Sunday, March 06, 2011

How wood petrifies

by Steph

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Hello class! A stop at the Petrified Forest National Park in Arizona’s painted desert reveals our next lesson in how wood petrifies.

wood3Way back in the triassic period, this desert used to be a lush wet forest. No kidding! Dinosaurs were running around in the trees, eating leaves and ferns and each other on the banks of a river. Over time, trees fell (as they do). There could have been some catastrophic event that made them all fall at once, but anyway, they ended up down on the ground. Then the river deposited layers and layers of silt over the trees, cutting them off from any oxygen which might help them decay.

Here’s the key: silica has to be introduced to the mix. In some places this comes in the form of a volcano that erupts and spreads a layer of ash over the buried trees. In other places, there may have been a volcano upstream, which erupts its silica ash into the water, which is then mixed into the river’s sediment and deposited farther away.

The main point is this: trees, covered and protected from oxygen and therefore decay, with a little silica thrown in for good measure. Water seeps in, bringing minerals of various types and colors, which eventually (like over millions of years) replace the organic material of the wood, creating….presto change-o:

Petrified trees!

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