ABERDEEN ~ The Granite City #3 Sir James McGrigor EarthCache
ABERDEEN ~ The Granite City #3 Sir James McGrigor
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There are more than 30 Aberdeens scattered across the world, but there’s only one Granite City. The North East of Scotland’s geological base is granite and Aberdeen IS the Granite City. Granite buildings are everywhere, ranging from the grandest of monuments to the humblest of tenements. The city of Aberdeen, not only known as 'the granite city' but also the Grey City and the Silver City with the Golden Sands is built extensively of silver-grey granite,( which can sparkle like silver because of its high mica content) from quarries within and around the city.
Although most famous for its grey granite, pink granite could also be found at Aberdeen (Shire) quarries. Corrennie is a medium grained biotite granite with a salmon-red colour, Birsmore grey and pink very like Shap granite in appearance.Clinterty a reddish grey,Banchory a fine grained dark red colour, and most famously Peterheads red granite.
Situated beside the south pathway in Duthie Park near Riverside Drive is a 70ft obelisk to Sir James McGrigor. Built of pink polished Peterhead granite it stands on a square plan base and plinth, bearing recessed tooled grey granite panel to the north. Sir James McGrigor (1771 -1851) was a military surgeon who reformed the Army Medical Service and greatly improved sanitary conditions for soldiers. The monument once stood in the centre of Marischal College which he both attended and served as Rector. It was moved to its current location in 1905 when the college was extended
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. Its name comes from the Latin word granum which means “a grain” for the coarse grain crystalline structure of the rock. Individual mineral grains are visible to the naked eye in all types of granite. It is made up of quartz, mica, and feldspar. It comes in a wide variety of colours including reds, browns, and many shades of grey from almost black to nearly all white.
There are different types of 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 (Redish colour), Na-feldspar (White) and Biotite (Black).
If you look closely you will see the different materials making up the stone. You will also see a dark mark above the E in the words “The Connaught” in the grey granite plaque and another much larger down under the plaque on the bottom left hand side. 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.
To claim this earthcache, please send the answers to the questions below to us by email/message centre - 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 Estimate how big the xenolith below the plaque mentioned above is.
2 At the south side, on the 3rd row up from the ground, there is a large rectangular block which contains 3 xenonilths in a line (almost) are all 3 the same colour? Do you think all 3 came from the same original rock? Give reasons
3 a At the rear (facing the road) about 5ft up (head height) there is a crack in the rock where you can see inside ~ look closely and describe the crystals you can clearly see in there ~ shape, colour, size etc
b From what you have learnt in the text above, which mineral is it which gives these crystals you can see this particular colour ?
While not compulsory, it is always good to see a photo from your visit.
There is plenty of parking in the park near the boating pond, but a steep lot of steps up to the memorial !
Additional Hints
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