Naked Science Forum

Non Life Sciences => Geology, Palaeontology & Archaeology => Topic started by: nana67 on 23/06/2010 12:30:02

Title: How does quicksand form and what controls its depth?
Post by: nana67 on 23/06/2010 12:30:02

Kathleen  asked the Naked Scientists:
   
Hello, I just read your very interesting article about quicksand.  I have one more question: Is there a bottom to it and how does it originally form.  

Is it just a fluke of nature?
 
Thank you,
Kate        

What do you think?

Title: How does quicksand form and what controls its depth?
Post by: Mazurka on 02/07/2010 12:25:20

It does have a bottom - which varies from patch to patch.  It is generally accepted that it is caused by some form of spring beneath the sand forcing water into the gaps between the grains fo sand.  This is pore pressure and is quite normal in sediments.  Where there is a high pore pressure, it forces the grains of sand further apart so although it appears to be normal sand it has a lot more water in it which cannot support the weight of a peson, so they sink.

When there is lower pore pressure but there is a vibration (such as an earthquake) the vibration can cause the sediment to liquify which is why structures fall over rather than break up.  To demonstrate this - stand in a bowl of very wet sand and wiggle your toes. If you do you will sink, whereas if you don't you will not. 

Title: How does quicksand form and what controls its depth?
Post by: geo driver on 03/07/2010 22:49:59

oh for a happy little bit of kitchen science.

to see roughly the same thing, get a glass with dry rice in it..... stop me if you know this one, put a knife into it blade down, and wiggle the blade around, continu to do this and eventualy you will be able to puck the glass up with the knife.

i think there should also be less volume of rice in the glass, but no less rice.

thats a good one to do with kids, tryed it worked  [;D]

Title: How does quicksand form and what controls its depth?
Post by: Geezer on 04/07/2010 00:01:43

Quote from: geo driver on 03/07/2010 22:49:59

oh for a happy little bit of kitchen science.

to see roughly the same thing, get a glass with dry rice in it..... stop me if you know this one, put a knife into it blade down, and wiggle the blade around, continu to do this and eventualy you will be able to puck the glass up with the knife.

i think there should also be less volume of rice in the glass, but no less rice.

thats a good one to do with kids, tryed it worked  [;D]

That's a good one. I'll need to try it. Thanks Geo.