TESTING THE PHYSICAL AND CHEMICAL HOMOGENEITY OF A CINDER CONE: A CASE STUDY OF THE HENRY KNOLLS CINDER CONE VOLCANO IN SOUTH WEST UTAH

The Markagunt Plateau represents the western extent of the Colorado Plateau in southern Utah. The plateau contains numerous cinder cone volcanoes and lava fields that are associated with magmatic systems that were active from 5 Ma to as recent as 1,000 years before present. Models suggest that many cinder cones in the southwest U.S. are a part monogenetic fields. The southwestern most volcano of the Henry Knolls Cinder Cones, provides a rare opportunity on the Markagunt Plateau to test the hypothesis that cinder cone volcanoes represent short-lived magmatic events of a homogenous composition and that they are truly monogenetic.

The S.W. Henry Knolls Cinder Cone has a quarry pit on its south flank that exposes the stratigraphic layers, which represent eruptive events throughout its lifetime. While the timeframe of these volcanic strata has yet to be quantified, erosional surfaces have been observed suggesting substantial breaks in activity. Samples of pyroclastic bombs and unconsolidated scoria were taken from twenty-eight exposed horizons in the excavated flank. Preliminary whole rock chemistry has yielded remarkably similar major and trace element concentrations from 10 samples. The homogeneity is supported by the petrographic observations made from thin sections of the same horizons. From these preliminary observations, we can infer that the Southern Henry Knolls Cinder Cone was sourced from a chemically homogenous magma reservoir. Future work will include more detailed petrographic and chemical studies as well as comparisons of cinder cones throughout the field.