Update on Field Research: Mafic Lapilli Stone of the Talapus-Katsuk and Implications for Global-Mafic Volcanism

March 2 Presentation by Doug Robbins, GWU student and Foshag Awardee 2021

Synopsis by Andy Thompson, MSDC Secretary

MSDC has been a long-time supporter of geological research, including research done by the George Washington University Geological Sciences Department. This past year, Dr. Richard Tollo identified a promising recipient for the 2021 Foshag-Hronik-Dhyse award, namely Doug Robbins, pictured here.

Doug chose to write his senior thesis based on his original field research pertaining to the mineralogical formation of a specific area within the Cascade Mountain chain, which runs from California north into Canada. Doug’s focus was in Oregon as shown on the maps below. The Cascades resulted from the Pacific plate subducting under the North American plate, thereby generating volcanic activity and lifting the land that eventually became the Cascades Mountain chain.

Being originally from Hawaii, Doug had long a strong interest in volcanism. That interest, combined with his passion for geology and global development, and discussions with Dr. Tollo, led him to this particular research project. Prior published research (1990) documented the complexity of the mineral deposits which include basaltic andesite (orange), silicic deposits (grey), pre-glaciation basalts and basalt and basaltic andesite (red), shown below.

Specifically, Doug focused on the pyroclastic lapilli stone (little stones) deposit just north of Devils Lake. That deposit was within the outcrops of two plateaus or buttes, named the Talapus-Katsuk region. He wanted to analyze their mafic inclusions and understand how they may have influenced the geologic formation of that area. He expects his findings will contribute to a better understanding the greater Cascades region.

Doug worked out of the city of Bend, Oregon. His focus included the area just north of Devil’s Lake and Crater Lake National Park as noted by the blue ellipse shown above and the blue circles shown below.

The two illustrations above provide the geologic context (left) and geographic overview (right) of the Cascades and region Doug researched.  The schematic on the left shows the Cascades’ structural contour (red lines) and suggests their geologic complexity. The image on the right shows the vast sweep of the Cascades Mountain range (in blue) from Yosemite in the south through to northern Washington State and into Canada.

His thesis investigates this complicated geologic setting and its diverse mafic inclusions.

Look closely at the above photo to see if you can spot Doug collecting mineral samples.

What he discovered at his site was somewhat odd, Doug said, in that he found in the deposit not only the expected mafic minerals characteristic of lava, but also evidence of the earlier presence of water, presumably brought there by icy glaciers. He said this type of glacier-lava formation is not unheard of, but is somewhat rare.

The photo above shows one of the many contrasts of minerals found in the region, with the steep slope of gray, silica rich rhyolitic rock in the foreground on the right. Off in the distance, farther along the steep road, one sees a mafic lava deposit of magnesium and iron.

The following photo tells the story of the immediate area Doug explored, with arrows pointing out two lava flows, the earlier pulse one and later pulse two.

Below shows some of the more representative mineral samples he collected, of which the darker mineral, for Doug, was the most interesting.

As part of his research, Doug made thin-sections of that specimen. Below, the thin section shows vesicles (cavities) formed by the escaping gases, which then solidified into vugs (small- to medium-sized cavities in rocks). The solidified lava flows also contained the smaller fragmented pyroxene crystals, which here seem to have a green tint.

Doug’s findings are ongoing, so his March 2 presentation should be viewed an update of a work in progress. But he did present data to advance a theory consistent with the evidence of the early presence of water and how the mafic deposit influenced the region’s formation. In particular, he explored how the original mountainous depositions shaped the course of lava flows which have been retraced by later flows of glaciers.

Doug searched Google Earth to find current examples of how the geological formation he found in the Cascades could be exemplified elsewhere in other parts of the world. He showed an example of the Norwegian Supplelebreen glacier (below) skirting around a rock outcrop but depositing onto the rock plateau (shown in the blue circle) a small amount of ice. The presence of that water, he said, was necessary for and consistent with what Doug believed happened. When the hot lava met the glacier ice, the result was extensive gas bubbles, evidence of which is found in the thin section’s vesicle-filled mafic mineral.

One goal of this synopsis is to encourage readers to watch and listen to the video of Doug’s presentation. In it, he provides a fuller and more colorful explanation of his research. The video also contains lively Q and A interactions between Doug, Dr. Tollo, and members of MSDC. Given that Doug’s research is a work in progress, we look forward to the possibility of learning his further findings which we expect will be made available in a future publication or presentation. Stay tuned.