The document discusses the properties and classification of three main types of rocks: igneous, sedimentary, and metamorphic rocks. Igneous rocks form from cooling magma, sedimentary rocks form through accumulation and compaction of sediments, and metamorphic rocks form from changes to pre-existing rocks through heat, pressure, and chemically-active fluids. The document describes the formation processes, typical mineral compositions, and textural characteristics of each rock type. It also discusses classification schemes for igneous and metamorphic rocks based on silica content and foliation, respectively.
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Properties of Rocks.pptx
1. Properties of Rocks
Dr. K. Pavan Kumar
Associate Professor
Department of Civil Engineering
Vasavi College Engineering
2. Nature Of Rocks
Rocks are aggregate of one or more minerals
The nature and properties of rock are determined by the minerals in it and by
the manner in which the minerals are arranged relative to each other (i.e.,
texture of the rock)
An individual rock type or specimen is always described in terms of its mineral
composition and its texture
According to their manner of formation rocks are of three main types:
Igneous rocks
Sedimentary rocks
Metamorphic rocks
3. Igneous Rocks
Are formed from magma, which has originated well below the surface, has ascended
towards the surface, and has crystallised as solid rock either on the surface or deep within
the Earth’s crust as its temperature fell
Sedimentary Rocks
Are formed by the accumulation and compaction of:
- Fragments from pre-existing rocks which have been disintegrated by erosion
- Organic debris such as shell fragments or dead plants
- Material dissolved in surface water or groundwater which is precipitated in condition of
oversaturation
Metamorphic Rocks
Are formed from pre-existing rocks of any type, which have been subjected to increases of
temperature (T) or pressure (P) or both, such that the rocks undergo change
This change results in the metamorphic rock being different from the original parental
material in appearance, texture and mineral composition
4. Igneous Rocks
Mineralogy of Igneous Rocks
Igneous rocks are formed from the cooling and solidification of hot and fluid
mass of rock called magma that exists in the interior part of the earth
Only about eight minerals are normally present as essential constituents of
igneous rocks
Which of the eight are present is primarily controlled by the composition of
magma
Intrusion of magma from deep-seated source (plutonic) or from moderate
depth after solidification (hypabyssal) result in the formation of igneous bodies
such as batholiths, laccoliths, phacoliths, lopoliths, which extend over a large
part of the earth’s crust
5. D – Dyke – Discordant bodies that cut across
the bedding of country rock
(i.e., rock native to that area)
L Lava Flow
S – Sill- concordant bodies occurring parallel
to the bedding plane
6. The different types of intrusive igneous rocks can be described as follows:
oBatholith is a very large discordant igneous body ranging from tens to
hundreds of square kilometres in size. It can be spread irregularly over great
depths.
oPhacolith is a concordant igneous body that appears crescent shaped at its
surface.
oLaccolith is an intrusive igneous body that has a concordant relation with the
country rock (native rock) and appears like a dome at its upper part.
oLopolith is an intrusive igneous body that maintains a concordant relationship
with the country rock and exhibits saucer-shaped appearance at the ground.
7. • Most igneous rocks are composed of an interlocking collection of crystals that
grew as the magma cooled
• When the magma is deep seated inside the earth, cooling is slow and
crystallization takes considerable time; hence, the rock is coarse grained
• If the magma is closer to the surface and intrudes through cracks in the crust
(as in dykes), the cooling and crystallization process is comparatively rapid and
the rock thus formed is fi ne-to-medium grained
• Extrusive rocks – solidification of lava at the surface – cooling is very rapid; no
time for crystallization – very fine textured rocks
• Volcanic rocks – igneous rocks that originate from rapid cooling of lava under
surface conditions
• The vast area of Deccan Trap in India is formed of volcanic rocks created by
lava flows
8. Irrespective of the nature of texture, igneous rocks are classified on the basis
of the proportion of the total silica in their composition
The more silica a rock has, the more acidic it will be
Hence on the basis of silica content, igneous rocks are classified into:
Acidic (Felcic) Igneous Rocks
- Contain more than 66% of silica
- Constituent minerals of these rocks are: Feldspar and quartz in abundant amount;
muscovite, biotite and amphibole in subordinate amounts
- Example: Granite
Intermediate Igneous Rocks
- Silica content in these rocks is between 52% and 66%
- Feldspars are abundantly present; biotite and amphibole are present in smaller quantities
- Example: Diorite
9. Basic Igneous Rocks
- Silica content is less than 52% (quartz is absent)
- Rocks contain mostly feldspar and pyroxene with olivine
- Example: Gabbro and basalt
Ultrabasic Igneous Rocks
- Silica content is less than 45% and the minerals are mainly olivine and pyroxene
Granite Diorite Gabbro
10. Sedimentary Rocks
Derived from pre-existing igneous, sedimentary or metamorphic rocks
All existing rocks are subjected to the actions of water, wind, and ice that result in decay
and disintegration of the rocks and formation of sediments
This process is called denudation of rocks
Sedimentary rocks may be composed of clastic (fragments of pre-existing minerals and
rocks) or non-clastic (precipitated chemically from water) materials
Sedimentary rocks provide information on the history of deposition and palaeo-
environment
The knowledge of origin of plants, animals, and man is obtained from the study of fossils
embedded in sediments
The most prominent primary structure of sedimentary rocks is the bedding or stratification
of varying thickness as well as composition
11. The sedimentary rocks of Vindhyan formation in India are famous for
architectural works from the prehistoric time and are even now used as
construction material in various engineering projects
12. Metamorphic Rocks
Metamorphic rocks are formed by the metamorphism of pre-existing rocks under
high temperature and pressure conditions or by chemically-active fluids bringing
about both physical and chemical changes in the original rocks
Xenolith – a metamorphic rock which shows remnants of earlier rock from which it
has formed
The pattern of change depends upon the grade of metamorphism controlled by the
pressure–temperature conditions
In general, metamorphism takes place within a temperature range of 100°C to
900°C generated from the intrusion of plutonic rocks
The pressure comes from the huge thickness of overlying rocks or forceful intrusion
of plutons (a deep seated intrusion of igneous rocks) and plate collisions
Hot water and other fluids associated with magma react with surrounding rocks to
bring about chemical changes
13. Two most important forms of metamorphism:
Contact or thermal metamorphism
- Native rocks react with intrusive intrusive igneous bodies causing changes in
the surrounding rocks as a result of the heat generated from intrusion or
injection of magmatic fluids
- The rocks produced by contact metamorphism are restricted to areas close to
plutonic (deep-seated) bodies
Regional Metamorphism
- Regional metamorphism affects large regions such as the Precambrian shield
and also cores of folded mountains
- Example: Gneiss, schist, slate
14. During metamorphism, the effect of pressure causes growth of new crystals or
reorientation of previously existing crystals to layers commonly known as foliation
Foliation causes splits in rocks and hence has an important bearing in engineering geology
related to metamorphic rocks
On the basis of foliation, metamorphic rocks are classified into the following types:
Slate: Fine grained; foliation is very closely packed.
Phyllite: Fine grained; wavy or crenulated surfaces having sheen caused by small flakes of
mica or chlorite.
Schist: Coarse to medium coarse; undulating planes with abundant mica. Depending on the
constituent minerals, this rock is designated as hornblende schist or garnet mica schist.
Gneiss: Coarse; widely spaced foliation than schist, mainly occurring with bands of dark and
light minerals in thin layers.
Migmatite: Coarse; poor foliation showing signs of having begun to melt.