At the southern part of the church court yard of St Paul’s Cathedral is a discrepit statue. What happened to it and how did it ended up there?
This monument of St Andrew at St Paul's Cathedral was once one of four statues of Evangelists and Saints on the west front. It is believed to be a stone carving by an unknown artist from the original cathedral construction period under architect Christopher Wren. But due to erosion from the London atmosphere, it has been removed and is now displayed in the churchyard next to the cathedral, where visitors can see it up close and observe the effects of weathering. The monument is made of portland stone.
So what is portland stone?
Portland stone was formed in a marine environment, on the floor of a shallow, warm, sub-tropical sea probably near land. When seawater is warmed by the sun, its capacity to hold dissolved gas is reduced; consequently, dissolved carbon dioxide (CO2) is released into the atmosphere as a gas. Calcium and bicarbonate ions within the water are then able to combine, to form calcium carbonate (CaCO3) as a precipitate. The process of limescale build up in a kettle in hard-water areas is similar. Calcium carbonate is the principal constituent of most limestones. Billions of minute crystals of precipitated calcium carbonate (called calcite) accumulated forming lime mud (called micrite) which covered the sea floor. Small particles of sand or organic detritus, such as shell fragments, formed a nucleus, which became coated with layers of calcite as they were rolled around in the muddy micrite. Portland stone measures 3.5 on the Mohs scale of mineral hardness.
The calcite gradually accumulated (by accretion) around the fragments of shell in concentric layers, forming small balls (of less than 0.5 mm diameter). This process is similar to the way in which a snowball grows in size as it is rolled around in the snow. Over time, countless billions of these balls, known as "ooids" or "ooliths" (from the Greek for "egg-shaped" or "egg-stone"), became partially cemented together (or lithified) by more calcite, to form the oolitic limestone that is called Portland stone. The degree of cementation in Portland stone is such that the stone is sufficiently well cemented to allow it to resist weathering, but not so well cemented that it cannot be readily worked (cut and carved) by masons. This is one of the reasons why Portland stone is so favoured as a monumental and architectural stone.
Weathering
Weathering, the process of breaking down rocks and minerals on the Earth's surface, manifests in three primary forms: i.e. chemical weathering, physical weathering and biological weathering.
The weathering of limestone is a process where the rock's chemical and physical properties are altered by exposure to environmental factors. This occurs through chemical, physical, and biological processes, with chemical weathering being particularly prominent. This breakdown of limestone, which is primarily composed of calcium carbonate (CaCO3), can lead to the formation of distinctive karst landscapes characterised by caves and sinkholes.
Chemical weathering
Chemical weathering is the most significant form of limestone degradation and is primarily driven by carbonation.
The carbonation process can be by :
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Formation of carbonic acid: Rainwater is naturally slightly acidic because it absorbs carbon dioxide (CO2) from the atmosphere. The reaction forms a weak carbonic acid (H2CO3):
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Dissolution of limestone: As this acidic water seeps into cracks and fissures in the limestone, it reacts with the calcium carbonate, dissolving the rock and carrying it away in a soluble form called calcium bicarbonate.
The effect of acid rain
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Air pollution from the burning of fossil fuels releases additional sulfur dioxide (SO2) and nitrogen oxides into the atmosphere.
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These gases react with water to form stronger acids, such as sulfuric acid, which significantly increase the rate of chemical weathering on limestone and marble structures.
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When acid rain reacts with calcite (CaCO3) in limestone, it can form gypsum (CaSO4⋅2H2O), which is more soluble than calcite. This leads to blackened crusts on sheltered areas of buildings and accelerated erosion on exposed surfaces.
Physical weathering
While chemical weathering dominates, physical processes can also break down limestone, often working together with chemical processes.
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Freeze-thaw: In colder climates, water enters cracks and joints in the limestone. When the temperature drops, the water freezes and expands, exerting pressure that widens the cracks and breaks the rock apart.
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Thermal expansion: In areas with large daily temperature fluctuations, the rock's surface repeatedly expands and contracts. This can cause stress and the formation of cracks, which leaves the rock more vulnerable to chemical weathering.
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Abrasion: The wind and waves can carry sand and other rock particles that physically wear away the limestone's surface over long periods.
Biological weathering
Living organisms also contribute to the weathering of limestone through both physical and chemical means.
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Root wedging: Tree and plant roots can grow into the fractures of limestone and exert pressure as they grow, physically breaking the rock apart.
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Organic acids: Lichens and other microorganisms can grow on rock surfaces. They release weak organic acids that help to dissolve the minerals in the limestone, accelerating the chemical breakdown.
Logging Requirements:
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Look closely at monument in front of you. Describe its appearance. What form(s) of weathering do you think is present here? Which form of weathering do you think has had the greatest impact?
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Does weathering appear to be evenly distributed, or are some areas more weathered than others? If so, why do you think this is the case? Also, is the rate of weathering consistent on whole monument, or does one part appear more weathered than the other?
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How do you see the old statue evolve as opposed to the new one?
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Upload a photo taken with the new St Andrew monument, at the coordinates N 51° 30.800 W 000° 05.894. You don't have to be in the photo, though it is strongly encouraged but your hand, your geocaching name or your personal item should be inside.