The obelisk, named Cleopatra's Needle has a twin in London, both examples are made of red granite, stand about 21 metres (69 ft) high, weigh about 224 tons and are inscribed with Egyptian hieroglyphs. They were originally erected in the Egyptian city of Heliopolis on the orders of Thutmose III, around 1450 BC. The granite was cut from the quarries of Aswan, near the first cataract of the Nile.
The inscriptions on the obelisks were added around 1250 BC by Ramesses II to commemorate his military victories. The obelisks were moved to Alexandria and set up in the Caesareum – a temple built by Cleopatra in honour of Mark Antony or Julius Caesar – by the Romans in 12 BC, during the reign of Augustus, but were toppled some time later. This had the fortuitous effect of burying their faces and so preserving most of the hieroglyphs.
In the late 19th century, during a surge of renewed international interest in Egyptian antiquities, the Egyptian government offered one obelisk to England and the other to the United States to further diplomatic relations. Transporting the Central Park Obelisk from Alexandria to New York City was an extraordinary undertaking – it was shipped aboard a cargo vessel called the SS Denton, built especially for the task in England. The Obelisk was rolled on cannonballs through a 360-square-foot hole cut in the starboard hull of the ship; impressively, it took only eight hours to load the 220-ton monument! After a month at sea, the Obelisk arrived in Staten Island. Its journey was far from over, however. It took six months to move the Obelisk from a Staten Island dock to its current location in Central Park, and a specially built railroad was built just for the task. Thousands of New Yorkers gathered to watch as the Obelisk was erected on January 22, 1881.
There has been much speculation about the chemical and physical weathering of the obelisk since its arrival in New York. The London obelisk was “sealed” with paraffin wax just after arrival, while the same measure was not carried out in New York until some time later. The condition of the New York obelisk is much worse than the one in London. Typical average temperatures in London range from 5 - 23 degreees celsius over the course of a year whereas New York's range from -2 - 30 degrees celsius. (See gallery pictures for graphs of temperature changes).
Weathering and erosion are often confused. Weathering is the wearing away of rocks. Erosion is the movement of the broken pieces away from the site of weathering. There are two types of weathering to consider here:
The weathering of rocks by chemicals is called chemical weathering. Rainwater is naturally slightly acidic because carbon dioxide from the air dissolves in it. Minerals in rocks may react with the rainwater, causing the rock to be weathered. Some types of rock are not easily weathered by chemicals. For example, granite and gabbro are hard rocks that are weathered only slowly. Still some of their minerals do react with the acids in rainwater to form new, weaker substances that crumble and fall away.
When fossil fuels such as coal, oil and natural gas are burned, carbon dioxide and sulphur dioxide escape into the air. These dissolve in the water in the clouds and make the rainwater more acidic than normal. When this happens, we call the rain 'acid rain'.
Acid rain makes chemical weathering happen more quickly. Buildings and statues made from rock are damaged as a result. This is worse when the rock is limestone rather than granite.
Physical weathering is caused by physical changes such as changes in temperature, freezing and thawing, and the effects of wind, rain and waves.
When a rock gets hot it expands a little, and when a rock gets cold it contracts a little. If a rock is heated and cooled many times, cracks form and pieces of rock fall away.
Wind, rain and waves can all cause weathering. The wind can blow tiny grains of sand against a rock. These wear the rock away and weather it. Rain and waves can also wear away rock over long periods of time.
Water expands slightly when it freezes into ice. If water gets into a crack in a rock and then freezes, it expands andpushes the crack further apart. When the ice melts later, water can get further into the crack. When the rock freezes again, it expands and makes the crack even bigger. This process of freezing and thawing can continue until the crack becomes so big that a piece of rock falls off.
- Eventually all trace of the hieroglyphs will be lost (unless the needle is preserved). Will this be caused by erosion or weathering?
- The twin to this obelisk can be found in London. The hieroglyphs on that one have suffered less than this example despite being made from the same granite. Why do you think that is? (Hint think about what the London one had done to it on arrival and any differences in physical conditions between the two cities).
- Describe the nature of the damage as you see it (at the top, middle, bottom etc). Are all faces damaged to the same extent?
As an optional extra post a picture of yourself and or your GPSr in front of the needle to prove your visit.
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