ABERDEEN ~ The Granite City #4 Hilton Quarry
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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.
The most famous city quarry is Rubislaw which produced a fine grey muscovite-biotite granite. More than 50 per cent of Aberdeen’s buildings are estimated to have come from the Rubislaw Quarry alone,but there were (including sand and gravel pits) well over 100 quarries in Aberdeen city and shire. One of the older quarries is Hilton, situated south of Hilton Road but its northern part lies across the road in what is now Stewart Park. Quarrying was undertaken until the 19thC. It was filled in during the first half of the 20thC when the surrounding houses were built. In 1814 the quarry was occupied by a James Bruce,mason. Today it is grass and trees popular with dog walkers but you can still find the cliff face type granite formations coming up out of the ground.
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-sized crystalline structure of the rock. Individual mineral crystals 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) and sometimes black amphibole hornblende.
There are three types of weathering, biological,physical and chemical, which cause the breakdown of the rock.
Living organisms contribute to the weathering process in many ways trees put down roots through joints or cracks in the rock in order to find moisture. As the tree grows, the roots gradually prize the rock apart. Even the tiniest bacteria, algae and lichens produce chemicals that help break down the rock on which they live, so they can get the nutrients they need.
There are two main types of physical weathering:
Wind, water and ice cause the rock to expand and contract in reaction to changes in temperature. It shatters on the surface and the fractures widen to form crevices. Freeze-thaw occurs when water continually seeps into cracks, freezes and expands, eventually breaking the rock apart.
Exfoliation occurs as cracks develop parallel to the land surface a consequence of the reduction in pressure during uplift and erosion.
Chemical weathering is caused by rain water reacting with the mineral grains in rocks to form new minerals (clays) and soluble salts. These reactions occur particularly when the water is slightly acidic.
Different minerals weather at different rates. Mafic silicates like olivine and pyroxene tend to weather much faster than felsic minerals like quartz and feldspar. Different minerals show different degrees of solubility in water in that some minerals dissolve much more readily than others. Water dissolves calcite more readily than it does feldspar, so calcite is considered to be more soluble than feldspar. A rock’s structure also affects its susceptibility to weathering. Massive rocks like granite generally do not contain planes of weakness whereas layered sedimentary rocks have bedding planes that can be easily pulled apart and infiltrated by water. Weathering therefore occurs more slowly in granite than in layered sedimentary rocks. Chemical weathering may involve the disintegration of a rock into rock fragments due to conversion of silicates into clays. For example, interlocking silicate grains in fresh granite gradually decay along crystal boundaries due to conversion to clays. Eventually cracks open around the boundaries, the rock weakens and easily disintegrates.
Unweathered granite contains the following minerals:
• Sodium Plagioclase feldspar (Na feldspar)
• Potassium feldspar (K feldspar)
• Accessory biotite, amphibole, or muscovite
The quartz (and muscovite if present) remains as residual minerals due to their high weathering resistance. Biotite and/or amphibole undergo hydrolysis to form clay, and oxidation to form iron oxides. The weathered rock fragments become constituents of the soil.
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. It is helpful if you send your answers at the same time as you 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 What type of weathering would you say is the main one present here and why do you think this?
2 What do you think will happen here in the future if weathering continues as it is today?
3 How high is the tallest out crop of granite here?
4 Looking closely at the rock, how would you describe its colour?
5 Feel the rock, can you feel the individual crystals, have a close look how big are they?
6 While not compulsory, it is always good to see a photo from your visit
Enjoy your visit and if you haven’t already found it, GC4D5EZ
Is very close by!
(No hints available.)