High Impact Forces of Nature Hazards in and Around Rocky Mountain National Park

Transcription

1 Stop 1 High Impact Forces of Nature Hazards in and Around Rocky Mountain National Park Trip Leader Vince Matthews, Colorado Geological Survey RMAG on the Rocks July 28, 2007

2 Welcome to the July RMAG on the Rocks Trip! Today s trip travels northward from Boulder along the mountain front to the town where we enter the mountains, Lyons famous for its sandstone quarries. Winding our way up through the mountains, we eventually break out into beautiful Estes Park, gateway to Rocky Mountain National Park (RMNP), home of one of Colorado s 58 fourteeners, Longs Peak (14,255 ). In RMNP we pass through Horseshoe Park, a glacial valley, before beginning our ascent up Trail Ridge Road. Our first stop is at Rainbow Curve where we will get a spectacular overview of glacial features and the fan created when Lawn Lake s dam burst. Continuing on to the crest of Trail Ridge (12,183 ), we will get increasingly spectacular panoramas of the Front Range, Gore Range, and Never Summer Range. Our second stop is on the tundra where we will view sackungen and learn about the difficulties in locating the source of Colorado s strongest earthquake Mw 6.6. We will continue on to the Alpine Visitor Center for lunch and views down into the cirque that fed the glacier that flowed to Horseshoe Park. After lunch and after purchasing some genuine, made-in -China souvenirs, we will retrace our path through Estes Park and descend into the Big Thompson Canyon. Our final stop will be in Drake which was heavily damaged by the Big Thompson Flood of We will exit the canyon through the Narrows and pass by the Loveland anticline and Devil s Backbone. From Loveland we head south on US-287 to Longmont and then SW on 119 to Boulder. Photograph of the Flatirons by John A. Karachewski. Geologist for scale at apex of flatiron. Flatirons are comprised of Pennsylvanian arkoses shed from the Ancestral Rocky Mountains.

3 STOP 1 RAINBOW CURVE From this vantage point we can see a flat-floored, glacial valley with a meandering stream. The valley was dammed by an end moraine and floored by sediments from the resulting lake. Lateral moraines are beautifully displayed. One of the moraines blocks and diverts the drainage from a side stream creating a boggy wetland. At STOP 3 we will see the cirque where the ice accumulated and flowed down to the park entrance. The striking light-colored feature across the valley is an alluvial fan deposited when Lawn Lake Dam (built in 1902) failed in 1982 killing three people and flooding Estes Park. The water from the Lawn Lake failure (228 million cubic feet) overtopped a dam at the Horseshoe Park, end moraine causing it to fail and release an additional 4 million gallons to the raging flood. Damage was $30 million in Estes Park and $2.5 million in RMNP. Counter clock-wise from lower left: Lawn Lake (USGS), Alluvial fan (Matthews), damage in Estes Park (USGS), flooding in Estes Park (USGS).

4 STOP 2 SACKUNGEN and the 1882 EARTHQUAKE 1882 Earthquake and Introduction to Sackungen Colorado s strongest historical earthquake was Mw 6.6 in Its epicenter is interpreted to be about ten miles north of Estes Park. A deterministic HAZUS analysis indicates economic losses of $240 million could be expected should a similar earthquake recur in the same place. Sackungen are trenches or scarps that generally parallel ridge crests and form in alpine areas that were glaciated during the Pleistocene. Unless one is aware of sackungen, the scarps can be misinterpreted as young tectonic faults. Statistically the scarps are uphill-facing. A sackung is interpreted to be the result of body forces within a ridge reacting to over-steepened glacially carved valleys on either side. Movement on sackungen may be triggered by strong earthquakes on nearby faults Sackungen 23 m.y.a. ash flow STOP 2 Photograph Bob von Norman

5 STOP 3- ALPINE VISITOR CENTER (LUNCH) Take an hour here to eat lunch, view the cirque, walk around, view the exhibits, use a non-swaying rest room, buy trinkets, and relax. STOP 4- DRAKE: BIG THOMPSON FLOOD OF 1976 As much as twelve inches of rain fell on the Big Thompson River Basin on July 31, 1976 creating a night of terror and death. Peak discharge measured at Drake was 31,200 cubic feet per second. Flood height at Drake was 11 feet and 23 feet downstream in the Narrows. Property damage totaled $16 million and 144 people lost their lives. Clock-wise from upper right: Drake before the flood (USGS), Drake after the flood (USGS), flood damage (Gruntfest, UCCS), flood damage (Hansen, USGS),

6 As we leave the mountains we will cross the nose of the Loveland anticline, a basement cored plunging anticline. This classic structure was highlighted in the Hayden Survey (1873) as an excellent example of a plunging fold. It has also been used in popular, Physical Geology laboratory manuals. Route of Trip