About This Project

At Santiam Pass, Oregon, eruptions of magma beneath thick ice sheets produced remarkable flat topped mesas known as 'tuyas' - spectacular landforms that may also be the key for understanding the timing and the size of Pleistocene ice sheets in this area. This project will use new measurements of the age and chemistry of tuya lavas to investigate our hypothesis that tuyas represent an early and less well known glacial event.

Ask the Scientists

What is the context of this research?

Many mountain ranges, including the volcanoes of the Oregon Cascade range, experienced glaciation during the Pleistocene (11,700 thousand to 2.58 million years ago). In mountainous areas, glaciers typically grow in an ice age, and shrink in interglacial times (like now). However, although the timing of global glacial cycles are well known, it can be much harder to understand when mountain glaciers expand or retreat because deposits left behind by glaciers are difficult to date accurately, and glacial advances obscure evidence of earlier events.

We will apply a new method - studying volcanic rocks that erupt under thick ice sheets - to constrain the timing and depth of ice at Santiam Pass in the Oregon Cascades. We hypothesize that sub-ice volcanoes record evidence of ice sheets that predate the most recent Pleistocene glacial maximum - opening a window into the earlier glacial history of this region.

What is the significance of this project?

We aim to provide new constraints on the timing and size of glacial ice sheets at Santiam Pass, Oregon, by studying tuyas - unique flat topped hills produced by volcanic eruptions under thick ice. The age of magmatic intrusion shows unequivocally when ice was present. These rocks will also be easier to date using radiometric dating techniques, and will produce more accurate results than dating moraines and other glacial sedimentary deposits which have been used to date.

We can then go even further and use the chemical compositions of tuya magma samples to estimate the thickness of the ice. This relies on measurements of water and carbon dioxide in glasses from tuya samples. The abundance of these elements depends on the pressure at which the glass formed when hot silicate magma quenched against ice. The thicker the ice the greater the pressure and the more water and carbon dioxide we will see.

What are the goals of the project?

This project will be conducted in collaboration with undergraduate students at Oregon State University. We have already collected some samples of two tuyas that we will study. These samples will be prepared for radiometric dating, and we will also document the chemical composition of minerals, and glasses that are present, as well as the bulk rock samples. We will use this data to study the evolution of the magmas and compare the tuya magmas to other magmas from the Santiam Pass area.

The age of our samples will be measured using the 40Ar-39Ar technique at Oregon State University, an approach that offers excellent accuracy and precision. Although an older K-Ar age was reported for Hogg Rock in the 1990's, that method is now known to be less reliable. We will also measure water and carbon dioxide on glass samples using infrared spectroscopy at the University of Oregon. The amounts of these species will be used to calculate how much ice overlay the tuyas when they formed.