In the Earth’s surface, rocks extend up to as much as 20 km depth.
Types of Rock
The major rock types are categorized as igneous, sedimentary, and metamorphic.
- Igneous rocks: formed from crystalline bodies of cooled magma.
- Sedimentary rocks: formed from layers of cemented sediments.
- Metamorphic rocks: formed by the alteration of existing rocks due to heat from igneous intrusions or pressure due to crustal movement.
Soils are formed from materials that have resulted from the disintegration of rocks by various processes of physical and chemical weathering. The nature and structure of a given soil depends on the processes and conditions that formed it:
- Breakdown of parent rock: weathering, decomposition, erosion.
- Transportation to site of final deposition: gravity, flowing water, ice, wind.
- Environment of final deposition: flood plain, river terrace, glacial moraine, lacustrine or marine.
- Subsequent conditions of loading and drainage: little or no surcharge, heavy surcharge due to ice or overlying deposits, change from saline to freshwater, leaching, contamination.
All soils originate, directly or indirectly, from different rock types.
Physical Weathering
Physical weathering reduces the size of the parent rock material, without any change in the original composition of the parent rock. Physical or mechanical processes taking place on the earth’s surface include the actions of water, frost, temperature changes, wind and ice. They cause disintegration and the products are mainly coarse soils.
The main processes involved are exfoliation, unloading, erosion, freezing, and thawing. The principal cause is climatic change. In exfoliation, the outer shell separates from the main rock. Heavy rain and wind cause erosion of the rock surface. Adverse temperarture changes produce fragments due to different thermal coefficients of rock minerals. The effect is more for freeze-thaw cycles.
Chemical Weathering
Chemical weathering not only breaks up the material into smaller particles but alters the nature of the original parent rock itself. The main processes responsible are hydration, oxidation, and carbonation. New compounds are formed due to the chemical alterations.
Rain water that comes in contact with the rock surface reacts to form hydrated oxides, carbonates and sulphates. If there is a volume increase, the disintegration continues. Due to leaching, water-soluble materials are washed away and rocks lose their cementing properties.
Chemical weathering occurs in wet and warm conditions and consists of degradation by decomposition and/or alteration. The results of chemical weathering are generally fine soils with altered mineral grains.
The effects of weathering and transportation mainly determine the basic nature of the soil (size, shape, composition and distribution of the particles).
The environment into which deposition takes place, and the subsequent geological events that take place there, determine the state of the soil (density, moisture content) and the structure or fabric of the soil (bedding, stratification, occurrence of joints or fissures)
Transportation agencies can be combinations of gravity, flowing water or air, and moving ice. In water or air, the grains become sub-rounded or rounded, and the grain sizes get sorted so as to form poorly-graded deposits. In moving ice, grinding and crushing occur, size distribution becomes wider forming well-graded deposits.
In running water, soil can be transported in the form of suspended particles, or by rolling and sliding along the bottom. Coarser particles settle when a decrease in velocity occurs, whereas finer particles are deposited further downstream. In still water, horizontal layers of successive sediments are formed, which may change with time, even seasonally or daily.
Wind can erode, transport and deposit fine-grained soils. Wind-blown soil is generally uniformly-graded.
A glacier moves slowly but scours the bedrock surface over which it passes.
Gravity transports materials along slopes without causing much alteration.