URBAN EARTH - Weathering or Erosion? EarthCache

Introduction

Rocks don't last forever. Eventually processes which geologists call weathering and erosion will dissolve and disintegrate even the mightiest of mountains.

Evidence of weathering and erosion can be found in most environments where rocks are present. The impacts of weathering and erosion you will see in the process of completing this EarthCache are on rocks found in an urban environment.

At the published coordinates you will find yourself in front of an art gallery, built in the Neoclassical style, which has been on this site for over 140 years. At waypoint R1 you'll be standing at the front of a former courthouse. Both buildings are Grade II listed.

Logging Tasks

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The two statues at the published coordinates are made of Carrara marble. In the fabric of the building you will see two colours of sandstone, an unidentified Triassic age red sandstone below and the pale brown Darley Dale sandstone above. The balustrade is also sandstone. From your observations, the information on this page and possibly a small amount of research you should be able to tell me:

1. What evidence of weathering can you see on the statues and which type or types of weathering do you think are in effect here? It's worth remembering that when freshly sculpted this rock is bright white.
   
   
2. What evidence of erosion can you see on the statues? Are there any parts which are obviously missing? Is there any eroded material left behind or have the agents of erosion carried any eroded material completely away?
   
   
3. Which type of weathering is most obvious on the balustrade - especially on the uppermost surfaces of the bannister rail?
   
   
4. Both the red and pale brown sandstone of the main building display various signs of both weathering and erosion. Please describe some of them and tell me which are signs of weathering and which are signs of erosion. Consider properties such as changes in colouration, surface textures, edges and corners.
   
   

Next, make your way to waypoint R1 a short distance away where you will see more balustrading. The balustrade is made up of three main parts; the bottom rail along the bottom (unsurprisingly), the bannister rail along the top and the ornately turned vertical cylindrical columns between the two which are known as balusters.

5. Some of the balusters and some of the other stonework in the balustrade have been replaced fairly recently (bearing in mind that the courthouse was originally built over 130 years ago). Comparing the new stonework with the old, would you say that the impact of weathering / erosion has been minimal or would you say that it's been extensive?
   
   
6. Which balustrading has suffered most from weathering and erosion - that in front of the former courthouse or that in front of the art gallery?
   
   
7. Optional task: feel free to add any photographs of your visit that do not show the specific features from the logging tasks - no spoilers please. In the interests of allowing everyone to experience the EarthCache fully for themselves obvious spoiler photographs will be deleted.
   
   

Background

The terms weathering and erosion are often used interchangeably but they are actually different processes.

Weathering is a collection of physical and chemical processes that pulverize rocks over time. Weathering makes rocks softer and weaker and more easily eroded. It can involve the physical disintegration and chemical decomposition of rocks into soil, loose clasts (rock fragments), dissolved chemical components (ions), and solid chemical residues. In other words, weathering is so powerful that it can break rocks down into their fundamental molecular structures.

Rocks are most stable in the conditions in which they were formed. When rocks are moved to a new location where conditions differ, weathering will break down the rocks into substances which are stable in those new conditions.

Weathering is an in-place process, breaking down rocks into substances which are more easily transported but not actually transporting those substances anywhere. When those products of weathering are transported to a different location then erosion has taken place.

Transport refers to the processes by which the products of weathering are moved along – for example, pebbles rolled along a river-bed or sea shore, sand grains whipped up by the wind, salts carried in solution or just loose material sliding downhill under the force of gravity.

Softening The Edges

Rocks found in nature come in all sorts of shapes and sizes making it more difficult, for us non-experts at least, to tell whether weathering (if you'll pardon the pun) or erosion have affected the rock and to what degree. Rock carved by man though tends to be more uniform and regular with definite identifiable shapes, edges or patterns - especially when freshly carved - and this makes it easier for us to see the impact of weathering and erosion over time as changes in the appearance and structure of the rock.

Architectural stonework, stonework used in buildings, tends to have certain well defined features like flat surfaces and sharp corners, or when the stonework is rounded, it tends to be in uniform ways - cylindrical columns for example. Statues, another type of man-made rock carving also tend to follow easily recognisable forms like people and animals. And then there's architectural statues, which are statue forms carved into the architectural stonework of buildings. All three of these types are available to see around the locations which make up this EarthCache, if you take a little time to locate them .

Because we can expect these types of stonework to look a certain way when they are freshly carved, it's easy for us to see the effects of weathering and erosion including things like discolouration of the rock, the softening of sharp edges and easily recognisable features like faces and hands, flaking and fracturing of rock surfaces and, as weathering gives way to erosion, the loss of parts of the rocks themselves, carried away by one or more of the agents of transport - gravity, wind, water and ice.

All Rocks Are Not Created Equal

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 later exposed to.

The way the rock is formed will affect things like:

• The chemical composition of the rock
  
  
• The hardness of the rock
  
  
• The porosity of the rock (the amount of space between the particles or grains which make up the rock)
  

These characteristics in turn impact on the rate of weathering of individual rock types exposed to the same environment, and even to different parts of a single rock which has a mixture of harder and softer parts. 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 noticable - especially on carved stones. This aspect of weathering is known as preferential weathering.

Types of Weathering

There are three types of weathering:

• 
  Physical - caused by the effects on the rocks of changing temperature. It may be that the rock itself expands each time the temperature rises and then contracts again when the temperature falls. It may also be that cracks or spaces in the rock fill with water which freezes, expands and acts like a wedge, widening the spaces or cracks. When the temperature rises the ice melts, the enlarged spaces left behind fill up with even more water and the process repeats. Either way, the forces arising from this repeated expansion and contraction cause the rock to gradually break apart. Rock which has been subjected to physical weathering will typically be fractured or might have flaking layers.
  
  
  
  
• 
  Chemical -- caused by rainwater reacting with the minerals the rocks are made of. The rain absorbs carbon dioxide and other pollutants found in the atmosphere as a result of burning fossil fuels such as coal, oil and gas. Absorbing these substances turns the rainwater into a weak carbonic acid which then dissolves the rocks it comes into contact with. Rocks made from carbonate minerals, chalk, limestone and marble for example, are particularly susceptible to chemical weathering. Marble is the material of choice for statues because of its smooth, white surface which makes the effects of chemical weathering easy to spot as the surfaces become discoloured and disfigured.
  
  
  
  
• 
  Biological - living organisms contribute toward the weathering process. Examples include plant roots growing into and expanding cracks in rocks, and the slow impact of algae and lichens which extract vital minerals from the rock by releasing chemicals which weaken and then break down the exposed surfaces of the rock. Biological weathering may also be caused by animals burrowing into the rock and even, although to a much lesser degree, by human footsteps passing repeatedly over rock surfaces.
  
  

Weathered rock may show any or all of these types of weathering, in any combination and to varying degrees.

Erosion

In a funny way, measuring the impact of erosion is sometimes more about what you cannot see than about what you can see. Weathering breaks down rock and alters its structure and appearance, but the altered rock doesn't go anywhere - it's still all there and the impact of that weathering remains in place to be seen - discolouration for example arising from polluting chemicals in the atmosphere. Erosion on the other hand reduces the amount of rock that's there or, more accurately, some of the rock that was there gets taken away, transported and deposited elsewhere by the agents of erosion - wind, water, ice and gravity. The evidence left behind that erosion has taken place is often the absence of rock i.e. the fact that some of the rock that was there originally isn't there any longer.


Looking at rock which has been impacted by erosion, we could be forgiven for wondering where the missing rock has gone - surely the remnants of the eroded rock should still be visible, perhaps accumulated in piles below the rock they have been eroded from? The products of erosion remaining to be seen though depends largely on the type and degree of weathering that took place before erosion took over and how effective the agents of transport have been. Frost wedging for example can lead to the erosion of large chunks of rock, too large and heavy to be easily carried away by wind or water and these do indeed pile up pretty much where they fall (or roll to if on a slope). Where the weathered and subsequently eroded products are on a much smaller scale though it can be that little or nothing of the eroded material remains for long.

The examples of weathering and erosion that you need to investigate in order to provide good logging task responses are obvious and easy to locate and a visit during daylight hours on a dry day should contribute significantly to a successful outcome

Please submit your logging task responses before posting your log.