Church Micro 10509...Migdale EarthCache
Church Micro 10509...Migdale
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The present church is from 1881, built on site of the church erected in 1843. An interesting group of Free Church buildings to the left of the church, enhanced by connection with Dr Gustavaus Aird, a local pioneer within the Free Church at the time of the 1843 Disruption, whose magnificent memorial stands in the church grounds and it is this memorial we need to have a closer look at.
The grey and the red stone of this memorial are both granite ~ The type of granite a particular specimen is, depends upon the percentages of minerals that make up the rock, especially quartz (Greyish colour), K-feldspar (Reddish colour), Na-feldspar (White) and Biotite (Black) and sometimes black amphibole hornblende. Local Migdale granite is red in colour, made up of pink and white feldspar, clear quartz and black biotite. It is cut by narrow veins (2/3 cm’s) of a coarser granite rock called pegamite which represent the last water rich products of the cooling granite.
So what is granite?
Granite is one of the most commonly known types of rocks, used in everything from buildings to sculptures. It has been used for thousands of years and is regarded as a symbol of status, strength, and durability. Granite is a common intrusive plutonic igneous rock. Intrusive means that it has moved into other rocks by force coming up from the mantle. Plutonic means that it is magma that does not reach the surface of the earth and so cools very slowly underground. Igneous (derived from the Latin word ignis meaning fire) 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. Because it cools slowly crystals have time to form. And the slower it cools the bigger the crystals Its name comes from the Latin word granum which means “a grain” for the coarse grain crystalline structure of the rock. Individual crystals are visible to the naked eye in all types of granite. It is made up of quartz, mica, and feldspar. Granite must contain at least 20% quartz within a rock to make it granite. Quartz is one of the last minerals to form in granite; it acts as a bond between other minerals. Although quartz is colourless, it often appears grey because it reflects the colours of dark and light minerals around it.
You will notice on the left hand side panel (NE), some larger dark marks, different to the rest of the grey in the panel. These marks are called xenoliths, they are part of the original rock which did not melt when the rock was formed.
Below the surface of the earth lies molten rock magma. Floating on this sea of magma, are plates of solid rock. They are the tectonic plates which are in constant movement. Where the plates meet, one plate will slide on top of another plate forcing it down into the sea of magma. The downward pressure causes some of the molten magma to swell up in bubbles and melt some of the rock above. The molten rock under great pressure from the solid plate above cools slowly to form large crystals. Not all the original rock melts and fragments of it remain in the rock which is how these dark patches (xenoliths) are formed.
Granite comes in a wide variety of colours including reds, browns, and many shades of grey from almost black to nearly all white. Granite is a mix of minerals and rocks, primarily quartz, potassium feldspar, mica, amphiboles, and other trace minerals. Granite typically contains 20-60% quartz, 10-65% feldspar, and 5-15% micas (biotite or muscovite). The minerals that make up granite give it the unique colours we see in different types of granite.
The relative proportion of different coloured minerals in a granite is largely due to the original source of molten rock that cooled to form the granite. If the molten rock was abundant in potassium feldspar, the granite is more likely to take on a salmon pink colour. On the other hand, if the molten rock is abundant in quartz and minerals that make up amphibole, you will likely get black and white speckled granite.
• Quartz – typically milky white colour
• Feldspar – typically off-white colour
• Potassium Feldspar – typically salmon pink colour
• Biotite – typically black or dark brown colour
• Muscovite – typically metallic gold or yellow colour
• Amphibole – typically black or dark green colour
The combination of the minerals above make up most of the colours we typically see in granite.
Red granite is a variation of pink potassium feldspar abundant granite, where the k-feldspar takes on a redder than pinker colour. Also, you can get red colouring from iron oxide in hematite grains or inclusion within feldspar, essentially the same process that makes rusted metal red coloured.
To claim this earthcache, please send the answers to the questions below to us by email or messenger- do not post in your online log. Your log may be deleted if these criteria are not met. Educational guidelines for Earthcaches are set by Geocaching.com and GeoSociety.org (Earthcache) and have to be adhered to.
1 Looking at the grey granite, you see both polished and unpolished granite ~ of which do you find the crystals easier to see and why?
2 Which of the upper 4 panels (those surrounded by the red granite pillars) do you find contains most xenoliths, how many can you see and what is there average size?
3 Compare the size of the crystals in the red and the grey granite, how much larger are the crystals in the red pillars compared to the grey panels, and why do you think this is?
4 Although not granite, you sometimes find another rock referred to as ‘Blue Granite’, what is this rock and where is it commonly found? (Google may help)
5 Why not compulsory it is always good to see photos of your visit
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