Friday, April 8, 2016

Petrified Forests



If you’ve never seen a petrified forest, you're in for an eye-catching treat. When I was young, having book-ends made from a petrified tree was a mark of sophistication. Never mind that petrified “wood” is close to 8 in hardness on the Mhos Scale (a scale where talc is 1 and diamond is 10), and cutting all those slabs must have cost more than a few diamond-edged rock-saw blades.

Q: I am a sixth grader at Kennedy Middle School. Our school is participating in something called Genius Hour which allows us to research a topic that interests us. We are required to interview a professional who is knowledgeable about our topic. My topic is the Petrified Forest
If you could please answer the questions below as soon as you get the chance, it would be greatly appreciated. Thank you for taking time out of your day to respond to this email.
Thank you for your time,
- Quinn C

My questions are:
Q: How was the Petrified Forest formed?

A: The one in NE Arizona was formed over 200 million years ago from fallen trees that had accumulated in log-jams along ancient river drainages. Sometime later ash from nearby volcanoes ​fell on the trees, and slowly over time the lignin and woody contents were replaced with silica from the overlying ash.

Q: What type of rock coated the trees?

A: The trees were not coated. Individual cells throughout their volumes have been replaced by silica leaching out of volcanic ash overlying them. ​

Q: How long did the process take?

​A: The time necessary for the process is poorly defined by actual data. In some cases the original tree material has not yet been completely replaced by the silica, and some lignin still remains in a petrified log after 211 million years. One example (below) appeared to have happened in only a few thousand years.
Q: Might this happen somewhere else?

A: There are petrified forests in North Dakota, Argentina, and Egypt, and likely other places. I have personally encountered bushes that are no more than 10,000 - 17,000 years old on the edges of former lakes in the southern Empty Quarter of southern Saudi Arabia and northern Yemen. The ​individual branches, in 5-cm to 15 cm fragments now, have been completely fossilized (silicified) this way.

 Q: How might this process affect the environment?

​A: It certainly preserves trees, and likely even some animals that have died in that time period. In the sense that it preserves trees over 200 million years old, it could be viewed as stopping or arresting the natural landscape evolution of these particular areas.​

 Q: Could this happen in the future?

​A: Yes. From the example above in north Yemen it is clearly happening now in some places. ​
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