At GZ you will find yourself in front of the ANU Research School of Earth Science's rock garden. Here you will find some fossilized wood among some other very interesting rocks. Please do not leave the footpath and enter the garden, and DO NOT disturb any of the rocks.
Petrified wood is the name given to a special type of fossilized remains of terrestrial vegetation. Petrifaction is the result of a tree or tree-like plants having transitioned to stone by a mineralization process. Petrified wood can preserve the original structure of the tree in all its detail, down to the microscopic level.
Petrification Process:
Petrified wood is part of a long-dead tree that was turned to stone and yet still retained its basic structure. This transformation can only take place under the right set of circumstances. When an organism dies, it usually decomposes - 'rotting' is a type of decay which sets in as microorganisms breaks down organic matter. Usually, a dead fallen tree will be subjected to this process. A newly-deceased tree may be rapidly buried by mud, silt or volcanic ash. This blanketing material then shields the dead tree from oxygen and because oxygen is the main driving force behind the decaying process, the smothered plant will begin to decompose far more slowly than it normally would. Meanwhile, mineral-lade water or mud seeps into the dead tree's pores and other openings. As the plants internal structure gradually breaks down, its organic material (wood fibres) gets replaced by silica and other minerals. Over a period of a few million years, those minerals will crystallize. The end result is a rock that appropriates the shape and structure of the original tree.
The final composition of petrified wood will depend on what minerals are present in the groundwater. Red and pink colours are produced by hematite, a form of oxidized iron. Yellow, brown and orange colours a produced by the presence of goethite. Goethite is a hydrated iron oxide that is derived by weathering from iron-bearing minerals. Green coloured petrified wood is produced by pure reduced iron (native iron). White is produced by pure silica which occurs most commonly as quartz. Shades of black are associated with iron sulfide or pyrite (fools gold). Sometimes a particular specimen will have thin veins of quartz when silicon and calcium carbonate were present in the groundwater solution and there was room for the minerals to crystallize.
The level of detail found in some specimens is amazing. Petrified wood can preserve the original structure of the stem in all its detail, down to the microscopic level. Structures such as tree rings and the various tissues are often observed features.
Logging Tasks
To earn credit for this earthcache, send answers for questions 1&2 to me via e-mail or messenger to the contact details on my geocaching profile. The photograph needs to be attached to your log. You can log the cache immediately but need to follow up with answers within ten days or so. Your Found log may be deleted if you do not send me the answers or attach a photo to your log.
1. Of the minerals mentioned above, which appears to be the most abundant in the fossilized wood?
2. Is there any evidence of quartz appearing on this piece of petrified wood?
3. Take a photograph of one of the other interesting rocks in the garden and include yourself and/or your caching name on a piece of paper/card. NOTE: As per newly published earthcache guidelines, the photo can be required to claim the find.
Examples of other Rocks in the Garden:
A piece of basalt column, a rock with brachiopods and crynoid stem fossils, a folded u-shaped rock with obsydian, and a limestone rock with three nautilus shell fossils.
Parking can be difficult during business hours and study semesters - there is good access for bicycles.
Thanks to Cankid for providing scientific assistance in submitting this earthcache