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. 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). 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.
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