Orbicular volcanic rocks of Cerro Panizos: their origin and implications for orb formation

M. H. Ort

Research output: Contribution to journalArticlepeer-review

22 Scopus citations


The orbs consist of two to five crystalline rings surrounding a xenolithic or orthopyroxene core. The rings alternate between bands of large, radially oriented plagioclase and orthopyroxene crystals and bands of small, tangentially or radially oriented biotite and ilmenite crystals. The ignimbrite orbs are associated with two types of pumice: 1) a coarsely porphyritic biotite-quartz-plagioclase dacite with 35-40% crystals found throughout the ignimbrite, and 2) a finely porphyritic biotite-plagioclase quartz dacite with 75-80% crystals found only in association with the orbs. The major- and trace-element and isotopic compositions of the two pumice types are identical. The orbs formed in a water-rich cupola along the roof of the magma body, where the magma was superheated and most crystals were resorbed. Pressure release related to eruption caused exsolution of water, leading to large degrees of undercooling. Orbs formed rapidly around the few available nuclei. As limited mixing with the surrounding coarsely porphyritic magma occurred, heterogeneous nucleation in the supercooled magma began, forming abundant small crystals seen in the finely porphyritic pumice. Eruption of the orbicular dacite occurred when a ring vent conduit tapped the magma in the cupola. -from Author

Original languageEnglish (US)
Pages (from-to)1048-1058
Number of pages11
JournalGeological Society of America Bulletin
Issue number8
StatePublished - 1992

ASJC Scopus subject areas

  • Geology


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