Download 1. Origin of Soils and Rocks (Das, Chapter 2) - Icivil-Hu

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1. Origin of Soils and Rocks
(Das, Chapter 2)
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Introduction
• The mineral grains that form the solid phase of a
soil aggregate are the product of rock
weathering.
• The physical properties of soil are dictated by the
size, shape, and chemical composition of the
grains, and hence the rock from which is
derived.
• Rocks are compact, semi-hard to hard mass
composed of one or several minerals.
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On the basis of their mode of origin, rocks can be
divided into three basic types:
•
•
•
Igneous rocks
Sedimentary rocks
Metamorphic rocks
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1. Igneous Rocks
Formed by solidification of molten Magma ejected
from deep within earth’s mantle.
Magma has originated well below the surface and
ascended towards the surface, and has crystallized as
solid rock either on the surface or deep within the
earth's crust as temperature fell.
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2. Sedimentary Rocks
Sedimentary rocks are widely spread over the surface of earth.
They constitute about 75% of the area of the continents.
Development
 Weathering
reduces the exposed rock mass to fragmented
particles which can be more easily transported more easily by
wind, water, and ice.
 When dropped by the agents of transportation, they are called
SEDIMENTS.
 Sediments
are typically deposited in layers or beds termed
STRATA.
 When
compacted and cemented together (a process called
LITHIFICATION), they formed sedimentary rocks.
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 The
process through which sediments are converted into
sedimentary rocks is called DIAGENSIS. It includes the
following phases:
1. Cementation
Water percolating through the voids (or pores) between the
particles of sediment carries mineral matter which coats the grain
and acts as cement that bind them together.
2. Compaction
The weight of top layers compacts sediments and expels water
out.
3. Crystallization
Sometimes grains of sediments are joined together due to
crystallization of some of their constituents due to pressure.
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3. Metamorphic Rocks
 Metamorphic rocks are formed if a rock is subjected to
increase in temperature, pressure, or both, to such degree
that a new TEXTURE or possibly a new MINERAL
composition is produced.
 The
process of
METAMORPHISM.
change
is
referred
to
as
 The
original rock may be igneous, sedimentary or
metamorphic.
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Rock Cycle
The formation cycle of
different types of rock
and the processes
associated with them.
This is called the rock
cycle.
Sedimentary
Rock
Metamorphic
Rock
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Igneous
Rock
The rock cycle is
important in any
discussion of soil
formation,
especially if we
know that soils are
nothing but
disintegrated
rocks.
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2. FORMATION OF SOILS
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Soil: some definitions
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Soils are formed from rock, loose unconsolidated
materials (may be transported), or organic
residues.
•
The word ‘soil’ means different things to different
people but basically it may be defined as the solid
material on the earth’s surface that results from
the interaction of weathering and biological
activity on the soil parent material or underlying
hard rock.
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Weathering
This refers to the breakdown and decomposition of •
rocks, soils and minerals into smaller pieces
through contact with the atmosphere, biota and
waters.
Weathering include comprise of mechanical and •
chemical weathering
Weathering
I. Mechanical Weathering:
It is the physical disintegration of the original rock mass
into smaller particles without any change in the
chemical composition.
Mechanical Weathering Processes:
1.
2.
3.
4.
Unloading or pressure release (e.g. uplift, erosion)
Thermal expansion and contraction (thermal stresses)
Frost action
Biological and Organic effects (e.g. the growth of plant
roots)
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Weathering
Photos of Mechanical Weathering:
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II. Chemical Weathering
1. It involves the direct effect of atmospheric chemicals
or biologically produced chemicals (also known as
biological weathering) in the breakdown of rocks,
soils and minerals
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Rates of Weathering of Rocks

Fine-grained rocks weather more slowly than
coarse-grained rocks of the same mineral
composition.

Igneous rocks (excluding certain volcanic rocks
that weather rapidly) and quartzite are the most
resistant.
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Parent Rock
Residual soils
~ in situ weathering (by
physical & chemical
agents) of parent rock
Transported soils
~ weathered and
transported far
away
by wind, water, ice, gravity.
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B. Transported Soils
These are soils which were formed from rock
weathering at one site and are now found at another
site. The transporting agent may be:
1.Water (Principal transporting agent)
2.Glaciers
3.Wind
4.Gravity
Transported soils are very important in
engineering because nearly all major cities
are located, at least in part, on flood plains,
deltas, and coastal plains.
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The transported soils may be classified into several
groups, depending on their mode of transportation and
deposition:
1. Glacial soils
—
deposition of glaciers
2.
by
transportation
and
Alluvial soils
—transported by running water and
deposited along streams
3. Lacustrine
4. Marine
soils —formed by deposition in quiet lakes
soils —formed by deposition in the seas
5. Aeolian
6.
formed
soils —transported and deposited by wind
Colluvial soils —formed by movement of soil from its
original place by gravity, such as during landslides
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