Lava Natural Arch
arch, possible Mars arch
This type of natural arch results when portions of the roof
of a lava tube collapse. The remaining roof segments become lintels.
Lava natural arches have the same observable attributes as the subterranean
variety of waterfall natural bridge,
except that flowing lava rather than water created the underground passageway.
Thus, a geologic attribute, the type of rock, is used to discriminate
a lava natural arch from a waterfall natural bridge. A lava natural
arch always has a rock frame made of basalt or lava. Like the waterfall
natural bridge, they frequently occur in series and clusters, but can
also be isolated. A maturity attribute should be based on the ratio
of lintel area to the area of collapsed roof rather than on the age
of the lava flow.