Lava holds clues to geology |

Lava holds clues to geology

Dennis WebbGSPI News Editor

Geologists have lawmakers and lava flows to thank for making possible the discovery over the last decade of a major geological collapse that centers around Glenwood Springs.An ambitious local mapping project funded by Congress helped substantiate the theory of the collapse, and offers a better idea of the age of the Glenwood Canyon (see related story, page 6).The theory holds that groundwater has been dissolving underlying deposits of gypsum and salt, called evaporites, causing much of the lower Roaring Fork Valley and western Eagle County to collapse by thousands of feet over the last several million years.Without the volcanic activity that once predominated in this region, “This story, nobody probably would believe,” said Vince Matthews, senior science advisor for the Colorado Geological Survey.”The dating of the volcanic rocks was really crucial to this.”Volcanic activity on this part of the Western Slope ranges from 8 million to 10 million years ago on the Grand Mesa, to 4,000 years ago at Dotsero, where a volcanic crater still can be seen today. Those lava flows can be studied to determine geological activity timelines with great precision.”There’s just a wonderful story that can be put together by dating them,” Matthews said.

The story began unfolding in earnest in the mid-1990s. Earlier, states lacked the funding to do basic geological mapping, said geologist Bob Kirkham. But then Congress provided states matching funding for this purpose. Kirkham, who recently retired as a geologist with the Colorado Geological Survey, said geologists immediately began mapping the Glenwood Springs area because so many geological issues needed to be better understood.Congress hoped the mapping would result in practical applications. Geologists figured that mapping could provide a better understanding of mudflows, landslides, sinkholes and other problems in the region.Even before the mapping work began, Kirkham said, geologists knew there was something odd going on in the area. Localized collapses were already identified over the last several decades. But only after the mapping project started did evidence of large-scale collapse became apparent.”The information developed as we were going along,” said Matthews. “The whole picture sort of fell together as the maps were finally finished.”I think we’ve learned quite a bit through that program.”By 1995, the U.S. Geological Survey joined the state in geological mapping that provided support for the collapse phenomenon.The geologists realized that to understand when the collapse started and how it progressed, they needed to know about about volcanic rocks in the area.Geologists subjected lava flows to a form of isotopic dating that is based on argon rather than carbon, and works for rocks millions of years old.They also conducted paleomagnetic analysis on lava flows. The magnetic field of the earth is constantly changing. When lava cools, it locks in the magnetic field of the earth at that time, which is useful in dating.Paleomagnetic analysis also can help determine whether a hardened lava flow has tilted over time – as might occur in a collapse.

Geologists mapped quadrangles around Glenwood Springs, dating volcanic rock across the region.”As you get out there and walk around, you discover a lot of things in looking at the rocks that people hadn’t described before,” said Matthews.Geologists saw how rock forms interrelated, suggesting a large-scale collapse.Or two collapses, as it were. Geologists identified one collapse generally in the lower Roaring Fork Valley. It extends almost to Basalt, goes south of Carbondale to about the base of Mount Sopris, and carries north to Glenwood Springs and southwest to Sunlight Peak. It also follows the Grand Hogback on its northwest path well beyond New Castle.Geologists call this the Carbondale Collapse Center. It covers more than 450 square miles.A second, much bigger collapse area runs from eastern Glenwood Canyon almost all the way to Vail, north of State Bridge, and south almost to Triangle Peak east of Basalt. The Eagle Collapse Center takes in about 965 square miles.Matthews said hummocky topography is one sign of collapse in the Carbondale Collapse Center. There are others.”What you see is that the lava flows on the mountains actually tilt into this area that was collapsed,” he said.Another indicator of collapse is Spring Valley southeast of Glenwood Springs. It’s a giant depression that once was largely filled by a lake, before early settlers drained it in order to use the area for agriculture.A new sinkhole 30 feet wide and 20 feet deep suddenly opened up just weeks ago at Colorado Mountain College’s Spring Valley campus, due to dissolving of underlying evaporites and the resulting collapse of the soil above them.Another sinkhole near the college is more than 200 feet wide.Spring Park Reservoir, between Carbondale and Basalt Mountain, rapidly drained twice during the 20th century when sinkholes appeared in its floor.Portions of the Roaring Fork Valley floor between Glenwood and Carbondale – particularly from Cattle Creek south – have an average of one sinkhole every 1.5 square miles.Kirkham said collapses have forced some irrigated areas out of agricultural production.”Moving water around has some effect on where collapse is occurring,” he said.But collapses occur on dry ground as well, he said. It’s just less likely to be noticed than on land tended by ranchers and farmers.Contact Dennis Webb: 945-8515, ext.

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