THE PETRIFIED TREE
Within a few kilometres of Scone on Moobi Rd, there is a petrified tree stump that is estimated to be around 200,000,000 years old. Regarded as one of the finest remaining specimens of petrified wood in situ in Australia (according to Professor Sir Edgeworth David), the tree was a type of Araucaria pine which is now extinct. The diameter of the tree was approximately 1 metre and was probably about 40 metres high. The remains of this ancient specimen show some vertical strands of the bark along it's rough surface.
In 1934 the Upper Hunter Shire Council decided to protect the petrified stump by putting a cage around it, as people were chipping off pieces of the tree as souvenirs. The remains of the stump are lying behind the cage.
There is a faded plaque next to the petrified stump. The text on the plaque reads:
WHAT IS PETRIFIED WOOD?
Petrified wood is a type of fossil. The word petrified comes from the Greek root "petro" meaning "rock" or "stone", literally "wood turned into stone". it consists of fossilised wood where all the organic materials have been replaced with minerals (most often a silicate, such as quartz), while retaining the original structure of the wood.
HOW DOES PETRIFIED WOOD FORM?
It forms when plant material is rapidly buried by sediment like volcanic ash or mud. This deprives the tree of oxygen and organisms, preventing decay. This allows mineral-rich groundwater to seep through the sediment, replacing the original wood's organic material with minerals such as silica, calcite, pyrite, or another inorganic material such as opal. Over several millions of years it essentially turns the wood into stone, while preserving its cellular structure and shape. The result is a fossil of the original woody material that often exhibits preserved details of the bark, wood, and cellular structures. This process is called petrification.
Some specimens of petrified wood are such accurate preservations that people do not realise they are fossils until they pick them up and are shocked by their weight and hardness. Specimens with near-perfect preservation are unusual; however, specimens that exhibit clearly recognisable bark and woody structures, such as the one on Moobi Rd, are very common.
The colour of petrified wood can vary depending on the minerals present during the process, with iron and manganese often creating red, yellow, or orange hues. You can see some of the colours in this specimen. Here is a list of minerals and related colour hues that is present in petrifed trees:
- Copper - green/blue
- Cobalt - green/blue
- Chromium - green/blue
- Manganese - pink
- Carbon - black
- Iron Oxides - red, brown, yellow
- Manganese Oxides - black
- Silica - white, grey
Petrified wood is not rare. It is found in volcanic deposits and sedimentary rocks at many of locations worldwide. It is sometimes found where volcanic activity covered plant material with ash, mudflows or pyroclastic debris. It is found where wood in sedimentary deposits was replaced by minerals precipitated from groundwater. It is especially abundant around coal seams, although many of the wood specimens in these locations are casts and molds rather than petrifications.
Since petrified logs are composed of quartz, they are hard and brittle, they can break easily when subjected to stress. The hard logs are surrounded by softer sedimentary layers. As the sediments shifted and settled, as well as tectonic movement, stress on the rigid logs causes fractures. As erosion continues, the rain and wind wear away the land, uncovering the logs, exposing them to the elements. They in turn, will be eroded over millions of years.
You can watch a video of the process using this link.
NB: Please take care at GZ, as you will be very close to the road.
GEO-DAD JOKE
I showed my dad some petrified wood.
His response was "That's funny, it doesn't look that scared to me".
You may log your visit immediately to track your geocaching progress. However, to complete your EarthCache experience, please send your answers to the logging tasks via email or GC Messenger within 7 to 10 days. Failure to do so may result in your log getting deleted.
Go to the posted coordinates to make your observations. Complete the following tasks.
Logging tasks:
1. Describe the petrified tree stump in relation to its height, shape, colours and texture.
2. What is the approximate age of this petrified tree stump?
3. Briefly explain how you think this petrified stump formed?
4. What mineral(s) might be present in the petrified tree stump?
OPTIONAL: Upload a photo of yourself or something with your geocaching name on it, with the petrified tree in the background.
RESOURCES:
https://geology.com/stories/13/petrified-wood/
https://www.npshistory.com/publications/pefo/brochures/trees-to-stone-2011.pdf
https://trove.nla.gov.au/newspaper/article/17099724
https://www.uky.edu/KGS/fossils/fossil-of-the-month_2022-03-Callixylon.php
CONGRATULATIONS: FTF goes to ilook4geo & goingplaces2
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