Montana Earth Science Picture of the Week

Fossils Revealed As Glacier Melts Away

Photo Courtesy of Callan Bentley, Assistant Professor of Geology
Northern Virginia Community College

Geologist Callan Bentley lies on the surface of the Grinnell Glacier cirque within several hundred feet of the Continental Divide in this 2007 photo. The strange circular shapes are ancient colonies of algae called stromatolites, which until recently, were covered with ice. (photo)

Cabbage Heads . . .
The photo below was taken just few miles away from the top photo, on the opposite side of the divide along the Highline Trail in Glacier Park. Although this location is approximately 5,000 feet above sea level, it contains fossils of algae colonies (blue-green algae; aka cyanobacteria) that lived in a shallow sea .8 to 1.6 billion years ago. The algae formed in an environment similar to what exists in the Florida Keys today. The fossil stromatolites have shapes and internal structures similar to ones that live in present-day seas. The outcrop in the photo below contains excellent examples of stromatolites that resemble heads of cabbage.. CLICK HERE to see a photo of modern stromatolites in Shark Bay, Australia.

A Sticky Situation . . .
Stromatolites are mound-like, multi-layered colonies of algae (blue-green algae; aka cyanobacteria). The formation of the layered mounds has to do with the way they change the chemistry of the shallow water they live in. Photosynthesis in the cyanobacteria removes carbon dioxide from the surrounding water, causing calcium carbonate to precipitate onto their slimy, mat-like colonies. The calcium carbonate, along with grains of sediment (silt, etc.), stick to the bio-film layer that covers the colonies. As they continue to grow upwards through the sediment, a new layer forms. This process occurs over and over again, creating layered mounds, columns, or sheets.

Gases and Rocks . . .
Fossils of different species of stromatolites can be found in different areas of the park. It is believed that stromatolites that lived in the Precambrian played a major role in increasing the amount of oxygen in the atmosphere of the primeval Earth ("The Great Oxygenation Event"). Cyanobacteria such as these would removed carbon dioxide from seawater and released oxygen as a waste product of photosynthesis. It is believed that stromatolites that lived in the Precambrian played a major role in increasing the amount of oxygen in the atmosphere of the primeval Earth ("The Great Oxygenation Event"). Living stromatolites can be found today at Shark Bay in western Australia. The removal of carbon dioxide from the water and the formation of large quantities of calcium carbonate also contributed to the formation of great thicknesses of carbonate rocks in the park.

Blame it on Plate Tectonics . . .
So how did these sea-dwelling organisms end up on a mountaintop in Montana? . . . Blame it on plate tectonics. About 100 million years ago, massive segments of Earth's crust (crustal plates) moved eastward from the area of the Pacific Ocean, pushing into the western edge of North America. This caused the rocks containing the cabbage heads to rise from sea level, forming huge mountains such as those found in this part of Montana. A similar process is happening today in the Himalayas where fossils of ancient sea creatures can be found among the world's highest mountains.

Source: Geology Along Going-to-the-Sun Road, Glacier National Park, Montana, by Omer B. Raup, Robert L, Earhart, James W. Whipple, and Paula E. Carrara: prepared by the USGS in cooperation with the National Park Service, Published by Glacier National Park Historical Association in 1983

Terms: carbonate (rock)


*Plate Tectonics (USGS site)
Callan Bentley's Web Site (cool travel photos)
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By Rod Benson
Earth Science Teacher at Helena High School

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