Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

small ridges that resemble ropes, as the
hot and plastic crust is dragged along by
molten lava beneath. These ropes in turn
may be dragged and stretched out into
attenuated lobate forms (fig. 5). Near the
....
Figure 1. Index map showing Lava
Beds National Monument, Medicine
Lake volcano, and distribution of basalt
of Mammoth Crater (shaded pattern),
which is host to most of the lava-tube
caves in the monument.
end of the period of consolidation some
ropy pahoehoe may be cut by closely
spaced vertical shears to form laminated
or cauliflower pahoehoe.
Subtle transitional changes in a pahoehoe
surface can be recognized where
pahoehoe changes to aa downstream
(Peterson and Tilling, 1980). The
smooth to ropy forms begin to lose the
glassy luster that formed as a thin skin of
chilled basalt glass, and the small spherical
bubbles confined beneath this glass
skin increase in number, grow larger,
and become visible on the surface as
bumps and broken bulges of the glass
crust. As the bubbles grow larger and
more irregular in shape, many of them
explode outward through the sticky glass
crust, and with further movement this
prickly surface breaks up into small
discrete blocks, completing the transition
to aa lava (fig. 6).
In lava tubes the transition from
pahoehoe to aa is frequently found downstream
from an area where molten lava
was violently churned up while tumbling
Figure 2. View of Medicine Lake shield volcano from northern edge of Lava Beds National Monument. Gillem Bluff to right.
Field of view is about 15 mi across.
Figure 3. Late afternoon view across Lava Beds National Monument with upper part of Medicine Lake shield volcano on left
skyline. Snowcapped Mount Shasta is about 40 mi distant on right skyline. View is southwestward.
Introduction 3