The Berrima Courthouse is built from local, hand-hewn sandstone, a common material for historic convict-era buildings in New South Wales, often referred to as "convict sandstone," though the specific geological type is generally a mature sandstone (quartz arenite) quarried from the region.
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Questions
- Based on the information contained in the write-up and your observations on site, do you feel that this weathering is primarily physical, chemical, biological or a combination of two or more types? Give examples
- Looking at the columns along the front of the court house, which direction ( North, East, South or West) is the majority of the weathering occurring?
- Based on your observations and the information in the write-up, do you support the removal of the black crust on the columns? Why or why not?
- Mandatory: Please post a photo of yourself or a signature item ( GPS, Name on a piece of paper, thumbs up) on your found log.
Weathering: is defined as a breakdown of rock in situ, or without it being moved. Weathering processes depend on the presence of water, temperature, mineral and chemical composition of the rock.
Weathering vs erosion: While weathering is often confused with erosion, the word erosion implies that the rock or stone has been subject to transport processes which have worn it down. Rocks are typically transported in rivers, streams or can be carried by glaciers or rock falls. Rock that remains on site that wears down is considered to be affected by weathering processes.
Types of weathering
1. Physical weathering
A) Expansion-contraction weathering, often called exfoliation or onion skin weathering, depends on expansion and contraction caused by heating of the rock from the sun. The outer layer of the rocks surface expands during high day temperatures and cools rapidly at night. This stress on the rocks surface will open up stress cracks which continue to erode. This type of weathering is typically more likely to occur in hot, arid climates. https://youtu.be/nL8ZwPOPMcc
B) Freeze-thaw weathering, sometimes called frost-shattering depends on the expansion of water when it freezes into ice. This is a common type of weathering in climates where temperatures fluctuate around the freezing point. Water will sit in a pore, crack or joint, and when it freezes will expand, further enlarging the crack or joint, pushing the rock apart. This can cause rock to break off from the main stone. Sandstones break into individual grains or into blocks along joints, while mudstones break into small angular fragments. Crystalline rocks including limestone and igneous rocks break into blocks controlled by joint patterns.
2. Chemical Weathering
The processes at work from Chemical weathering are more difficult to observe as rocks will react to chemical processes in differently depending on the type of stone, composition, and durability. Typically, there are three types of chemical weathering
Oxidation occurs when iron in the minerals of the rock will oxidize or rust if water is present. Iron is normally orange, yellow or brown in colour when it oxidizes on the outer skin of the rock.
Hydrolysis or rain water, which can be slightly acidic, will react to some minerals causing them to decompose. An example of this is found in feldspar minerals common in granite, which decompose readily to clay minerals and are then washed away by rainfall. Granites typically weather along joints where water can flow and dissolve the minerals.
Carbonation is a form of hydrolysis, which occurs when rainwater reacts with any minerals containing carbonate. Limestone and sedimentary rock, chalk, and marble are typically affected by carbonation. Marble is a metamorphic rock consisting of carbonate minerals that recrystallize under the influence of heat, pressure, and aqueous solutions (most commonly calcite (CaCO3) or dolomite (CaMg(CO3)2)
Black Crust – Pros and con’s
Black crust on building materials can form as a result of both chemical and physical reactions between the surface of the stone and environmental factors, including acid aerosols, and air born particulate matter. Chemical composition varies depending on the minerals in the sandstone which also affect the colour variations from black to reddish brown.
Black crusts can act as a protective layer of pourous building materials or as barriers against environmental stressors, which react with the original material, promoting its degradation (Torok, 2003). However there can also be negative and destructive consequences on building stones where this form of weathering can lead to exfoliations in the stone. Typically, black crust on historical buildings is viewed from an aesthetic point of view, and restoration is often considered which can lead to issues down the road where the black crust removal also removes the protective layer, promoting deterioration in the long term.
3. Biological Weathering
Similar in some ways to Chemical weathering, Biological weathering occurs when lichens and mosses attach themselves to rock surfaces, using moisture and nutrients in the air as well as reacting to chemicals on the rock. The roots of plants can also affect monuments including ivy as their roots can get into small spaces and cause the breakdown of minerals on the rock such as feldspar. Roots from trees can push up from below and topple stones leading to physical weathering.
References:
- https://www.thoughtco.com/examples-of-chemical-weathering-607608
- https://www.yarrabeestone.com.au/blog/understanding-the-effects-of-weathering-on-sandstone
- https://link.springer.com/article/10.1007/s00254-007-0871-y - Abstract
- https://pmc.ncbi.nlm.nih.gov/articles/PMC4236627/
- https://www.sciencedirect.com/science/article/abs/pii/S0013935116300561
