OREGON STATE UNIVERSITY

Pahoehoe

 

 

Pahoehoe flows differ from 'a'a flows in almost all ways imaginable.

The first and most obvious difference is that pahoehoe flows are smooth down to a scale of a few mm. Instead of consisting of only 1-2 large flow units, a pahoehoe flow consists of thousands on thousands of small flow units called toes. Each toe is usually <30 cm thick, 1-2 m long, and 30-50 cm wide.

 

Close-up photo of an active pahoehoe toe. This toe is about 30 cm wide at its widest. Note how it has erupted out of a crack in a previous toe and is flowing over yet another previous toe (with the ropy texture). Note also that with the sun shining on it, one side of the active toe doesn't look all that different from the surfaces of the older inactive toes; late afternoon and early morning (and night) are the best times for observing lava flows.

Pahoehoe flows are associated with low-effusion rate eruptions and are emplaced at low volumetric flow rates (2-5 cubic meters per second) and slow flow front velocities (1-10 m/hour) [See the A'a page for a velocity comparison chart]. Pahoehoe flows can be just as long as 'a'a flows. The longest post-contact flow was also erupted from Mauna Loa in 1859 (forming the second half of the "paired flow"; Rowland & Walker 1990), and is 47 km long. This strongly contradicts the notion that flow length is directly determined by effusion rate.

The low velocity of pahoehoe flows means that the skin that forms by air-cooling is not disrupted during flow and can maintain its smooth, unbroken, well-insulating surface. Thus the temperature and viscosity of lava do not change very much even tens of kilometers from the vent. The advancing front of a pahoehoe flow consists of hundreds or thousands of active toes. Each stops flowing after a few minutes and becomes inflated (with lava) as the eruption continues. Eventually the cooled skin fractures, often at the seam between two toes, and a new toe forms.

 

Cross-section of a pahoehoe flow exposed in a sea cliff. Some of the individual flow units have been outlined in white but you can see many others. Two particularly large ones were probably flowing as small lava tubes; the one labeled 'd' drained out at the end of the eruption and the one labeled 'f' solidified full. The dashed pink lines mark the top and bottom of the pahoehoe flow; above and below are 'a'a flows.