Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

over a lava fall or down a series of lava
cascades.
Two additional varieties of pahoehoe
are commonly recorded by observers of
actively erupting flows in Hawaii but are
more difficult to recognize in the congealed
flows at Lava Beds National
Monument. Pahoehoe toes from 1 to
several feet in length may sprout forward
all along the front of some advancing
lava flows. In places the chief manner of
forward movement is by the extending
and overriding of successive pahoehoe
toes. Shelly pahoehoe congeals where
large hollow lava blisters 3 ft or more in
diameter have formed beneath a thin
crust of erupting volatile-rich lava.
These large lava blisters flow out, flatten,
and override one another. Fleener
Chimneys, in the monument, erupted
shelly pahoehoe as the last part of the
eruption that produced the Devils Homestead
flow. Much of this flow is aa (fig.
7), particularly farther from the vents at
Fleener Chimneys.
The bubbles and blisters that form in
molten lava are produced by release of
water and other gases from the molten
rock. When pressure is lowered by rise of
molten liquid, called magma, to the
surface, or by the turbulence of tumbling
over a cascade, the lava may froth just as
the dissolved carbon dioxide in beer will
froth and form bubbles as you open the
can and tumble the beer into a glass.
Not all collapses of lava-tube roofs
took place after volcanism ceased. Many
lava tubes contain easily decipherable
records of breakdown that occurred
when molten lava was flowing through a
tube. Careful examination of the congealed
surface of the last flow of lava
down a tube is likely to reveal both small
and large blocks of rock that tumbled
from the roof of the cave and were then
rafted downstream on the molten flood
until it, in turn, congealed into rock.
Large rafted blocks are shown on the
maps of this report .
If a large segment of a tube roof
collapses while the tube is still filled or
half-filled with flowing lava, a number
of events can occur that leave their record
in the rocks to be examined long after
volcanism ceased. If the tube is only half
full of flowing lava, and the thickness of
collapse debris is nearly equal to the
Figure 5. Ropy pahoehoe. A pasty redhot
rind of partly congealed magma at
the surface of a lava flow was folded and
twisted into rope-like ridges as it was
dragged forward by the molten rock
beneath. Chilled by air, the lava surface
congealed into lustrous black glass.
Near Giant Crater, south flank of Medicine
Lake volcano (see fig. 1 ).
Pahoehoe
•
Spiny
•
pahoehoe
A a
Figure 6. Increasing vesiculation and
turbulence may cause a lava flow to
change from pahoehoe to aa downstream.
....
Figure 4. Location map of Lava Beds
National Monument showing major lava-tube
systems, cave locations, and
other selected features. Basalt of Mammoth
Crater shown in blue. Other lava
flows shown in red.
Figure 7. Example of aa lava, broken surface of Devils Homestead lava flow (see fig.
4).
Introduction 5