In my previous post about hiking up Bunsen Peak I had wondered about a stunning white rock outcrop along with an ugly windowless wooden structure with lots of antennae protruding from it. Faithful blog follower THE AMAZING TRAVEL AGENT nailed the identity of the structure in his well-researched and timely comment. Sadly, no one ventured any ideas about the outcrop. I was pretty sure it was not of volcanic origin (but hey, what do I know?). It was up to me to peel away the layers of my own mental uncertainty concerning this mysterious geological deposit.
|Travertine outcrop on Bunsen Peak trail|
I didn’t pay much attention to the outcrop on the ascent but simply marched right through it, intent on my two–mile assault of Bunsen Peak. However, a huge landslide of white boulders tumbling down from the mountainside across the Grand Loop Road did catch my attention. These large jumbled masses of rocky blocks are in an area called Silver Gate and are referred to as “the Hoodoos.” Earlier in the season I had learned that these rocks are travertine, a form of limestone or calcium carbonate which is common in hot springs located outside of the Yellowstone caldera, particularly in the Mammoth Hot Springs area. I could swing a cat from Bunsen Peak and it would land in Mammoth Hot Springs. Kitty cat might even go flying through the park superintendent’s window and land on his desk, since Mammoth is where park headquarters is located. I need to be careful with my aim if I choose to be swinging cats anywhere in the park, particularly on peaks.
|Mammoth Hot Springs from Bunsen Peak trail|
Where was I before I started talking about swinging cats?
Oh, right. Travertine.
Inside the caldera, thermal features such as geysers and hot springs come up through a silica–rich volcanic rock called rhyolite. As the water of the hot spring cools enough for dissolved minerals to come out of solution, deposits of “siliceous sinter” or “geyserite” form. This geyserite is actually quartz with some extra water in it.
However, outside the caldera (where Mammoth Hot Springs is located) the hot springs come up not through any silica–rich volcanic rock but through bedrock limestone. Deep underground the hot, acidic water has the ability to dissolve the calcium carbonate of the bedrock limestone. As the hot, calcium carbonate–rich water rises near the surface and the pressure on it drops, carbon dioxide gas escapes from solution (and in its own peculiar way contributes to global warming). It is at this point that the calcium carbonate precipitates as travertine terrace deposits.
So as I head down the trail from Bunsen Peak I notice and examine this outcrop of white rock. I look across to the mountain across the road. I look back at the outcrop. I look back and forth for a few more minutes. Eureka! It’s got to be the same rock.
|Terrace Mountain and landslide deposits|
|Travertine outcrop on Bunsen Peak trail - Jan for scale|
After I got home I searched around in my own personal heap of Yellowstone reference material that covers nearly every horizontal surface in my apartment to see if I could find some sort of age for this travertine. I discover that the mountain I was gazing at across Grand Loop Road is called Terrace Mountain. It is an extinct travertine terrace; the travertine deposits there have been dated to around 374,000 years ago while the travertine deposits at Mammoth Hot Springs and nearby Gardiner, Montana have younger dates ranging from about 5,000–57,000 years ago. These are “disequilibrium ages” – if anyone can explain what that means I would really like to know!
Two favorite Yellowstone references:
Christiansen, R.L., 2001, The Quaternary and Pliocene Yellowstone Plateau Volcanic Field of Wyoming, Idaho, and Montana, USGS Professional Paper 729-G.
Smith, R.B. and Siegel, L.J., 2000, Windows into the Earth – The Geologic Story of Yellowstone and Grand Teton National Parks