Re: A Natural Compass - Rock Cracks
- From: "North" <north@xxxxxxxxxxxxx>
- Date: Mon, 30 Jan 2006 19:03:32 -0500
GREENHORNS BEWARE !!
Don't use cracks in rocks to find N,S,E,and W
n.
Too_Many_Tools <too_many_tools@xxxxxxxxx> wrote in message
news:1138549903.016150.234190@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> FYI
>
> http://www.livescience.com/forcesofnature/050121_rock_cracks.html
>
> A Natural Compass: Rock Cracks Point North
> By Michael Schirber
> LiveScience Staff Writer
>
> Night has fallen, and you are lost in the middle of an unfamiliar
> desert. There are ways to find your bearings by looking up at the
> stars. But how about looking down at the rocks?
>
> According to Leslie McFadden of the University of New Mexico, there may
> be a kind of compass in the alignment of cracks in certain rocks.
>
> In trying to explain how a boulder falls apart when water is scarce,
> McFadden has incorporated the power of the Sun, and the simple fact
> that it rises in East and sets in the West, roughly speaking.
>
>
> "It dawned on me that Nature might exhibit the effect of solar heating
> by having cracks line up in a North-South direction," McFadden told
> LiveScience in a telephone interview.
>
> McFadden and his colleagues have confirmed that a majority of cracks in
> some desert rocks are oriented in this non-random way. They go on to
> suggest that this weathering pattern could show up on other planets or
> moons.
>
> Cracks in the pavement
>
> McFadden's primary interest is weathering in the world's arid and
> semi-arid climates. One peculiar feature found in these dry areas is a
> desert pavement - a flat, gravel-strewn stretch of land, with little
> or no vegetation.
>
> "They often have a dark color," McFadden said. "They almost look like a
> big parking lot."
>
> Coalition forces trudged over desert pavements in Iraq, and the barren
> lots are common in the Southwestern United States. The thin layer of
> gravel that covers a pavement arises from the break down over millennia
> of the boulders that dot the landscape. How this weathering occurs has
> been a bit of a mystery.
>
> Water can split a rock apart when it gets inside and freezes. But
> deserts generally do not get cold enough for this to happen, so
> geologists have speculated that salt weathering, in which salt grains
> form out of water that has penetrated into a rock, is the dominant
> action.
>
> Salt weathering occurs along coastlines, where sea spray causes rocks
> to crumble apart. But McFadden does not think this process could fully
> explain the weathering seen on the desert pavements.
>
> "To make salts work, you have to get salts into the interior of the
> rocks," he said.
>
> McFadden and his colleagues contend that salt weathering is more
> effective at opening up cracks that are already there. To explain the
> initial splitting, the scientists have revisited an old idea that had
> previously fallen out of favor.
>
> Hot rocks
>
> Heat can be a significant factor in breaking down rocks. This is
> evident to anyone who has ever put a rock into a campfire or looked at
> the aftermath of a forest fire.
>
> "When you have a fire, silicate rocks are broken down because they are
> inefficient conductors of heat," McFadden said.
>
> Because of this poor conduction, the outside of a rock becomes
> extremely hot in a fire, while the inside can remain relatively cool.
> The temperature difference causes the rock to rupture, as the outer
> layers expand away from the interior.
>
> Although there are not many fires out in the middle of the desert,
> there is the blazing heat of the Sun. Some dark rocks in the desert can
> reach 176 degrees Fahrenheit (80 degrees Celsius), according to
> McFadden.
>
> In the 1930s, researchers looked at the effects of solar heating in the
> lab, but they could not reproduce the weathering seen in Nature, so the
> Sun was abandoned as an explanation.
>
> The Sun is back
>
> But desert cracks are a different breed. A split rock implies there was
> a strain between the two sides - a situation distinct from a fire,
> where the strain is between layers.
>
> McFadden realized that the Sun, shining on only one side, could create
> such a strain due to the temperature differences. He said previous
> research had failed to take into account the rock's shadow.
>
> "The largest surface temperature gradients will occur in the morning,"
> McFadden said, when the shaded half of the rock is still cool from the
> night.
>
> Therefore, if McFadden was right, the cracks should line up along the
> line between morning Sun and shade. On a relatively round rock, this
> line should point North-South. To test this hypothesis, McFadden and
> his colleagues went to a half dozen desert pavements in New Mexico,
> Arizona and California. They found that a majority of the cracks on
> round, uniform boulders lined up in a North-South direction.
>
> "We have evidence that pulls the Sun back into the game," McFadden
> said.
>
> The results were published in the current issue of the Geological
> Society of America Bulletin.
>
> Not just rocks
>
> The authors point out that solar heating could explain other kinds of
> weathering outside of desert pavements. Buildings and other man-made
> objects may form cracks that reflect the movement of light and shade
> across their walls.
>
> And the effect may not be limited to our planet.
>
> "There's a possibility that it might work on Mars where some signs of
> physical weathering have been seen," McFadden said.
>
> Mars has a day that is only 40 minutes longer than Earth's. A similar
> heat-cooling cycle might explain the breakup of some of the rocks on
> the Red Planet.
>
.
- References:
- A Natural Compass - Rock Cracks
- From: Too_Many_Tools
- A Natural Compass - Rock Cracks
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