Questions Based on Lecture 6
1. Describe the process of top soil formation
Regolith is a layer of loose (un-lithified) soil and rock debris that overlies and blankets bed
rock. Regolith is formed by physical and chemical weathering of bedrock. Regolith consists
of mainly primary minerals. Further weathering of the top part regolith leads to formation of
clay minerals, sand and metal oxides and hydroxides. This is the sub-soil, which is a mixture
of primary and mostly secondary minerals. The top part of the sub soil is enriched by
organic matter for dead and decaying plants and animals (humus). The mixture of humus
and organic matter is the top soil.
2. Describe various layers (horizons) formed in soil
O and A Horizon…..Topsoil
Only a few meters deep; provides the richest amount of organic matter for germinating
seeds since plants and animals decay on the ground level. As decomposition occurs in the
topsoil, microorganisms, from bacteria to fungi, populate the soil to keep the decaying
process active.
E and B Horizon…..Subsoil
Subsoil does not have high organic matter concentrations but offers rich minerals for root
systems. Minerals from the topsoil find their way down into the subsoil through water
runoff; consists of mainly secondary minerals with some primary minerals.
C Horizon: Regolith. Mainly primary minerals
R Horizon: Bedrock
3. What are the main inorganic components of soil and discuss the composition of these
components
Main inorganic components of soil are sand, silt and clay. Sand is the secondary mineral
derived from the primary mineral quartzite. Sand size varies between 50 µm – 1000 µm.
Silt is a mixture of fine sand and feldspar (primary) minerals. Silt size varies from 2 µm – 50
µm. Clay consists of secondary alumino-silicate (clay) minerals and metal oxide/hydroxide
solids. Size of clay is < 2 µm.
4. Describe subsoil and topsoil and point out the difference between them
Sub-soil, mainly consist of sand, silt and clay, which are secondary minerals derived from
weathering of rocks. The subsoil is rich in minerals, but poor in nutrients. The top part of
the sub soil is enriched by organic matter (humus) from dead and decaying plants and
animals (humus). The mixture of humus and sand, silt and clay is the top soil. Top soil is
rich in both minerals and nutrients and hence suitable for growth of plants.
5. Sediment transport is one of the main functions of a river. Discuss this statement and
point out the importance of sediment transport.
Weathered materials like subsoil and top soil being relatively small in size can be carried
easily by flowing water. Thus soil produced on mountains due to weathering is transported
by rivers to the plains. This sediment transport from the hills to the plains is one of the most
important functions of a river. For example, the soil found in the indo-gangetic plain is not
native soil, but it is soil eroded from the himal
himalaya that has been deposited to form the indo-
indo
gangetic plains.
6. Answer
nswer the questions below on soil classification system based on texture
a. b.
Using the label and scales in the precious
Label the axes and draw the scale of the problem determine the approximate
triangular soil classification system on the percentages of sand, silt and clay in the three
triangle above samples shown in the figure
7. Clearly distinguish between geological and geo geo-morphological
morphological features of the earth’s
landscape.
Topography, i.e., elevations of the earth at various locations, is shaped by, 1) tectonic forces,
originating below earth’s surface and 2) agents of erosion including water, ice, wind and
tectonic forces are called Geological Features,
gravity. Topographical features caused by tecton Features
while topographical features caused by erosion forces are called Geo-morphological
Geo
Features. The tectonic forces generally increase elevation differences, while erosion
erosi tends
to equalize elevation differences.
8. Describe the rock cycle using a suitable diagram
Igneous rock is formed due to cooling of
magma.
ma. Igneous rock can get transformed
transf into
sedimentary rock. Both
th igneous and
sedimentary rock can get transformed into int
metamorphic rock. All types of rock undergo
weathering to form soils / sediments. Soils /
sediments can be transformed to t sedimentary
rock. All three types of rocks can undergo
subduction and get re-transformed
transformed into magma.
It is a cycle. Rock is created, as well as
destroyed by geological and geo-morphological
geo
processes.
