The large rock at GZ weighs 28 tonnes and was mined from Stoney Pinch Quarry which is situated approximately 35km west of Renmark. It is primarily basalt in nature.
Basalt is an igneous rock formed from magma (molten rock) and a mixture of other materials. If magma cools beneath the surface of the Earth, it forms intrusive rocks such as granite which can take many years to cool. Magma which reaches the surface cools much faster, forming extrusive rocks such as basalt which are very fine grained because the rapid cooling does not allow time for large crystals to grow.
Basalt is the most common volcanic rock on earth, and is generally found in a variety of dark colours such as grey or black. This particular rock is unusually coloured as a result of the magma mixing with local yellow sand as it burst to the surface. It has then been further discoloured by exposure to the environment. In arid regions, exposed basalt surfaces often develop a white limey encrustation. In humid areas the iron in basalt oxidises, colouring the surface a rusty brown. Although basalt is a hard durable rock it has limited building use because of the staining that occurs from the iron oxide. However basalt is excellent as a crushed rock and has many uses in this form.
To log this Earthcache:
Visit GZ and examine the large rock near the information board. Message me your answers to the following:
1. Examine the stone up close and describe what you see. Is the colouring uniform or mottled? Is the surface texture consistent or varied?
2. Given the colouring of this rock, and the information above, do you think this rock was exposed to an arid environment or a humid one?
3. How many other large rocks of a similar nature are there in this rest area?
You may post your Found log immediately without waiting for my response. I may not contact you unless I feel your answers require clarification. If I don't receive correct answers within 7 days of your Found log being posted, I may delete your log. Please don't post answers to the questions above in your log.