200 C, then progressively slow down as they cool
Newtonian at highest temperature (yield strength approaches 0)
Becomes Pseudoplastic at lower T and higher crystallinity

Some measurement terminology of lava flows:
Effusion rate: volume/second at vent
• Typical values for basalt: 0.5 to 5000 cubic meters/second
• Historic flows from Mt. Etna: ~0.5 cubic meters/second
• Fissure-generated flows associated with Icelandic Laki eruption in 1783: ~5000 cubic meters/second
• Effusion rates for andesite and dacite are much lower (0.05 cubic meters/second) due to their higher viscosities

Flow rate: meter/second anywhere in flow
Flow front advance rate: speed of lava across the surface

Description of Pahoehoe vs. Aa

Pahoehoe - least viscous of all lavas - 10 to 100 poise


Pahoehoe flows can thicken by inflationg (internal injection of lava under a crust) so that final flow thickness can be > than flowing lava


Aa - more viscous; jumble of loose, irregularly shaped cindery blocks, razor sharpt edges

• usually upper rubbly part (clinkers) and a lower, massive part consisting of solid lava that cooled slowly
• pasty lava core moves downslope, clinkers carried along surface
• at leading edge, clinkers roll off front end, get overrun, so flow has layers of fragments at top and bottom
  
  
Thickness of flowing aa is thicknes of final lava bed



Effects of viscosity/yield strngth and rate of strain (or flow)

Development of crust:
• low stress/strain ratio forms smooth skin = pahoehoe
• higher stress/strain ratio forms ruptured crust = aa texture
  
  
Pahoehoe to aa transition
• Can go from pahoehoe to aa, but not the reverse
  
• Same chemical composition
  
• Caused by change (increase) in viscosity. What can cause that change? (loss of gas, decrease T, increase crystallinity)
  
• Also can be caused by change (increase) in velocity as when a lava flows over a pali.
  
• Governed by graph of rate of flow vs. viscosity

1. Pahoehoe lava features
• Lava tubes
  
• Tumuli
• Tree molds and tree casts
  
• shelly pahoehoe - thin skin overlying large cavities
• ropy pahoehoe
• entrail pahoehoe
• pahoehoe blister
• pahoehoe toe
  
• Lava terraces - as magma deflates, get "bathtub rings" around canyon edges
  
  
2. Aa lava features
• Accretionary lava balls
  

• Planar Jointing - may mark flow foliation
  
• Columnar Jointing - cooling *after* solidification