Erosion and Weathering.
Rocks are susceptible to processes geologists call weathering and erosion. Evidence of weather and erosion can be found in most environments where rocks are found. While the two terms are often used interchangeably, they actually describe two different (but interlinked) processes that affect rocks.
At the published coordinates, you will find yourself at Fiveways Junction, an old railway junction that’s now part of the Edinburgh cycle routes. This area is partially framed by some low stone walls. In the centre of one of these stone walls is a stone-sculpted lion.
In order to complete this EarthCache, you will need to locate the lion and answer the following questions. Please send your answers to the cache owner via either email or the message centre before you log your cache. Once you have submitted your answers you may log your find. If there are any problems with your answers, I will contact you.
- Based on the text below, do you see any evidence of weathering on the stone lion? Describe any type of weathering you see.
- What evidence of erosion can you observe on the lion? Are there any parts that are missing? Which agents of erosion do you suspect have happened here? Explain how you came to that answer?
- Look at the wall surrounding the lion, is there evidence of weathering/erosion on the wall? What type of weathering/erosion can you see? Is the weathering/erosion as pronounced on these stones as it is on the lion? What might explain any differences between the animal and the surrounding rocks?
- How many stone walls are at this junction? Are there any other sculpted animals in the walls? If yes, please name them.
The terms weathering and erosion are often used interchangeably but they actually describe different processes.
Weathering
Weathering describes the destructive processes where exposed rocks demonstrate a change in colour, texture, composition, firmness, or form, with little or no transport of the loosened or altered material. Most weathering occurs on the surface, but it may also happen deeper in the rock. Weathering causes a rock to deteriorate over time, and can involve the physical disintegration and chemical decomposition of rocks into soil and rock fragments.
Weathering happens in place. It breaks rocks down into substances which may be more easily transported (ie: rock fragments), but are not actually transported during the process of weathering. When those products of weathering are transported to a different location then we may say that erosion has taken place.
Identifying the impact of weathering is sometimes easier with sculpted or carved rock. Because we can expect carved stonework to look a certain way when they are freshly carved, it is easy for us to see the effects of weathering and erosion including things like discolouration of the rock, the softening of sharp edges, and changes in easily recognizable features like faces and hands. As weathering gives way to erosion, the lost parts of the rock are carried away by agents of erosion -- wind, water, ice, and gravity.
Weathering is a very slow process but the rate of weathering of an individual rock can, and does, vary, depending on how the rock was formed and what environmental conditions it is exposed to. The chemical composition of the rock, how hard the rock is, and how porous the rock is all impact on the rate of weathering of individual rock types in the same environment, and even to different parts of a single rock. This variation in weathering rate is known as differential weathering.
Weathering also preferentially attacks edges and corners which is why these are often the areas where weathering is most noticeable - this is especially true on carved stones. This is known as preferential weathering.
Types of Weathering
There are three types of weathering:
Physical weathering is caused by changing temperatures and causes rocks to fracture. It is most common in mountain regions or deserts. One example is when a crack in the rock fills with water. When it freezes, it expands and acts like a wedge widening the crack. As the temperature rises, the ice melts and the enlarged spaces left behind fill with water to continue the process.
Chemical weathering is caused by a chemical reaction with the mineral grains in the rock. One example of chemical weathering is a rock that blackens with pollution or reddens with oxidation. Rain, air pollution, oxidation are all examples of chemical weathering.
Biological weathering is caused by living organisms. Examples include plant roots growing into and expanding cracks in rocks. Algae and lichens that live on the rocks and produce chemicals that help break down rocks. Biological weathering may also be caused by animals burrowing into the rock.
Erosion
Erosion is the process by which soil and rock particles are worn away and moved elsewhere by gravity, or by a moving transport agent – wind, water, or ice. The evidence that erosion has taken place is often the absence of rock where there should be rock.
Whereas weathering is the breakdown of rock into fragments, erosion involves the removal of solid material by a transporting agent. Rock may be eroded in several ways:
Water: moving water is a significant agent of erosion. Whether it is water in rivers or streams, rain, flooding, or waves all contribute to erosion by water. Fast-flowing streams and rivers or heavy flooding are capable of carrying away large pieces of rock and soil; however, slower streams or light rain can only transport smaller fragments. Erosion by water is responsible for the changes in coastlines as well as the transport of light topsoil from farmers’ fields.
Wind carries dust, sand, volcanic ash, and finer particles of rock from one place to another. Wind is the primary agent responsible for sand dunes. In drier areas, wind can act like a sandblaster of considerable force. It can wear away softer rock which is then also transported via wind or another agent of erosion. It can also polish rocks until they appear smooth.
In colder climates, Ice plays a considerable role in the erosion of the landscape. As glaciers move downhill and across the landscape, they pick up rocks along their route; these rocks may be as small as tiny grains of sand, or as large as huge boulders. The rocks carried by glacier scrape against the ground and erode both the ground below and the rocks in the glacier. Eventually the rocks carried by the glacier are deposited elsewhere, sometimes hundreds of kilometres from their place of origin. Material deposited by a glacier is called moraine.
Gravity is the final agent of erosion. It works by pulling rock and soil downward. Gravity may work together with another agent of erosion, for example a glacier being pulled down hill slowly, thereby causing erosion by gravity indirectly. Gravity also works directly by pulling soil or rock down hill, such as with rock slides/landslides. Slump (sometimes called rotational slide)is the movement of rock debris a short distance downward and occurs when the base of the mountain has eroded away, thus offering no support for the material above. Creep is the slow movement of rock debris that happens on all slopes covered with loose, weathered material.
This lion, Lopez, was commissioned for the five junctions area and was created by local sculptor Joyce Campbell.
As Edinburgh is a common tourist city and there are people from all over the world caching here, if you're not a native English-speaker, and you feel more comfortable writing in your own language, please know that I will accept answers in any language so long as I am able to translate them online. If you're having any difficulty, just send me a message; I'll be glad to help!