Manchester Marble.... The Touchstone. EarthCache
Manchester Marble.... The Touchstone.
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Manchester, is the city of variety, delight and interest. Within walking distance of the city centre highlights is what can only be described as a geological wonder : Manchester's own bit of marble. The marble, otherwise known as the Touchstone, is situated on the public path near to the Bridgewater Hall. It is made from Italian Carrara marble.
Marble is a metamorphic rock resulting from the recrystallization of limestone.Limestone is a sedimentary rock composed largely of the minerals calcite and/or aragonite, which are different crystal forms of calcium carbonate (CaCO3). Marble is composed of recrystallized carbonate minerals, most commonly calcite or dolomite.Heat and pressure over time cause the carbonate minerals to recrystallize into an interlocking mosaic of crystals, the different colours due to the various mineral impurities such as clay, silt and sand. The resulting marble rock is typically composed of an interlocking mosaic of carbonate crystals. Primary sedimentary textures and structures of the original carbonate rock (protolith) have typically been modified or destroyed. Pure white marble is the result of metamorphism of a very pure (silicate-poor) limestone or dolomite protolith. The characteristic swirls and veins of many colored marble varieties are usually due to various mineral impurities such as clay, silt, sand, iron oxides, or chert which were originally present as grains or layers in the limestone. Green coloration is often due to serpentine resulting from originally high magnesium limestone or dolostone with silica impurities. These various impurities have been mobilized and recrystallized by the intense pressure and heat of the metamorphism. Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure (temperatures greater than 150 to 200 °C and pressures of 1500 bars) causing profound physical and/or chemical change. The protolith may be sedimentary rock, igneous rock or another older metamorphic rock. Metamorphic rocks make up a large part of the Earth's crust and are classified by texture and by chemical and mineral assemblage (metamorphic facies). They may be formed simply by being deep beneath the Earth's surface, subjected to high temperatures and the great pressure of the rock layers above it. They can form from tectonic processes such as continental collisions, which cause horizontal pressure, friction and distortion. They are also formed when rock is heated up by the intrusion of hot molten rock called magma from the Earth's interior. The study of metamorphic rocks (now exposed at the Earth's surface following erosion and uplift) provides us with information about the temperatures and pressures that occur at great depths within the Earth's crust. Some examples of metamorphic rocks are gneiss, slate, marble, schist, and quartzite Because of its specific colours and textural variations, marble is highly valued as a decorative stone. The quarries of Carrara, Italy, are where Carrara marble comes from. The marble from here has had many famous uses, such as Michaelangelo's David and the Marble Arch in London. When logging this find, please include a picture of you or your GPS at the site. Please also contact Hillgorilla, with the answers to the following: (1) How long is the Touchstone? (2) What does it feel like? (3) Describe the colour(s) of this piece of marble and indicate what impurities (if any) would be found in it. (4) How as this type of rock formed? Please don't include this answers in your log. Overall please have fun. It is wheelchair accesible.
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