Rocks and Soils

There are five factors for soil formation – parent material, time, climate, biota and relief/topography.
Among these factors, climate and biota are called the active agents because they act on the rest of the three factors, which are called ‘passive agents’ for the formation of soil.

Rocks are made up of various minerals. Out of about 2000 different minerals, only 12 minerals are common all over the world. These 12 rock forming minerals are called 'rock formers'.

Igneous rocks are also called ‘primary rocks’ because they were the first ones to be formed.
‘Extrusive igneous rock’ is the name given to magma erupting and solidifying on reaching the earth’s surface. As magma cools down rapidly on reaching the earth’s surface, so these rocks are very fine-grained.
Basalt is a typical example of extrusive igneous rocks.

‘Soil profile’ is a vertical section of the earth's crust showing the different horizons or layers.
Soil profile consists of various layers of soil; horizons A, B, and C. ‘Horizon A’ refers to the upper layer of soil nearest to the surface.
The layer below horizon A, of course, has to be horizon B. Litter is not present in horizon B and therefore, there is much less humus.
Horizon C must be below horizon B. Horizon C consists mostly of weathered big rocks.
It must be understood that ‘soil profile’ does not include the ‘bedrock’, which is non-weathered.

Grey desert soil is also called sierozem and it is the typical soil of the mid-latitude desert regions. Due to lack of the natural vegetation in these regions of dry climate, the grey desert soil is phenomenally poor in humus.
Red desert soil is a characteristic of still more arid regions, and their colour ranges from pale reddish grey to deep red. The humus content reaches its lowest possible limits and the role of plants as soil formers is also reduced to the minimum.
Chernozem soil is the most widespread among all zonal soils of the semi-arid climate. It is rich in humus.
Chestnut soil lies on the arid side of the chernozem soils. It is lighter in colour and contains less amount of humus.

Soil organisms are the creatures that spend all or part of their lives in the soil. There are several criteria that can be used to classify soil organisms. For example, soil organisms are classified according to their size into:
(1) macro-organisms (> 2 mm in width),
(2) meso-organisms (0.2 to 2 mm in width), and
(3) micro-organisms (< 0.2 mm in width)
Arthropods belong to the meso group. They are a diverse group of spiders, mites, pseudo-scorpions and insects. Many arthropods prey on disease causing pests. Others help to shape soil structure and thus, can improve root development, water infiltration, drainage and help in aerating the soil.

Loam is soil composed of sand, silt and clay in relatively even concentration (about 40-40-20% concentration respectively). It is considered ideal for gardening and agricultural uses. Loam soils generally contain more nutrients and humus than sandy soils, which have better infiltration and drainage than silty soils and are easier to till than clay soils.

No doubt, rocks help to know the history of the earth through carbon dating method, but that is not a geographer’s area of study. Rather, geologists are interested and involved in exploring the history of the earth.
Geographers are concerned with the study of elements and minerals that the rocks are made up of as these rocks, after disintegration and erosion, leads to formation of soils, which are important for geographers from agricultural point of view.
Geographers are also concerned with the presence of various resources in association with the rocks.

The first three are sedimentary soils; quartzite is a metamorphic rock.

Availability of various rock forming elements (in percent) in earth’s crust (in % mass) is
Oxygen – 46.6
Silicon – 27.7
Aluminium – 8.1
Iron – 5.0

Statement (1) is correct. Igneous rocks are the first one to be formed. All other rocks are formed by subsequent work of various rock forming processes and factors over these rocks. Thus, igneous rocks are also called parent rocks.
Statement (2) is correct. Unlike sedimentary rocks, igneous rocks do not have any strata.
Statement (4) is also correct. In the absence of pores in the igneous rocks, water does not penetrate in these rocks easily. Hence, igneous rocks are less affected by chemical weathering.
Statement (3) is wrong. Sedimentary rocks are not the most widely distributed rocks on the earth’s surface.

Igneous rocks are divided into two main categories: plutonic rock and volcanic rock. Plutonic or intrusive rocks result when magma cools and crystallises slowly within the earth's crust. A common example of this type is granite. Volcanic or extrusive rocks result from magma reaching the surface either as lava or fragmental ejecta forming minerals such as pumice or basalt.

Soil texture determines the water retaining capacity of the soil. The larger the pore spaces in the soil, the lesser is its water retaining capacity and vice-versa.

The scientific study of rocks is called petrology; pedology is the scientific study of soils.

Weathering and erosion are geological processes that act together to shape the surface of the earth. In simple terms, weathering is a set of processes that break solid rocks at the place of their location into fragments. Erosion, on the other hand, is a group of processes that involve running water, blowing wind or moving ice that pick up and move these eroded fragments to a different location. Thus, weathering is in situ, while erosion is ex situ.