Mathematics – Logic
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.v22b1046w&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #V22B-1046
Mathematics
Logic
1000 Geochemistry, 3640 Igneous Petrology, 8414 Eruption Mechanisms
Scientific paper
Lava erosion is thought to have been responsible for the modification of some lava tubes and channels on the Earth, the Moon, Mars, Venus, and Jupiter's moon Io. It may include both thermal erosion (melting and assimilation of country rock into lava) and mechanical erosion (physical removal of unconsolidated rocky materials by flowing lava). The search for unequivocal morphological and geochemical evidence of erosion has been hampered by the inaccessibility of lava/substrate contacts in active (and most inactive) tubes and the potential for deformation, metamorphism, and mobilization of elements in ancient lava flows. Thus, the best circumstances for identifying evidence of lava erosion are in Holocene tubes and channels where lava/substrate contacts can be studied and sampled, and where the rocks have not been greatly altered. We collected samples from lava/substrate contacts in the lava tubes of the ~1900 year old, relatively unaltered Cave Basalt flows, Mt. St. Helens, to assess whether the lavas contain geochemical evidence of lava erosion. Previous studies have found morphological evidence indicating that both thermal and mechanical erosion occurred during tube formation. Basaltic tube linings in contact with the dacitic wall rock have whole-rock major and trace element compositions intermediate between those of uncontaminated lava and the substrate, and therefore appear to be contaminated by the wall rocks. In contrast, basaltic tube linings that are closest to the tube interior appear to be uncontaminated. There is no evidence that the lavas become increasingly contaminated downstream. Some distal linings contain wall rock xenoliths and others contain wall rock xenomelts, indicating that mechanical and thermal erosion were operating simultaneously. There are two possible interpretations of these relationships: 1) the lava may have been flowing laminarly, resulting in contamination of only the boundary layer adjacent to the contact, or the 2) the lava may have initially been flowing turbulently, resulting in a well-mixed and contaminated interior, but that the interior of the tube was replenished by uncontaminated lava, preserving contaminated lava only along chilled margins. Thermal and fluid dynamic models support the former interpretation. Regardless, the interior tube flows that have been sampled obviously represent the waning stages of the eruption, in which flows erupted at lower flow rates only partially fill pre-existing tubes and have not been able to melt through existing basaltic tube linings to come in contact with the underlying substrate.
Greeley Ronald
Kadel Steven D.
Lesher Michael C.
Williams David. A.
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