[meteorite-list] Why Are Death Valley's Rocks Moving Themselves? --not off-topic at all!

From: Göran Axelsson <axelsson_at_meteoritecentral.com>
Date: Sun, 20 Feb 2011 13:34:53 +0100
Message-ID: <4D610A6D.9090204_at_acc.umu.se>

If I might make a guess here....

The area is covered under a thin layer of water, not enough to cover the
rocks. Then the temperature drops, forming a thin layer of ice, trapping
the rocks. Then it doesn't need a lot of wind to drag along an ice sheet
with frosen in rocks. As the rocks is still protruding from the ice, it
forms tracks in the underlying surface.
If more than one rock is trapped in the ice, then they will form
parallel tracks, turning at the same time if the wind direction shifts.

I've seen the tremendous force a loose ice sheet and a modest wind can
achieve. Many years ago in spring I was at our cabin near a lake. The
weather had been warm and calm so the ice sheet of the lake (3 km x 1 km
big) was thawed a couple of meters around the edges. Then it started to
blow straight towards us and the ice started to move. First slowly,
hardly noticeable, then it started to creep up onto the beach. When it
started I stood at the edge of the water but after fifteen minutes I had
been pushed over a meter back by the ice and along the beach was a half
meter wall of ice. Then it all stopped and I've never experienced it
again, but I've seen the wall of ice deposited along beaches a couple of
times since then.

/G?ran

Stuart McDaniel wrote:
> Would you please explain what is meant by "ice-rafting" in laymen's
> terms??
>
>
>
> Stuart McDaniel
> Lawndale, NC
> Secr.,
> Cleve. Co. Astronomical Society
> -----Original Message----- From: Rob Matson
> Sent: Saturday, February 19, 2011 3:06 PM
> To: Michael Groetz ; Meteorite List
> Subject: [meteorite-list] Why Are Death Valley's Rocks Moving
> Themselves? --not off-topic at all!
>
> Hi Michael,
>
> IMHO, it's definitely ice-rafting. It happens on any desert playa that
> is hard enough, receives sufficient winter rains, and gets cold enough
> to freeze at night. In California, I've seen the rock furrows at Silver
> Dry Lake, Superior Dry Lake, Cuddeback Dry Lake and (most recently)
> Coyote Dry Lake. I've also seen them on some Nevada playas as well
> as the Alvord Desert in Oregon, and they occur in Arizona, as well.
>
> SoCal got a lot of rain this past December -- so much so that even
> after three weeks of dry weather in January the northern third of
> Coyote was under water. I've never seen this in the decade I've been
> going there, and sure enough I saw rock trails there for the first
> time last month.
>
> Bob Verish, Nick Gessler and I coauthored an abstract and presentation
> on this subject, and in particular its implications for meteorite
> recovery, at the 65th Annual Meeting of the Meteoritical Society
> in 2002:
>
> http://adsabs.harvard.edu/full/2002M%26PSA..37Q..51G
>
> Cheers,
> Rob
>
>
> -----Original Message-----
> From: meteorite-list-bounces at meteoritecentral.com
> [mailto:meteorite-list-bounces at meteoritecentral.com]On Behalf Of Michael
> Groetz
> Sent: Friday, February 18, 2011 5:24 PM
> To: Meteorite List
> Subject: [meteorite-list] OT (Sorta...) Why Are Death Valley's Rocks
> MovingThemselves?
>
>
> Interesting photo- wish I could crawl out of my chair in Ohio and
> go check those rocks out.
> I know this has been discussed on the list before.
> Have a good night.
> Mike
>
> http://www.foxnews.com/scitech/2011/02/18/death-valleys-rocks-moving-racetra
>
> ck-playa/
>
> Why Are Death Valley's Rocks Moving Themselves?
>
> By Philip Schewe
>
> Published February 18, 2011 | Inside Science News Service
>
> Death Valley National Park contains many mysteries, including one of
> nature's strangest phenomena: rocks that seem to move around all on
> their own.
>
> In the remote, almost totally dry lakebed called Racetrack Playa, some
> of the rocks move themselves across the desert floor when people
> aren't watching.
>
> Scientists know the rocks move because they leave narrow tracks
> trailing behind them, but they haven't actually seen it happen. And
> although one can't entirely rule out the possibility of some prank
> being played, at least some of the rocks appear to be moving under
> natural circumstances.
>
> It doesn't rain often in Racetrack Playa, and when it does the lakebed
> can flood. The rocks don't float exactly, but the main explanation for
> their movement is that moisture can make the mud on which the rocks
> sit more slick, making it easier for high winds to push the rocks
> along. Another explanation offered is that the temporary deposit of
> water, chilled to form extensive sheets of ice, might help to reflect
> and focus the winds, making it easier for the rocks to move.
>
> The winds required to move rocks in this way would seem to be at the
> level of 100 mph or more. That's why the rocks are sometimes referred
> to as "sailing stones." They are rare but they have been noticed in
> Racetrack Playa and a few other arid places around the world subject
> to occasional floods
>
> Ralph Lorenz, a scientist at Johns Hopkins University, offers a new
> explanation. The rocks are actually lifted up by the ice, or at least
> made more buoyant by the ice, making it easier for the rocks to
> migrate. If the rocks are moving about on ice rafts, the ground below
> cannot offer as much resistance against their motion and the winds
> needed for movement wouldn't have to be as great, he argued.
>
> So why hasn't the motion been observed?
>
> "Movement happens for only tens of seconds, at intervals spaced
> typically by several years," said Lorenz. "This would demand
> exceptional patience as well as luck."
>
> So, the rocks are probably traveling on the coldest and windiest days
> that occur over a period of several years. The most likely time would
> be in the very early dawn. Little wonder no one is around to witness
> the event.
>
> Lorenz and his colleagues would like to install inexpensive time-lapse
> monitoring of the Playa area, using digital cameras. The lakebed is
> about 2.5 miles long and 1.25 miles wide. They have also performed
> some laboratory tests by blowing on ice-assisted rocks. These simple
> tests support the ice-raft hypothesis. The results appear in the
> January 2011 issue of the American Journal of Physics.
>
> An additional reason for studying the rocks of Racetrack Playa is that
> its qualities resemble those at a drying-up lake on Saturn's moon
> Titan. Pictures taken by the Cassini-Huygens mission reveal what look
> like river channels, cobblestones, and lake beds or mud flats. Only at
> Titan's "Ontario Lacus," as one interesting site is called, the runoff
> consists of liquid hydrocarbons, not water. Some pictures even seem to
> be showing a "bathtub ring" left by what is probably a drying lake.
>
> One of Lorenz's colleagues, Brian K. Jackson, who works at NASA's
> Goddard Space Flight Center, also likes the idea that their research
> at Racetrack Playa has a dual purpose.
>
> "It's been exciting trying to solve a mystery that has resisted
> solution for sixty years," Jackson said. "Scientific accounts of the
> Racetrack Playa rocks go back to at least 1948, and there were
> certainly stories about the playa long before that.
>
> And Jackson also believes discoveries in Death Valley, here on Earth,
> will help us to better understand similar real estate on Titan or
> Mars.
>
Received on Sun 20 Feb 2011 07:34:53 AM PST