Date of Award

Summer 6-28-1991

Document Type


Degree Name

Master of Science (MS)


Geology and Geophysics

First Advisor

Williams, Stanley

Second Advisor

Byerly, Gary

Third Advisor

Henry, Darrell


Field measurements and petrographic analyses of pumices from the most recent tephra deposits of Nevado del Ruiz volcano provide a context in which to view the disastrous eruption of 13 November, 1985 (R0). The pumices are all crystal-rich, two-pyroxene andesites to dacites with whole rock SiO2 contents of 62 - 66 wt.%, and magma water contents estimated to be between 1 - 4 wt.%. The 1985 tephra has greater petrologic diversity than earlier deposits which may be related to an input of a hotter, more basic magma into the Ruiz system, as suggested by xenocrystic olivine which is most abundant in the tephras of R1 (1845) and R0.

Maximum pumice and lithic isopleth maps were used to define dispersal axes, classify eruptive styles and estimate theoretical column heights. Isopach maps were used to estimate individual deposit volumes. Results indicate that the pre-1985 eruptions were less intense (lower column heights) than R0, but of greater magnitude (total volume), producing up to 15 times the volume of the 1985 deposit. R6 is petrographically and petrologically unique, and may have originated from a currently inactive crater. Calculated volumes for eruptions R8 - R0 range from 0.02 - 0.29 km3 with estimated column heights of 18 -26 km. All eruptions, except R1 and R6, can be classified as plinian in nature.

A cumulative isopach map of the tephra layers from the active Arenas crater reveals that the deposits have accumulated in a nearly circular pattern around the volcano, which is slightly elongate along an axis directed S-SW from the summit. We propose that a revision of the current Ruiz ashfall hazard map is necessary, and should reflect a more balanced consideration of the distribution and volumes of the pre-1985 deposits.