The stone that makes up the war memorial is made up of granite.
So what is granite?
Granite is a common type of igneous rock. Igneous rock is one of the three main rock types , the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava, which then forms crystals which make up the rock. We all look at building stones everyday without much though most of the time, but if we look a bit closer, we can then look at its micro scale, some of which can be seen by the naked eye, others which may be more difficult to see with the unaided eye. Looking at the microstructures and macrostructures can help provide information on how quickly it cooled, and what it is formed of. Granite is formed of crystals, of which some can be described as phenocrysts. Igneous rock forms by crystallisation either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. Intrusive rock forms within the crust from the crystallization of magma. As the magma pushes up from deep, it eventually slowly cools into a solid, and forms rock. An intrusive rock is any form of igneous rock that forms within the crust of the planet. Large areas of magma that solidify underground before they reach the surface are known as plutons .Granitic rocks mainly consist of feldspar, quartz, mica and amphibole minerals, which form an interlocking matrix of feldspar and quartz with scattered darker biotite mica and amphibole (often hornblende) peppering the lighter colour minerals.
A phenocryst is a crystal in igneous rock, that is larger than the surrounding crystals of the rock that it forms part of.
Phenocrysts can have different forms :
They can be euhedral, these crystals are well defined, sharp and easily recognisable edges.
They can be anhedral, which do not have well defined edges.
When describing the microstructure and macrostructures of igneous rocks, the texture of the rock can be described.
Pegmatitic: this means it contains very large crystals.
Phaneritic: this means the crystals can be seen by yours without any aid such as a microscope.
Aphanitic, this means that the rock has a uniform, fine grained crystals , which arises from rapid cooling.
Porphyritic: this means it contains phenocrysts.
Vesicular: this means there are holes in the igneous rock , which are formed by gas bubbles that were trapped as the rock cooled.
Vitreous : this means the rock as a glassy appearance which have crystals so small they cannot be seen by the naked eye.
Equigranular : this means that the rock crystals are all the same size.
So basically phenocryts are crystals, now these can be classified in terms of shape.
This being an EarthCache, in order to log it, I ask that you answer the below questions . Please send them to me, and do not include them in your log. You can send them to me by using the message facility or email, both of which can be found by looking at my profile. Alternatively, they can also be given face to face should we meet.
(1) Please look at the base of the war memorial, here you can see a number of phenocrysts. Please describe their shape, and colour.
(2) Using the above EarthCache lesson, please tell me whether you think this granitic rock cooled quickly or slowly, what is the rationale for your answer?
(3) Are the phenocrysts, anhedral or euhedral?
(4) Using the above diagram, please state what predominant shape are the phenocrysts.
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