This earthcache aims to teach you about the formation of intrusive rocks and the formation their internal grain structure.
At the coordinates there is a monument, a flat polished granite rock with the word “Remember” etched on it.
Nearby there are some chucky “bluestone” rocks, one of which has the word “respect” etched on it.
A phanerite is an igneous rock with a microstructure that is made up of crystals large enough to be distinguished with the unaided human eye. (In contrast, the crystals in an aphanitic rock are too small to be seen with the naked eye). Phaneritic texture forms when magma deep underground in the plutonic environment cools slowly, giving the crystals time to grow.
Granite is a type of felsic intrusive igneous rock that is granular and phaneritic in texture. Granites can be predominantly white, pink, or grey in colour, depending on their mineralogy. Strictly speaking, granite is an igneous rock with between 20% and 60% quartz by volume, and at least 35% of the total feldspar consisting of alkali feldspar, although commonly the term "granite" is used to refer to a wider range of coarse grained igneous rocks containing quartz and feldspar.
Bluestone usually refers to basalt elsewhere in the world, however, in South Australia, the name bluestone is given to a form of slate which is much less durable than basalt but is valued for its decorative appearance. The interior of the stone is usually pale grey or beige in colour, but is given attractively coloured surfaces by ferric oxide and other minerals deposited in joints and bedding planes. The slate is laid in masonry with the mineralised surfaces exposed. Bluestone was most popular from about the 1850s to the 1920s, quarried in the Adelaide Hills.
Slate is a fine-grained, homogeneous metemorphic rock derived from an original shale type sedimentary rock composed of clay or volcanic ashthrough low-grade regional metamorphism. It is the finest grained foliated metamorphic rock..Foliation may not correspond to the original sedimentary layering, but instead is in planes perpendicular to the direction of metamorphic compression.
Both Granite and Bluestone are examples of Intrusive Rocks as both are formed within the earth's crust.
Granite is formed from cooling magma deep in Earth's crust. This cooling magma crystalizes to form the crystals that you can observe in Granite. A rock that cools within Earth's crust will cool very slowly and form larger crystals and is called an intrusive rock.
Granite can have grains over 25mm in size, however, the faster the granite cools the the smaller the grain size
Bluestone is formed when volcanic material intrudes into shale (compressed clay) within in the earths crust. The extreme heat causes the shale to change form.
To log this Earthcache, please answer the following questions, which aim to enhance your understanding of how intrusive rocks are formed.
Please send your answers to the cache owner using the messenger feature on geocaching.com.
1. Please describe the colour, texture and grain size of the “Remember” granite.
2. Based on the grain size, do you think it cooled slowly (deep within the earths crust) or rapidly (closer to the surface)?
3. “Respect” looks very different to “Remember” and is local “bluestone”. Describe the shape, size, colour and grain size of “Respect”.
4. Based on your observations, is the Bluestone a Phanerite?
5. There is a particular foliation pattern on “Respect”. How you think this pattern was formed?
You may log you visit as soon as your answers are submitted. The cache owner will respond confirming your approval to log the cache or asking for additional information if there are concerns about your answer.
Acknowledgements – Wikipedia.