A puzzling pile of stone
You should be standing in front of ‘a puzzling pile of stone’, one of the remnants of the Secret City of the Royal Arsenal. This is a steam-engine base from the early 1840s, which was excavated from the Dial Arch workshops nearby. The first steam engine was installed in the Royal Arsenal in 1805, and it is likely this base was from the steam-powered boring-mill introduced in 1842.
Your tasks are to identify the type of rock this is and consider the impact of its use on the way it looks now (weathering).
Steam-power
Static steam engines were transformational during the Industrial Revolution. They produced far more power than had previously been available. Steam engines use a boiler to boil water and produce steam. The pressure of the steam exerts a force on a piston, which can be used to drive machinery. A variety of heat sources can be used to boil water, including wood, coal and oil. These blocks of stone provided a stable base to support the heavy, moving machinery. Steam engines were used at the Royal Arsenal to power machine tools, which greatly increased manufacturing efficiency and productivity.
Types of rock
There are three types of rock: sedimentary, igneous and metamorphic. An example of each of these is described below. The rock you are looking at is one of these.
Sedimentary – Sandstone
Sedimentary rocks are formed on the surface of the Earth by layers of sediment pressing together. In the case of sandstone, the sediment is sand. Layers are often visible in sandstone, and the grains are usually very small (less than 2 mm).
Igneous – Granite
Igneous rocks are formed by molten rock which has cooled and hardened. Sometimes this happens under the Earth’s surface, and other times when molten rock comes to the surface (e.g. in a volcanic eruption). Granite is formed under the Earth’s surface and generally has large, visible, interlocking crystals which form as it cools slowly.
Metamorphic – Marble
Metamorphic rocks are formed when existing rocks are subjected to extreme heat or pressure. This happens deep below the Earth’s surface. These rocks are very hard and often have visible crystals. Marble is formed when limestone (a sedimentary rock) melts and recrystallises.
Weathering
Just as rocks form, they can also degrade. There are several ways that rocks can erode or weather. These are described simply below. All weathering can be categorised as either mechanical (sometimes called physical) and chemical. Different types of rock are more or less susceptible to each of these types of weathering, and their use and location can also impact on the degradational forces acting on them.
Mechanical
Forms of mechanical weathering include the action of rain, flowing water and wind, which can wear away the rock over time. Rocks can also be weathered by movement in the landscape, such as landslides and debris flow on steep slopes. Temperature changes can cause weathering as the rock expands and contracts with heat and cooling, causing it to weaken and fracture. Weathering can also be caused by direct force applied to the surface of the rock.
Chemical
Chemical weathering takes place when water, oxygen, carbon dioxide, and other chemical substances react with rock. These include chemical reactions when water dissolves chemicals within the rock (dissolution). This can be more pronounced if there is carbon dioxide present. Sulphur dioxide, present in fossil fuels, causes water to become much more acidic and can have an even greater effect on dissolution. Oxidation acts on rock that have a high iron content, causing the rock to become weak and crumbly. Biological weathering occurs when plants live on the rock and excrete acidic substances through their roots, which cause chemical changes to the rock.
Your Tasks
Stand with your back to the building facing the river, with the blocks in front of you.
- Look at the lowest course of blocks, sitting on the ground. How many blocks across is the steam-engine base?
Let the blocks be Block 1 – Block X from left to right.
- Look at the column above Block 3. Describe the blocks here in terms of texture and colour. What type of rock do you think this is?
- Using the numbering described above, which two blocks have the most significant damage?
- There are two types of weathering most evident here. What are they, and how do you think they were caused?
(Optional) If you wish to take a photograph at GZ, please take it from a different side of the steam-engine base (i.e. not the side nearest the building).