URBAN EARTH - All Rocks Are Not Created Equal EarthCache

Introduction

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

On one hand weathering can be described as a slow and unremarkable process, but on the other hand it's also one of the most destructive forces on planet Earth - and has been going on as long as weather has existed.

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

At the published coordinates you'll be standing at the foot of a carved sandstone gateway arch which has stood at this spot for a little over 100 years. The sandstone used for the statues in the front and rear of the arch differs from the sandstone used for the arch itself.

Logging Tasks

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Based on your observations at the EarthCache location and the information on this page you should be able to tell me:

1. Describe any weathering on the sandstone the body of the arch is made from - what type or types of weathering do you see and what do they look like?
   
   
2. Describe any weathering on the statues - what type or types of weathering do you see and what do they look like? REMEMBER - there are four statues.
   
   
3. Compare the sandstone of the arch with the sandstone used for the statues - which is weathered more and which property of the two types of sandstone could cause that?
   
   
4. Now compare the three statues on the east-facing aspect of the arch with the one statue on the west-facing aspect. Which statue or statues are most weathered?
   
   
5. Take a look around and consider the location and environment the arch is in. Describe two environmental factors that impact on the amount of weathering seen on the statues on either side of the arch.
   
   
6. 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

I've admired this arch and the statues on it for many years now and I've always assumed, because the statues are white in colour, that they were carved from limestone or possibly marble.

The statues though have clearly suffered badly as a result of their exposure to the elements, to a degree not expected in marble for statues of this age. And it actually turns out that they aren't limestone either but are in fact carved from white sandstone which is, apparently, quite rare.

The arch itself, being carved from a different sandstone, has fared better than the statues but is still impacted by the effects of weathering.

The whole structure was given Grade II listed building status in February 1984 and described as GATEWAY TO ROMAN CATHOLIC CHURCH OF ST MARY. Sadly that status does not seem to have encouraged any investment in the restoration or protection of the structure, which is badly needed if it is to last another hundred years.

Weathering

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.

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) has 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 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.

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 including things like discolouration of the rock, the softening of sharp edges and easily recognisable features like faces and hands, and the flaking and fracturing of rock surfaces.

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 - which can vary within the body of the rock
  
  
• The hardness of the rock - significantly influenced by the strength of the mineral cement holding together the sedimentary grains which make up 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.

The examples of weathering 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.