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Surface features of pahoehoe flows
If they advance unimpeded, pahoehoe surfaces are often pushed into sleek, broad,
billowing folds which commonly develop a characteristic bluish sheen when the glassy surface crystals rcflcct the sky. Smooth surfaces are often covered with
filaments resembling an artist's brushwork, some fine, like a Gainsborough painting,
others with the rougher texture of a late Rembrandt. If the flow is impeded,
pahoehoe lavas develop festoons like coiled ropes, 1-5 cm in height and 2m in length,
which bulge down slope like ridges on hot moving tar. The Blue Dragon Flow in the Craters of the Moon displays these features well. Occasionally, lava is
squirted out in a tangled mass resembling entrails, which are well developed, for
example, on the northeastern side of the Craters of the Moon. When the molten lava exerts localized pressures on its crust, the surface is raised into "tumuli", humps
2m or 3m high and 20m broad, which sometimes crack open at their crests. Cracked tumuli compose "Captain jack's Stronghold" at the snout of the
Mammoth Crater flows in the Lava Beds National Monument, named after the Modoc chief who held off us cavalrymen for five months in 1873 by making full use of the
labyrinthine natural fortress.
Upward surges as the current progresses may also generate pressure ridges which
commonly rise about 10m above the general level and may be up to 1km in length. They usually occur in two or three festoons trending parallel to the margins of the
flow. The zones around tumuli and pressure ridges are often cracked into slabby
pahoehoe surfaces, which are well developed at Yaiza in the lavas erupted in 1730
in Lanzarote. Small emissions of molten lava may escape from the cracks onto the
surface and form toes rarely exceeding 1 m in length, which can be notably grooved
when they are extruded like toothpaste from a jagged orifice.
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