TY - JOUR
T1 - Erosion by flowing lava
T2 - Geochemical evidence in the Cave Basalt, Mount St. Helens, Washington
AU - Williams, David
AU - Kadel, Steven D.
AU - Greeley, Ronald
AU - Lesher, C. Michael
AU - Clynne, Michael A.
N1 - Funding Information:
Acknowledgments We are grateful to Jim Nieland (Gifford Pinchot National Forest Service office) for granting S.D.K. permission to collect samples in the Cave Basalt lava tubes and to Greg Michaels (ASU) for assistance with the fieldwork in 1994. We are also grateful to Paul Neumann (MERC) for preparing the samples for geochemical analyses, Ray Gorzynksi (MERC) for performing the dissolutions for ICP-MS analysis, and Marcus Burnham (OGL) for supervising the ICP-MS analyses. The authors would like to thank Jim Kauahikaua and Steve Self for very helpful reviews of the manuscript, which significantly improved the final version. Julie Donnelly-Nolan provided detailed editorial comments that have enhanced this paper. This work was funded by grants from the Planetary Geology and Geophysics Program (NASA) to RG and from the Natural Sciences and Engineering Research Council of Canada (NSERC) to CML.
PY - 2004/2
Y1 - 2004/2
N2 - We sampled basaltic lava flows and underlying dacitic tuff deposits in or near lava tubes of the Cave Basalt, Mount St. Helens, Washington to determine whether the Cave Basalt lavas contain geochemical evidence of substrate contamination by lava erosion. The samples were analyzed using a combination of wavelength-dispersive X-ray fluorescence spectrometry and inductively-coupled plasma mass spectrometry. The results indicate that the oldest, outer lava tube linings in direct contact with the dacitic substrate are contaminated, whereas the younger, inner lava tube linings are uncontaminated and apparently either more evolved or enriched in residual liquid. The most heavily contaminated lavas occur closer to the vent and in steeper parts of the tube system, and the amount of contamination decreases with increasing distance downstream. These results suggest that erosion by lava and contamination were limited to only the initially emplaced flows and that erosion was localized and enhanced by vigorous laminar flow over steeper slopes. After cooling, the initial Cave Basalt lava flows formed an insulating lining within the tubes that prevented further erosion by later flows. This interpretation is consistent with models of lava erosion that predict higher erosion rates closer to sources and over steeper slopes. A greater abundance of xenoliths and xenocrysts relative to xenomelts in hand samples indicates that mechanical erosion rather than thermal erosion was the dominant erosional process in the Cave Basalt, but further sampling and petrographic analyses must be performed to verify this hypothesis.
AB - We sampled basaltic lava flows and underlying dacitic tuff deposits in or near lava tubes of the Cave Basalt, Mount St. Helens, Washington to determine whether the Cave Basalt lavas contain geochemical evidence of substrate contamination by lava erosion. The samples were analyzed using a combination of wavelength-dispersive X-ray fluorescence spectrometry and inductively-coupled plasma mass spectrometry. The results indicate that the oldest, outer lava tube linings in direct contact with the dacitic substrate are contaminated, whereas the younger, inner lava tube linings are uncontaminated and apparently either more evolved or enriched in residual liquid. The most heavily contaminated lavas occur closer to the vent and in steeper parts of the tube system, and the amount of contamination decreases with increasing distance downstream. These results suggest that erosion by lava and contamination were limited to only the initially emplaced flows and that erosion was localized and enhanced by vigorous laminar flow over steeper slopes. After cooling, the initial Cave Basalt lava flows formed an insulating lining within the tubes that prevented further erosion by later flows. This interpretation is consistent with models of lava erosion that predict higher erosion rates closer to sources and over steeper slopes. A greater abundance of xenoliths and xenocrysts relative to xenomelts in hand samples indicates that mechanical erosion rather than thermal erosion was the dominant erosional process in the Cave Basalt, but further sampling and petrographic analyses must be performed to verify this hypothesis.
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U2 - 10.1007/s00445-003-0301-2
DO - 10.1007/s00445-003-0301-2
M3 - Article
AN - SCOPUS:1542347667
SN - 0258-8900
VL - 66
SP - 168
EP - 181
JO - Bulletin of Volcanology
JF - Bulletin of Volcanology
IS - 2
ER -