Visiting Derry, I walked past this attractive old building.
The mix of rock types used in it's construction shows how some rocks are more weatherproof than others.
1) Of the two types of rock (igneous/sedimentary), which is showing less signs of weathering? Explain how the way the particles are packed together may explain why this is.
2) Describe any weathering you can see on the front of the building, particularly at the base of the columns and the balcony.
3) Using the information below, what type or types of weathering do you think has taken place here?
4) Take a photo of yourself at GZ, or an identifying item, being careful not to reveal any of the answers to the questions.
Please submit your answers via message though the Geocaching website or by sending me an email - there is no need to wait for a response before logging your find.
Differing permeability of rocks
Most of the bulding is composed of sandstone, a sedimentary rock. Sedimentary rocks are typically composed of particles that are loosely packed together. These particles, such as sand, silt, and clay, settle and accumulate over time through processes like erosion and deposition. Due to their loose arrangement, sedimentary rocks often have higher permeability, allowing fluids like water to flow more readily through their pores and fractures. However, this also means they are easier to carve, which makes them a popular choice for more ornate flourishes on buildlings.
The pink columns are made from granite, an igneous rock. In contrast, igneous rocks form from molten magma that cools and solidifies either underground (intrusive) or on the Earth's surface (extrusive). This process usually results in a tightly interlocking crystalline structure, making igneous rocks less permeable compared to sedimentary rocks. The dense, crystalline nature of igneous rocks restricts the movement of fluids through them, resulting in lower permeability overall.
Weathers Effect on Rocks
Weathering is exactly that - the effect the wind and rain has on rocks. Rain can permeate rocks where the particles are loosely packed together, washing grains away. The repetition of this over time can be seen on the surface of the stone, the type of stone and the location it is placed in affect the speed this takes place.
Often in statues or carvings, one of the effects of weathering can be seen where the carvings are - what started out when the statue was new as a sharp edge becomes softer as the grains of the fine, sharp edge are removed by weather - think of a drawing with a blunt rather than a sharp pencil. Sometimes the stone can become stained by other chemicals in the air or water.
Whole sections can break off by strong wind or by water entering gaps, freezing and expanding, causing gaps to get wider and wider and eventually break over time.
More information on the various types of weather is below.
Types of weathering
Physical
Caused by changing temperature. Temperature causes rock to expand and contract as it gets warmer and colder. Repetition of this action can cause cracks and breaks. Freeze-thaw weathering also affects sandstone. When water seeps into the pores of sandstone and freezes during cold weather, it expands, exerting pressure on the rock. This expansion can cause the sandstone to crack and fragment over time. When the ice thaws, the water contracts, further stressing the rock. Over many freeze-thaw cycles, this gradual weakening and fragmentation can lead to the disintegration of sandstone into smaller pieces.
Chemical
Chemical weathering can significantly impact sandstone over time. When certain chemicals, such as acids from rainfall or organic acids from decaying vegetation, interact with the minerals in sandstone, they can cause chemical reactions that break down the rock.
Biological
Biological weathering involves the breakdown of sandstone by living organisms. One common mechanism is root wedging, where plant roots grow into cracks and crevices of the rock. As the roots expand, they exert pressure, causing the rock to fragment over time. Another form of biological weathering is caused by burrowing organisms like ants or worms, which create tunnels and crevices within the sandstone, facilitating the infiltration of water and further weathering processes. Additionally, organisms such as lichens and mosses can release acids that chemically weather the minerals in sandstone. Biological weathering, driven by the activities of organisms, plays a significant role in shaping landscapes and contributing to the erosion of sandstone formations over extended periods.