USGS/CVO Logo, click to link to National USGS Website
USGS/Cascades Volcano Observatory, Vancouver, Washington

Mount St. Helens, Washington, Eruption 2004
Dome Rock Comparison 1986 to 2004
1986 - Augite-Hornblende-Hypersthene Dacite
63.5 weight % SiO2.
  • Hypersthene is the dominant mafic mineral, with accessory augite
  • Distinctive hornblende reaction rims
  • Large plagioclase phenocrysts with sieve-textured cores common
2004 - Hypersthene-Hornblende Dacite
65.3 weight % SiO2, collected November 4, 2004.
  • Hornblende is the dominant mafic mineral
  • Hypersthene is smaller and less abundant and augite is not present
  • No significant hornblende reaction rims
  • Lacks large plagioclase phenocrysts with sieve-textured cores
Image, click to enlarge
MSH86_st_helens_rock_sample_dacite_from_1986.jpg
[Click image for larger size] 		Image, click to enlarge
MSH04_st_helens_rock_sample_dacite_11-04-04.jpg
[Click image for larger size]
Image, click to enlarge
MSH86_st_helens_rock_sample_hornblende_from_1986.jpg
[Click image for larger size] 		Image, click to enlarge
MSH04_st_helens_rock_sample_hornblende_11-04-04.jpg
[Click image for larger size]

Return to:
[Mount St. Helens Eruption 2004 - Petrologic Monitoring Menu] ...
[Mount St. Helens Eruption 2004 - Menu] ...
[Mount St. Helens General Menu] ...

[an error occurred while processing this directive]
URL for CVO HomePage is: <http://vulcan.wr.usgs.gov/home.html>
URL for this page is: <http://vulcan.wr.usgs.gov/Volcanoes/MSH/Eruption04/Petrology/dome_rock_comparison_1986_to_2004.html>
If you have questions or comments please contact: <GS-CVO-WEB@usgs.gov>
11/21/04, Lyn Topinka