Naked Science Forum
General Science => Question of the Week => Topic started by: thedoc on 14/01/2009 14:14:11
-
I've heard stories of wooden fence posts becoming petrified over time, but I'm not sure if this is scientifically possible. How is petrified wood formed, and what exactly does it consist of?
Asked by Erin, Texas
Hear this Question on our Podcast (http://www.thenakedscientists.com/HTML/podcasts/show/2009.01.18/)
-
We put this to Steve Laurie, Sedgwick Museum, University of Cambridge:
[img float=right]http://www.thenakedscientists.com/HTML/uploads/RTEmagicC_800px-Petrified_wood_closeup_2.jpg.jpg[/img]Petrified wood is literally where the wood has been replaced by minerals. Sometimes it has a vague sort of woody texture, it looks a bit fibrous or it’s got a ring structure. The best examples will have whole cell structures preserved, the cell walls and the cell interior’s filled with mineral. You need to have water of the right chemical composition moving through the wood. It tends to be silica is the best chemical for replacing the wood. It actually reacts with cellulose and leaves a cells structure and gets bound in. Over millions of years it gradually changes from this strange mixture of cellulose and silica into opal and into a more crystallised form of silica. If you just randomly bang fence posts into British soil then probably it would take thousands of years to petrify a piece of wood in anything like normal conditions. If you have a fence post and throw it into, for instance, some of the hot springs in Yellowstone National Park then, yes you might get a decent piece of petrified wood out the end of it. That’s very unusual.
-
Petrified wood rocks! Sorry but it does look cool. It happens when the organic material in wood leaches out and is replaced with minerals from the soil turning the wood into stuff like quartz with impurities that give it pretty colours. It retains a lot of the original tree's structure so you can still see the tree rings.
As far as I know fossilisation takes a very long time, although it has been done in the lab so under the right conditions I suppose it could have happened in the thousands of years that we have probably been using fences for?
-
I think the proper term is "permineralisation" whereby, exactly as Bryan suggests, silicates and other minerals from the surroundings saturate and gradually replace the organic matter.
As an interesting aside, on the beach at Walton on the Naze recently my brother and I sifted through some small stones and found many fragments of petrified wood (small sticks and stems).
Chris
-
Permineralisation is the correct term for this process. This happens as Carbon (C) in the wood is replaced by Silicon (Si). It turns out that Si has a +4 valance state the same as C, so because it has similar properties and is fairly similar in size this chemical replacement is sometimes possible. As I have heard this process only takes place in a few very specific settings where wood is rapidly buried before it has an opportunity to decompose and it must be buried in an Si rich setting. The best mechanism for this to occur is to have wood buried by material erupted from a volcano that has a chemical composition high in quartz (SiO2). When this eruption takes place the rock that is deposited from the volcano is probably going to be known as rhyolite, the extrusive equivalent to granite, if the rock remains warm/hot for a time the Si may be able to migrate into the structure of the cell in the wood where the Si will displace the C and leave behind permineralized wood.
-
Fossilised trees can sometimes be found in coal seams - although strictly speaking these are not petrified as they have undergone a kind of metamorphism rather than mineral replacement.
There are also certian circumstances where calcic waters and petrify materials - This will occur where travertine is being deposited. This can happen remarkably quickly.
-
Standing, fossilizing trees near hot springs in Yellowstone National Park are called "bobby socks" due to their white bases and roots. The trees take up silica (or calcite at Mammoth Hot Spring) which replaces the woody material from the bottom up.
I used to have a photo somewhere..........
-
So, I'm here researching an ongoing debate. I'm a science guy and a Christian with children in a conservative Baptist elementary school where they teach, well let's say they take the Bible quite literally. So, I'm trying to read their arguments as open-mindedly as possible (it's hard). Folks who believe as they do are good people who should still be respected and deserve to have their positions discussed in a rational respectful way. So, this is my due diligence. In their attempt to challenge accepted scientific beliefs regarding the actual age of the earth, they refer to a supposed event that occurred in 1973 in Canada.
Heat Contamination:
Another problem that calls into question the credibility of radiometric dating is heat contamination. For example, In 1973, in Alberta, Canada (near the town of Grand Prairie) a high voltage line fell which caused nearby tree roots to fossilize almost instantly. When scientists at the University of Regina, Saskatchewan were asked what the results would be if these roots were dated by Potassium Argon method. Their response was that the results:
"WOULD BE MEANINGLESS; it would indicate an age of millions of years BECAUSE HEAT WAS INVOLVED IN THE PETRIFICATION PROCESS." The Mysteries of Creation, by Dennis Petersen, p. 47."[/quote]
from http://www.earthage.org/
Thanks
-
Coach Ben - I sympathise with your situation. But I think you are in a no win situation. I think you'll find it is pointless to use scientific arguments. You may be able discredit one theory that is being used to justify a literal interpretation of the Bible, but as soon as you do, you will be presented with another and another. There are entire books full of, so called "scientific facts" that are quoted by literalists to support their position. Check out the Discovery Institute for example.
I have a lot of good friends who do take the Bible quite literally, but I find it best never to discuss science with them. They will take anything I say on the subject as a threat and a personal insult.
-
If you can put a fence post in a hot spring and pull it out a few years later and find that it is a petrified fence post--why is this such a difficult question? Some of the trees on Mt. St. Helens were also petrified in a relatively short time. Why is this a problem?