A GEOCHEMICAL ANALYSIS OF VOLCANIC ASH

Volcanic ash deposits have been used in the Midwest as chronological markers in determining the age of the deposits above and below the ash deposit. Volcanic eruptions in recent geologic time have been extensively studied and identified by several characteristics. Two unique characteristics are the chemical composition of the volcanic glass and the morphology of the glass shards. In order to use tephrachronology to identify the eruption it is critical to know were the volcanic ash deposit came from.

A volcanic ash deposit was recently discovered outside of Crescent, Iowa. The bed is in loess and is discontinuous along a length of thirty meters. Its thickness varies from several millimeters to twelve centimeters. The overall extent of the deposit is not known as it only occurs in outcrops in a quarry. The deposit was mapped, photographed, described and sampled. The samples were dried and sieved. The ash from each sieve size was then viewed under a stereoscopic microscope where glass shards were described and hand picked for future analysis. Shards were also viewed with a petrographic microscope to ensure the purity of the volcanic glass. After a significant number of shards were accumulated, they were prepared for analysis with an inductively coupled-plasma mass spectrometer (ICP-MS). This mass spectrometer provides an analysis of numerous elements.

Microscopic analysis is consistent with an interpretation that this volcanic ash is associated with the Lava Creek B eruption that produced deposits throughout the Midwest and would indicate an approximate age of 600,000 y.a. for this deposit. Features such as bubble walls and triple wall junctions are consistent with similar types of features reported elsewhere. Preliminary geochemical analysis of this ash by ICP-MS is also consistent with this interpretation.