Tuesday, February 11, 2014

Why is ice so slippery?

In Countdown to Olympic Curling, I calculated that the stones used in curling do not exert enough force to melt the ice.  Did anyone do the math and figure out the answer to "What's the maximum contact area for an ice skate so that a 60 kg figure skater will melt the ice under her?"

Well, Science Friday (or Robert Carpick, professor and curler) did the math and it would take a gargantuan ice skater.  So the textbooks are wrong.  UC Berkeley chemistry professor Gabor Somorjai and his lab also explain.  I just love the mental picture of every other water molecule vibrating up and down but not side to side.  Remember how people danced to punk rock?

Speaking of rock and ice, there is one instance where the pressure is absolutely positively large enough to melt ice--rock glaciers.

When I first moved to Boulder, I tried to hike up to Isabelle Glacier (pictured above in a 1920 photo from the Library of Congress) with a geomorphology grad student who had also recently relocated from Berkeley to Boulder for grad school.  I got altitude sick so we never made it up to Isabelle Glacier.

However, he pointed to a pile of dirty ice and rocks and announced that we had climbed high enough to reach a glacier.

"Where?", I asked.

"There."

I just saw a pile of rocks and a little patch of dirty ice/snow.

"It's a rock glacier.  It's mostly underground."

Who knew that glaciers can be like icebergs, lurking below the surface?  The pressure above melts the layer at the bottom, which then flows downhill and refreezes.  The toe of the glacier inches downhill underground, grinding up the rock into soil along the way.

I should have remembered that water moves even when you can't see it from the artesian well on CA highway 92, between San Mateo and Half Moon Bay.  In order for the water in the artesian wells to reach the surface, they must have flowed from somewhere higher.  The only thing higher than the coast mountain range is the Sierra Nevadas.

Wow, there is so much more than apparent to the eye.  I never thought about water and ice the same way after that day.

5 comments:

  1. Anonymous21:34

    An artesian well on the west side of the bay being driven by hydrostatic pressure from the Sierra Nevadas seems kind of unlikely to me. Are you sure the source of pressure wasn't just from rain fall that soaked into the ground somewhere nearby, somewhere perhaps a few hundred feet higher in elevation from the well, but still in the coastal range?

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  2. Anonymous21:36

    Above comment was from me, Eric.

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  3. Yup, the water was traced to the Sierras. Not only are the Sierra's the closest taller thing, but the porous rock layer was also mapped under the bay and east to the Sierras.

    Moreover, the water pressure at the Artesian wells correlates with Sierra water table height.

    I learned that in Field Biology class in HS.

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  4. Anonymous13:58

    Weird. That's very counterintuitive! I would have thought there would be "leaks" in between, that would short circuit the flow. Or that some enterprising famer in the central valley would drill himself a self-pumping irrigation well. I wonder how long it takes the water to make the trip? I.e., the water molecules you see in the coastal mountain well, when were they snow in the Sierras? Presumably one could do some sort of isotope dating to figure this out. I have a friend who works on dating water found in aquifers below the Sahara.
    Eric

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    1. Yes, central valley farmers have been using groundwater at an unsustainable rate. Some parts of the valley are sinking 11"/year.

      Read more:
      http://water.usgs.gov/ogw/pubs/fs00165/
      http://www.fresnobee.com/2013/11/21/3624593/usgs-over-pumping-of-valley-ground.html

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