Inflation rates, rifts, and bands in a pāhoehoe sheet flow

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The margins of sheet flows—pāhoehoe lavas emplaced on surfaces sloping <2°—are typically delineated by structures that form to accommodate vertical flow inflation. We refer to these structures as inflation rifts. The surfaces of inflation rifts almost always exhibit bands of varying color and texture. Various explanations for the bands have been proposed, but active band formation has never been documented. In order to test our hypothesis that banding is caused by changes in the inflation rate, we collected time-lapse photographs of the margin of an actively inflating flow and simultaneously measured the height of the flow with an extensometer. Data collected over a period of ∼1 d indicate that the height of the flow margin changed in a stepwise manner and that rate changes correlate with band formation. This confirms our hypothesis.

Inflation and rift-band formation is probably cyclic, because the pattern we observed suggests episodic or crude cyclic behavior. Furthermore, some inflation rifts contain numerous bands whose spacing and general appearances are remarkably similar.

We propose a conceptual model wherein the inferred cyclicity is due to the competition between the fluid pressure in the flow's liquid core and the tensile strength of the viscoelastic layer where it is weakest—in inflation rifts. The viscoelastic layer consists of lava that has cooled to temperatures between 800 and 1070 °C. This layer is the key parameter in our model because, in its absence, rift banding and stepwise changes in the flow height would not occur.

Publication type Article
Publication Subtype Journal Article
Title Inflation rates, rifts, and bands in a pāhoehoe sheet flow
Series title Geosphere
DOI 10.1130/GES00656.1
Volume 8
Issue 1
Year Published 2012
Language English
Publisher Geological Society of America
Publisher location Boulder, CO
Contributing office(s) Volcano Hazards Program
Description 17 p.
Larger Work Type Article
Larger Work Subtype Journal Article
Larger Work Title Geosphere
First page 179
Last page 195
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