Urban Geology is the geology of the built environment. This includes the building stones and other materials used in town and cities as well as the tantalising glimpses of the pre-urban landscape and underlying bedrock.
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QUESTION: Granite can be of different colours. Most kinds of granite are grey however granite can be black, dark-red and rarer green or bluish-grey colour. Two colours of granite have been used to make up the feature at GZ. Describe what two colours you see and what type of feldspar would have caused this colouring during the formation of the granite. Use the above examples to give an answer/answers as A B C D or E. The rest of the information can be found in the listing.
WHERE GRANITE CAME FROM
Granite is classified as igneous, arising from magma many miles below the surface. Granite formation begins when two tectonic plates collide. One plate will be pushed under the other creating a subduction zone. Rocks in the down-going plate are super-heated under extreme pressure, up to 1,600 degrees C, melting into magma which then rises upwards because it is hot and less dense than the surrounding material. About 10% of the magma will be pushed to the surface (extrusive), erupting as volcanoes where rapidly cooling lava creates a variety of fine-grained rocks from basalt to obsidian. The magma that remained below the surface (intrusive) slowly cools and crystallises, the slow speed of cooling allows larger crystals to form which gives granite its appearance. The granite collects in plutons and batholiths of igneous rock several kilometres below the surface.
Granite is found in all continents around the world and is generally the foundation of many orogenic belts or mountain chains. As it turns out, most of the ocean floor is basalt, and most of the continents are granite.
The granite is under great overhead pressure. These granite formations are then exposed to the surface through the process of erosion accelerated by continental uplift acting over many millions to hundreds of millions of years. This process removes several tens of kilometres of overlying rock in many areas, exposing the once deeply buried batholiths. The pressure on the granite is reduced and it then expands and forms fractures or sheet joints parallel to the surface.
What does the colour of granite depend on? It depends on mineral composition (components) of granite. The colour of feldspar especially strongly influences the decorativeness of granite. But feldspars are white. Feldspar breeds get grey and black colours from the admixture of dark-coloured minerals - mica or pyroxenes. Granite consists of three main minerals: quartz and two kinds of feldspars (potassium and calcium-sodium) and mica. They form the structure of granite with equal amounts (30%). They all are colourless or white. And pyroxenes, amphibole or mica, being dark minerals, make 10%; they will be noticed in granite as separate scales or grains. If you look at the stone from a distance everything merges into a grey colour. Obviously, the more dark minerals in the granite - the darker it is. The colour of some granite seems darker because of quartz which can be presented by morion. How does granite become red? Bright red or pink feldspar are included into their mineral composition. They can be met in all parts of the world. The smallest crystals of hematite (oxide of iron) and red iron ore, disseminated regularly on all crystal of feldspar, give colouring to feldspar. Painting abilities of hematite are exclusively great. Feldspar takes thick red colouring already when there is 0,7-1,0% of hematite in it. Granite feldspar acquires the pink colouring when maintenance of hematite does not exceed 0,3-0,4%. So, red colour of granite is a result of difficult and not always ordinary processes. And how about green granite? Such colour is given to granite due to inclusions of green potassium feldspar which has been known for a long time as a semiprecious stone. The Nature of the colouring is not clear up to this moment. There is a hypothesis that at the beginning the usual red and white feldspar was crystallised, which subsequently under the influence of the flying substances, accumulating in the central part of a vein, turned into amazone-stone (amazonite). Granite is a very desirable material. It is used on exteriors and interiors of homes and commercial buildings. The most common uses are for headstones and kitchen countertops. Some other examples include: fireplaces, bars, vanities, sinks, cabinet handles, outlet switch plate covers, tables, walls, benches, fountains, shower surrounds, flooring….If you can imagine it, you can use granite.The feature at GZ is a good example of granite in use as a decorative piece, it was presented to the Wimbledon Council in 1898
THE PARK
At the start of the 19th century Wimbledon was still mainly farmland, this was until 1838 when the railway station was built which brought many workers to the area leading to the building of roads and houses. In 1899 the council bought this land and turned it into The Gardens. There is a horse trough in the gardens that was used by deliverymen who sold milk and coal to water their horses. The Gardens have a number of original features and as such have been listed grade 2 on the English Heritage Register. The Gardens are a great place to come for a stroll, they provide a tranquil oasis for local residents and people working in the area.
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