Lunar Pyroclastic Volcanism Project

Lisa R. Gaddis
Principal Investigator
U.S. Geological Survey
Astrogeology Program
Flagstaff, AZ

Funded by NASA Planetary Geology and Geophysics Program

Collaborators:
B. Ray Hawke (PGD/SOEST, Univ. Hawaii, Honolulu, HI)
Mark S. Robinson (Geological Sciences, Northwestern Univ., Evanston, IL)
Cassandra R. Coombs (Geology, College of Charleston, Charleston, SC)
Christine Rosanova (U.S. Geological Survey, Flagstaff, AZ)
Julia Sable (Amherst College)
 

The products of explosive or pyroclastic volcanic eruptions on the Moon have intrigued lunar scientists for years. These deposits, nearly 100 in number and spread widely across the lunar surface, are generally dark and smooth-surfaced. Although pyroclastic materials were found in all lunar samples and thus they are known to be widely dispersed from their sources, the dark deposits are thought to mark the locations of volcanic vents on the Moon. Many of these vents may have also produced effusive volcanic deposits, or maria, which have obscured the older pyroclastic deposits. One of the most famous of the lunar pyroclastic deposits is that of the Taurus-Littrow region in southeast Mare Serenitatis. This very dark material was thought to represent a young volcanic deposit, and exploration of such a young material was a primary reason for selection of the Apollo 17 landing site. Astronaut observations and analyses of the returned samples quickly dispelled the idea that this was a recent deposit, and in fact it was determined that the age of the both the pyroclastic and mare deposits generally exceeded 3.5 BY.

The extreme darkness of the Taurus-Littrow pyroclastic deposit is due largely to the presence of black beads. Pyroclastic materials at this site and others on the Moon include volcanic glass beads in a variety of colors, as well as the crystallized beads such as the black ones found at Apollo 17. Orange glass beads were also found at the Apollo 17 site, primarily in the material excavated by Shorty Crater, and they are known to be the chemical equivalents of the crystallized black beads. Both types of pyroclastic materials are thought to have been produced in fire-fountain eruptions, possibly of several days or weeks duration. Variations in the rates of cooling of materials in the fountain are believed to have produced quenched glass (rapidly cooled) or crystallized (more slowly cooled) beads. The composition and distribution of these materials thus can be used to understand the type and duration of the eruption that formed them.

Laboratory analyses of the pyroclastic glasses and beads indicate that, unlike most lunar materials, they have volatile-element enriched coatings from their gas-rich source regions, they originated at great depths (<400 km) inside the Moon, and they represent the most basic or primitive of lunar volcanic materials. Studies of the lunar pyroclastic materials thus provide unique information on the composition of the lunar interior and the origin and evolution of basaltic magmatism on the Moon.

Remote sensing analyses of these deposits have helped lunar scientists to identify the characteristic components of some of these deposits, to constrain the distribution of lunar volcanic deposits, and to understand the styles of eruption and emplacement of basalts on the Moon. With the global coverage provided by the Clementine multispectral data, we now have the opportunity to fully characterize lunar pyroclastic deposits. To appreciate the role of pyroclastic volcanism on the Moon, we must understand the ranges of composition, spatial and temporal distribution, relationship to effusive volcanic deposits, and modes of formation of pyroclastic deposits. Until additional samples are available, remote analyses of lunar pyroclastic deposits are the only means of fully characterizing these deposits.

In recent years, we have been using the DOD/NASA Clementine data, acquired at 5 wavelengths, to study the compositions and distributions of the lunar pyroclastic deposits. In addition to this Overview, we have compiled a Database of the locations and attributes of all of the known lunar pyroclastic deposits. Also at this site, you will find Research Reports summarizing our results, Images of these deposits, Figures, and relevant References. Finally, you’ll find links to Meetings featuring this type of lunar science.
 
 

Contact: lgaddis@flagmail.wr.usgs.gov
Date: 5/24/99