The breaking down of rocks, or weathering, is defined as the process by which larger rocks and minerals are reduced to sediment and soil via natural processes, such as chemical reactions and atmospheric processes. Weathering is a distinct process from erosion, as weathering occurs without any significant movement of the rock or mineral in question. Erosion, on the other hand, involves movement of the rock by flowing media such as water or wind.
Weathering is an important process in the rock cycle as it breaks down larger rocks and minerals into soil, and also provides for the sediments that will eventually become sedimentary rocks under the right conditions. In this Earthcache, we will be looking at one particular type of rock weathering, known as chemical weathering.
Chemical weathering occurs when the mineral and compounds in the rock react with substances from the surrounding environment to form new minerals. In most cases of chemical weathering, the minerals in the rock will either dissolve or react with rain water (which is slightly acidic due to dissolution of carbon dioxide from the atmosphere). When rocks undergo chemical weathering, their chemical composition will change significantly, with the particular elements being enriched in the rock in the process, while other elements will be significantly reduced.
The chief processes in chemical weathering are dissolution, where particular compounds will dissolve in water and be carried away, and hydrolysis, where these compounds react with the rain water instead. There are other processes such as carbonation, where the compounds in the rock react with carbonic acid, an acid which is a result of rain water reacting with carbon dioxide in the air. The reactions that take place will form new compounds (and in turn, creating a chemical composition change in the rock) which will alter the chemical composition of the rock. These new compounds form what is known as the secondary minerals of the rock.
Naturally, in a rock, the compounds which tend to react and/or dissolve in water will be weathered first, as they tend to be washed away by rain water. The compounds which will be left behind in the rock are the compounds which are less reactive. This process will lead to a further alteration of the chemical composition of the rock, and with it, a significant change in the physical properties of the rock (as different compounds will exhibit different physical properties). Some of these compounds may be more brittle than the original rock, weakening it in the process, while others, may actually strengthen the rock.
Now that the basics of chemical weathering have been covered, let us look at the predominant form of chemical weathering that occurs in tropical weather conditions.
The tropical climate is marked by alternate periods of heavy rainfall and dry spells which causes rocks to weather in a very particular manner. This process, known as laterisation, takes on the name of the soil it will produce, that is, laterite. We will now look into the process of laterisation proper.
Fig.1: Cross section of soil showing where laterite forms in soil
Laterite is a broad term used to describe the product of intensive chemical weathering under hot and humid tropical conditions. Laterite are formed from a variety of rocks (which span all three families of rocks; igneous, sedimentary and metamorphic), and these rocks are known as the parent rock. Over time, the parent rock, which is exposed to the harsh tropical climate, will begin to undergo intense and prolong weathering caused by the alternate heat and rainfall which will begin to alter and remove certain minerals from the rock, while leaving others behind. This process, where certain minerals are removed through rainwater seeping through the rock is known as leaching.
Potassium, calcium and sodium minerals: The first minerals that will begin to react with the acidic rainwater are the minerals containing sodium, potassium and calcium, which react fairly easily with water and acids. During the rainy season, the reaction intensifies due to the large volume of water seeping through the rock. In the dry season, the water within the rock will begin to move to the surface of the rock and evaporate, leaving the easily soluble sodium, potassium and calcium salts behind, which will be quickly washed away during the next rainy season.
Silicon: The next minerals to react are the silicon-based minerals. These compounds are usually quite stable and do not dissolve in water, but the reaction of the sodium, potassium and calcium minerals can significantly alter the pH of the water in the rock due to the bases that these minerals can form. These will begin to convert the normally stable silicon dioxide (SiO2) to silicate compounds (SiO44-), which are susceptible to hydrolysis by rainwater, and, eventually, wash away, causing a depletion of the silicon minerals, like quartz. This process is, however, is slow, and hence, plenty of quartz will tend to remain in the original rock.
Another very important reaction of silicon leads to its reaction with water and aluminium, leading to the formation of kaolinite, (Al2Si2 O5(OH)4), a very important mineral with an interesting property that will be addressed in one of the tasks.
Fig. 2: A sample of kaolinite
What’s left: After the minerals that are more reactive elements have been weathered, the rock is left with a high proportion of insoluble minerals. Minerals like bauxite (aluminium ore), hematite (iron ore) and nickel, zirconium, tin and titanium ores become dominant within the rock, changing its physical properties. The richness of particular elements in the original rock will determine which of the above minerals (and therefore, physical properties) will dominate the remaining rock. One of the tasks of this Earthcache is concerned with this aspect of the rock.
Laterite derives its name from the word later meaning "brick" as it was first described by geologists in South Asia, where the stone is used in the construction of buildings and temples. Despite its humble appearance, laterite stones have a dual quality that makes its extremely attractive to masons. It is easy to cut and shape when first removed from the ground, but becomes stronger when exposed to sunlight and air. (does this remind you of something?). A great testament to this remarkable property of laterite is the fact that many of the temples of the Angkorian civilisation were built almost primarily out of laterite. The great Angkor Wat itself has a foundation of laterite, but tiled with sandstone, as it is easier to carve intricate reliefs on sandstone.
Laterite soil is also rich in particular minerals, and indeed, it is the soil which yields most of the Earth's iron, nickel and aluminium ore. It also provides the topsoil for most of the Earth's rainforest, and the preferred soil of many commercial tropical crops, such as rubber and oil palm.
Tasks for this Earthcache
At the published coordinates, you will see several instances of laterite boulders. Inspect the rocks to answer the following questions:
- What is the dominant colour of the laterite rocks? The colour should be familiar, one commonly associated with metal objects. Why is it of this colour and what can be said about the composition of the parent rock?
- Scratch the rock with your fingernails, followed by a coin and, if you have one, a sharp object, like a knife. Are any of the objects capable of scratching the rock? Is the rock hard or is it brittle (forms flakes and crumbles easily)? Explain your observation. [Hint: it has something to do with kaolinite, and its physical properties when left to dry in the sun]
- Are the rocks smooth or porous (full of holes)? Based on the information provided above, suggest a reason for this
Please email me the answers to the above tasks. Photologs as sole proof to claim a find will not be accepted. Have fun Earthcaching.