1. Origin of Soils and Rocks
(Das, Chapter 2)
Sections: 2.1
1
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.
2
On the basis of their mode of origin, rocks can be
divided into three basic types:
•
•
•
Igneous rocks
Sedimentary rocks
Metamorphic rocks
3
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
Igneous
Rock
4
The rock cycle is
important in any
discussion of soil
formation,
especially if we
know that soils are
nothing but
disintegrated
rocks.
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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II. Classification Based on Mode of Occurrence
A. Volcanic or Extrusive Rocks
They are formed when magma reaches the surface,
along either wide vertical fissures or pipe-like
openings in the earth crust.
B. Plutonic or Intrusive Rocks
Those rocks are formed when the magma rise
through the earth’s crust but does not reach the
surface.
Intrusions can be divided into two types:
1. Major Intrusions
2. Minor Intrusions
•Stocks
•Dykes
•Batholiths
•Sills
•Laccolith
•Sheets
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•phacolith
– Extrusive/Volcanic: Igneous rocks that form due to
the freezing of melts above the surface of the Earth.
7
•Intrusive/Plutonic:
Form by freezing of melts below the surface of the
Earth.
8
Igneous Intrusions
9
Types of Igneous Intrusions
Discordant: cut across pre-existing fabric of rock layers
Dikes: are small igneous intrusions that cut across rocks into
which the magma intrudes. They are commonly sheet-like, only a
few meters wide, but possibly laterally extensive.
Stocks: are fairly large (10’s of miles) igneous intrusions that cut
across pre-existing rock layers. In size, they are on the order of an
individual mountain peak.
Batholiths: are huge igneous intrusions made of many stocks.
Their size is on the scale of an entire mountain range (100’s of
miles).
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Types of Igneous Intrusions
Concordant: follow pre-existing fabric of rock layers without
interruption of layering.
Sills: are also small igneous intrusions. They are sheets of rock
that, unlike dikes, are parallel to pre-existing rocks. Think of
magma invading sedimentary rocks by spreading out between
rock layers. That magma would cool to form a sill.
Laccoliths: are small intrusions having a flat floor and doomed
roof.
Lopoliths: are also large, inverted mushroom-shaped (spoonshaped) intrusions that “sag down” in the middle because of
dense rocks.
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Tabular Intrusions: Plutons
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Non-Tabular Intrusions: Plutons
• Pluton: Irregular blob-shaped
discordant intrusions that range
in size from 10’s of m, to 100’s
of km
13
• Batholith: A pluton that is > 100 km2
in surface exposure
• Stock: A pluton that is <100 km2 in
surface exposure
Non-Tabular Intrusions: Plutons
• Laccolith: a dome-like sill that bends the layers above it into a
dome shape
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1. Igneous Rocks
The types of igneous rock formed by the cooling of
magma depends on the i) composition of the magma;
ii) the rate of cooling associated with it.
Bowen (1922) was able to explain the relation of the
rate of magma cooling to the formation of different
types of rock.
Bowen’s reaction principle: describes the order
crystal formation from magma as cooling occurs.
15
of
Bowen’s Reaction Principles
16
Bowen’s Reaction Series
Two series of minerals formed during crystallization of magma
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Low Silica Magma
1200oC
1000o
Intrus. Extrus.
Gabbro Basalt
Diorite Andesite
750o
Framework
Sheet
Double
Chain
Single
Chain
Isolated
Temperature of
Crystallization
Granite Rhyolite
High Silica Magma
Mineralogy of Igneous Rocks
Many different types of minerals occur in igneous rocks, but
only about eight are normally present as essential constituents
of a rock. They are:
•Quartz
•Orthoclase
•Plagioclase
•Muscovite
•Biotite
•Hornblende
•Augite
•Olivine
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Which of the eight are present is controlled primarily by the
composition of the magma.
Classification of igneous rocks (Summary)
19
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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Classification of Sedimentary Rocks
Sedimentary rocks are classified into two major categories:
I.Detrital Sedimentary Rocks
They formed from minerals or rock fragments derived from the
breakdown of pre-existing rock
Sediment
Gravel
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Particle Term
Boulder
Cobble
Pebble
granule
Sedimentary Rock
Conglomerate
Sand
Very course sand
Coarse sand
Medium sand
Fine sand
Very fine sand
Sandstone
Mud
Silt
Clay
Shale
Mudstone
Conglomerate
This is a rock consists of cemented boulder, cobbles, pebbles and
granules
Sandstone
This rock is composed essentially of pressure- cemented grains of
sand. The cement agent determines the degree of indurations, or
hardness, of sandstone.
Shale
It is formed from silts and clays which have hardened into rock,
with the principal indurations agent being pressure.
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REMARKS
1. Breccia is a variety of conglomerate, which has angular
rather than rounded fragments.
2. When the grains in sandstone are practically all quartz, the
rock is referred to as orthoquartzite.
3. For Sandstone, the three chief kinds of cement, in the order of
their importance are:
•Silica
•Iron oxides
•Carbonates
4. Mudstone has a blocky aspect; whereas, in the case of shale,
the rock is split into platy slabs.
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5. In mudstone and shale, size of the particles are generally
less than 1/16 mm.
6. Shale is the most abundant of the sedimentary rocks.
Nearly half of all the sedimentary rocks are shale.
7. Marl is a calcareous mudstone (expansive).
8. From engineering point of view, sedimentary rocks are the
most important and relevant because:
• They are the most abundant, and hence encountered.
• They are the most problematic.
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II. Chemical Sedimentary rocks
Sedimentary rock also can be formed by chemical processes.
Rocks of this type are classified as chemical sedimentary rock.
Examples of Chemical Sedimentary Rocks:
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Textures of Sedimentary Rocks
There are two main types of textures of Sedimentary Rocks:
1. Clastic Texture
Rocks formed by mechanical weathering have clastic texture
2. Non-Clastic Texture
Most chemical sedimentary rocks have a non clastic texture.
These rocks have somewhat the same appearance as igneous
rocks with crystalline texture.
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Summary of Sedimentary Rocks
28
29
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 change is referred to as
METAMORPHISM. It is derived from the Greek META
which means change and MORPH, shape or form.
 The
original rock may be igneous, sedimentary or
metamorphic.
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 The origin of metamorphic rocks is the least clear among
the three classes of rocks. Because no one has ever seen a
metamorphic rock been formed, and for that reason many
of our ideas about them are pure conjecture.
 During
metamorphism the minerals do not melt, but
remain largely in the solid state.
 The metamorphism process is isothermal process, i.e. the
overall chemical composition of the rock is nearly the
same before and after recrystallization.
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Classes of Metamorphism
There are three broad classes of metamorphism depending on
the control exercised by temperature and pressure.
1. Thermal or Contact Metamorphism
Increased temperature is the dominant agent producing
change. (Caused by igneous activity)
2. Dynamic or Dislocation Metamorphism
The dominant control is stress, extra heat is relatively
unimportant. (Associated with faults & earthquake zones)
3. Regional Metamorphism
Both temperature and pressure have operated over a large
(regional) area. (Associated with tectonic plate activity).
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 The degree of metamorphism is related to the conditions of
temperature and pressure under which the new metamorphic
rock has formed, and may be assessed by the appearance of
certain minerals.
 The
index minerals each of which indicates a particular
temperature and pressure at the time it formed are used to
define the metamorphic GRADE of the rock in which they
occur.
 One
important characteristic related to the metamorphic
grade is that grain size gets coarser with higher grade, i.e.
rocks formed at higher temperature have coarse grained
particle like GNEISS, and vise versa.
33
Some metamorphic rocks and their original rocks
Metamorphic
Original Rock
Rock
Mudstone, Shale Slate
Sedimentary
Sandstone
Quartzite
rocks
Limestone
Marble
Igneous
Rocks
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Dolerite
Diabase
Augite&Chlorite
Gabbro
Hornblende
Biotite
Plagioclase
Granulites
CLASSIFICATION OF METAMORPHIC ROCKS
I. Classification Based on Type of Metamorphism
1. Contact Metamorphic Rocks
2. Dynamic Metamorphic Rocks
3. Regional Metamorphic Rocks
35
CLASSIFICATION OF METAMORPHIC ROCKS
II. Classification According to Degree of Foliation (Textures)
Foliated
•Rock has distinct banding or layering
•Formed under direct pressure
Non-Foliated
•No distinct layering character
•Formed under uniform pressure
36
CLASSIFICATION OF METAMORPHIC ROCKS
II. Classification According to Degree of Foliation (Textures)
WellFoliated
Moderately
Foliated
NonFoliated
Slate
Gneiss
Quartzite
Phyllite
Migmatite
Marble
Schist
Granulite
Hornfels
Mylonite
Serpentine
Amphiblite
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Foliated Metamorphic Rocks
slate
phyllite
schist
38
gneiss
Classification of metamorphic rocks according to
metamorphic grade
39