Welcome to Kianinny bay the location for todays Earth Science Lesson, which is a quick and easy one about Eden Rhyolite. Although the area has some quiet complex geology and a variety or rocks available to discuss we decided to make it quick and easy as we are disappointed to hear many cachers ignore Earth Caches, as they find them difficult and confusing. We would like everyone to enjoy earth science as much as we do so this cache is a great place to start and hopefully encourages more Earth Caches to be found in the future.
Firstly the background to the area:
Tathra started out as a small jetty, known as the ‘Farmers’ Sea Wharf’. It was a shipping outlet for a group of local farmers, who were fed up with having to transport their produce to Merimbula 25 km away, the original jetty was at Kangarutha. In 1860, it was decided that Kianinny Bay was more sheltered for loading than Kangarutha, so a store was built at Kianinny. Cargo was still shipped from the beach by small boats to vessels and navigated in. Bad weather often held up the produce wagons on their way to the boats, so loading was uncertain and the freight costs high. In 1861 Tathra township was surveyed, the jetty was replaced by the now famous Tathra wharf, which was funded by donations from local farmers. Kianinny is now used as the local recreational boat ramp, angling club and the start of Kangarutha Walking Trail can also be found in the picnic area. It’s a very scenic area with some special geology visible.
Now lets look at what is Rhyolite?
Rhyolite is an extrusive igneous rock with a very high silica content. It is usually pink or grey in colour with grains so small that they are difficult to observe without a hand lens. Rhyolite is made up of quartz, plagioclase and sanidine, with minor amounts of hornblende and biotite. Trapped gases often produce vugs in the rock. These often contain crystals, opal or glassy material.
Many rhyolites form from granitic magma that has partially cooled in the subsurface. When these magmas erupt, a rock with two grain sizes can form. The large crystals that formed beneath the surface are called phenocrysts and the small crystals formed at the surface are called groundmass.
Rhyolite usually forms in continental or continent-margin volcanic eruptions where granitic magma reaches the surface. Rhyolite is rarely produced at oceanic eruptions.
So why is the Rhyolite here called Eden Rhyolite?
Basically Eden Rhyolite is just a name given by geologists to the rhyolite that is part of the geology of the Eden-Tathra region. It was called Eden Rhyolite because it was first studied at Eden. Rhyolite in the area consists largely of pyroclastics and is a potash Rhyolite which is usually Porphyritic.
What does Porphyritic mean?
Porphyritic is an adjective used in geology, specifically for igneous rocks, for a rock that has a distinct difference in the size of the crystals, with at least one group of crystals obviously larger than another group. Porphyritic rocks may be aphanites or extrusive, with large crystals or phenocrysts floating in a fine-grained groundmass of non-visible crystals, as in a porphyritic basalt, or phanerites or intrusive, with individual crystals of the groundmass easily distinguished with the eye, but one group of crystals clearly much bigger than the rest, as in a porphyritic granite. Most types of igneous rocks may display some degree of porphyritic texture.
Additional information:
We also need to point out that there are several Quartz veins easily visible at the published location; we need to point these out so you don’t get your answers confused. In geological terms a vein is a distinct sheet like body of crystallized minerals within a rock. Veins form when mineral constituents carried by an aqueous solution within the rock mass are deposited through precipitation. The hydraulic flow involved is usually due to hydrothermal circulation. An example of one of the local Veins is below.
To successfully log this cache please use your own judgment and the information provided whilst examining the area to answer the following questions and send us your answers to the best of your ability;
1. At the published location, find some Rhyolite what does it feel like and why does it feel this way?
2. Can you see any crystals? What Shape and Colour are they?
3. Did you happen to notice any of the Quartz veins we talked about? What was one characteristic of these that stood out to you?
4. A photo of your team, GPS near GZ with your log and answers. (optional)
You are welcome to log your answers straight away to keep your TB's and Stats in order but please message us with your answers. Cachers who do not fulfil the Earth Cache requirement will have their logs deleted.
Source: Wikipedia, Geology.com.
