Logging Requirements

This EarthCache is best completed (or may only be possible) at low tide.

1. Study the texture of the rocky outcrops in the ocean:
(a) Do they look like tree stumps, or are they more like rocky boulders?
(b) Are they soft like wood, or hard like rock?
(c) Do you think they are good samples of petrified wood, based on the description?

2. (a) A lack of what is required for petrified wood to form? [one word] [refer to Petrified Wood section]
(b) Subsequently from Question 2(a), why do you think petrified wood can be found in this location? Reference / consider your observation of the environment.

3. Take a photograph of you or your written name next to the rocky outcrops noted at the location. Make sure this is at GZ and contains a rocky outcrop. This photograph must be attached to your log.

Introduction

When the Southern Ocean was at the Continental Shelf many thousands of years ago, a forest of tall trees covered this stretch of country now covered by the sea. As the trees died acid from the rotting wood mixed with salt and calcium crystals, remineralised, creating a chemical front of karst earlier known as petrified wood. Some rock formations can be flint, others may be cemented sand and limestone rock weathered and carved by wave action.

Petrified Wood

Petrified wood is a fossil. It forms when plant material is buried by sediment and protected from decay due to oxygen and organisms. Then, groundwater rich in dissolved solids flows through the sediment, replacing the original plant material with silica, calcite, pyrite, or another inorganic material such as opal. The result is a fossil of the original woody material that often exhibits preserved details of the bark, wood, and cellular structures. Fossilisation only happens in the rarest of cases, and usually, the wood rots away before fossilation happens. To become a fossil, the tree needs to die in a location where it is protected from the elements. This means getting buried in sand, soil or mud, hence, the best place for that is on the seabed or a river bed. There it is preserved due to a lack of oxygen which inhibits aerobic decomposition. As time passes, the tree becomes buried deeper and deeper. What was mud or sand becomes compressed on its way to becoming rock.

Safely sealed away underground, chemicals and minerals percolate through the sediment and the original tree gradually recrystallizes. In extreme cases, the entire tree can dissolve away, leaving a hollow where it once was. The minerals from the enclosing rock can gradually fill the hollow to create a natural cast of the original. So sometimes a fossil doesn't contain anything of the original tree except its shape.

In other cases, minerals from the rocks gradually impregnate the wood, changing its chemical composition and making it capable of surviving for as long as, or longer than, the rock enclosing it. Eventually the rock enclosing the fossil is eroded away, and the fossil is revealed on the surface of the ground.

References

Geology.com - "What is Petrified Wood? How Does it Form?", https://geology.com/stories/13/petrified-wood/, accessed 29/01/2021.