Goulburn is home to two cathedrals. The newer St Saviour's, not far from here, is made of sandstone, like so many buildings in Goulburn and across NSW including Sydney's CBD. But here at GZ you will find Saints Peter and Paul Old Cathedral, which is much more interesting from a geological perspective. Its unusual appearance is due to the use of diorite porphyrite, an extremely hard greenstone mined locally near Bungonia. It is widely claimed that this is the only greenstone cathedral in the world.
Let's consider the name of this unusual green stone: diorite porphyrite. Diorite is an intrusive igneous rock, which means it is formed when magma, hot molten rock, pushes up from deep within the Earth into cracks or spaces, sometimes forcing other rocks out of the way. It then cools into a solid. Many mountain ranges and other geographic features were formed in this manner, often over millions of years. Different components present in the magma produce different igneous rocks. Diorite is composed of feldspar, biotite and hornblende. It sometimes contains pyroxene, which would give it a greenish colour cast.
When an igneous rock is described as porphyritic, this indicates a distinct difference in the sizes of the crystals that formed during the cooling process. Usually this occurs because the magma cools in two or more separate stages. It may cool slowly while still deep in the crust, forming large crystal grains of 2mm or more. These crystals are carried in the magma as it continues to push upwards to where it eventually cools rapidly near the surface or as it erupts. This more rapid cooling produces smaller grains which can be invisible to the naked eye. Thus porphyritic rock often has the appearance of larger grains embedded in a continuous background mass.
The coordinates for GZ are at the south-west entrance to the cathedral, where you can observe the building closely from the public footpath without needing to go through any gates. To complete this Earthcache, you must visit GZ and perform some observations based on the information above. Email your answers to the following questions to me via my profile link above.
1. Note that this building is heritage listed, so don't chip or scratch the stone. Examine the lighter sandstone and darker diorite and describe both kinds of stone. Consider overall colour, grain size, variation in grain size and colour of individual grains. Bear in mind the porphyritic nature of the diorite.
2. Now you are familiar with both types of stone, how would you characterise the use of sandstone and diorite in this building?
a) mostly diorite, with sandstone edging
b) mostly sandstone, with diorite edging
c) sandstone and diorite used interchangeably
3. Which of the two types of stone is used for the fine details such as the beautiful ornamentation around and above the door? Why do you think this was used in those places in preference to the other type of stone?
4. The effects of weathering and different light conditions can alter the appearance of the diorite, but bearing in mind that diorite from Bungonia is described as greenstone, do you think pyroxene was present during its formation?
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.
More information:
https://en.wikipedia.org/wiki/Diorite
https://en.wikipedia.org/wiki/Porphyritic
http://www.stspeterandpaulsoldcathedral.org/
** FTF honours go to: J+J.au **