SUMMARY (Petrology)

What is Petrology?
 From the Greek word “Petra” means rock and “Logos” means knowledge.
• It is the branch of geology that studies rocks and the conditions under which they form.
Petrology Terms
 Petrography- the branch of science concerned with the description and classification of
rocks, especially by microscopic study.
 Petrologist- A geologist who specializes in petrology.
Significance of Studying Rocks
The study of rocks is important for several reasons:
 Their minerals and global chemistry provide information about the composition of
the Earth's crust and mantle.
 Their ages can be calculated by various methods of radiometric dating, and a time
sequence of geological events can be put together.
 Their features are usually characteristic of a specific tectonic environment, allowing
scientists to reconstitute tectonic processes.
 Many rocks host important ores that provide valuable raw materials that we rely on for
our sustenance and technological development.
Three Major Divisions
Igneous Petrology
 It is the study of igneous rocks—those that are formed from magma.
Sedimentary Petrology
 Description and classification of sedimentary rocks.
 Interpretation of the processes of transportation deposition of the sedimentary
materials forming the rocks.
 The environment that prevailed at the time the sediments were deposited.
 The alteration (compaction, cementation, and chemical and mineralogical modification)
of the sediments after deposition.
Metamorphic Petrology
 Description and classification of sedimentary rocks
 Interpretation of the processes of transportation and deposition of the sedimentary
materials forming the rocks, the environment that prevailed at the time the sediments
were deposited.
 The alteration (compaction, cementation, and chemical and mineralogical modification)
of the sediments after deposition.
Minor Division
Experimental Petrology
 It is the field of research concerned with experimentally determining the physical and
chemical behaviour of rocks and their constituents.

Shell Structure of the Earth

Earth's Crust
 In geology, a crust is the outermost layer of a planet. The crust of the Earth is composed
of a great variety of igneous, metamorphic, and sedimentary rocks.
Mantle
 Mantle is a layer inside a planetary body bounded below by a core and above by a crust.
Mantles are made of rock or ices, and are generally the largest and most massive layer
of the planetary body.
Outer Core
 The outer core is the third layer of the Earth. It is the only liquid layer, and is mainly
made up of the metals iron and nickel, as well as small amounts of other substances.
Inner Core
 Earth's inner core is the innermost geologic layer of the Earth. It is primarily a solid ball
with a radius of about 1,220 kilometres, which is about 20% of the Earth's radius or 70%
of the Moon's radius.

Definition of Rock
 In geology, naturally occurring and coherent aggregate of one or more
minerals.Definition and Characteristics of Rocks
Three Major Classes of Rocks
 Igneous Rocks- Which have solidified from molten material called magma. Also known
as “fire rock” (derived from the Latin “ignus”)
 Sedimentary Rocks- are formed on or near the Earth’s surface, in contrast to
metamorphic and igneous rocks, which are formed deep within the Earth.
 Metamorphic Rocks- Which have been derived from either igneous or sedimentary
rocks under conditions that caused changes in mineralogical composition, texture, and
internal structure.
Characteristics
 Hardness- It is a measure of the mineral's resistance to being scratched.
 Luster- Some minerals are shiny and some are dull, lustre means metallic or non-
metallic.
  Color- helps identify a mineral/rock but doesn't totally identify it.
 Crystal Structure- It is another way to figure out what type of rock or mineral it is.
 Streak- It refers to the colour pouder that the mineral leaves after it scratches it.
 Cleavage and fracture- Cleavage is a way the mineral breaks and fracture is a breakage
is rough and has jagged edges.

Geological Classification of Rocks

Igneous rocks
Extrusive Rocks: ( Above the earth’s surface)
Intrusive Rocks: (Beneath the earth’s surface)
1. PLUTONIC ROCKS
2. HYPABYSSAL ROCKS
CHEMICAL COMPOSITION
1. ~ SiO2 40-75%, Al2O3 10-20%, others like Ca, Mg and Fe 10%.
TEXTURE
 CRYSTALLINITY
a) Holocrystalline
b) Holohyalite
c) Hemicrystalline
 GRANULARITY MINERAL
 SHAPE ROCK
CLASSIFICATION
 Euhedral
 Subhedral
 Anhedral

Sedimentary Rocks
STRUCTURE
 STRATIFICATION
 LAMINATION
 CROSS BEDDING
TEXTURE
 ORIGIN OF GRAINS- partly or wholly composed of clastic or non-clastic grains.
  SIZE OF GRAINS
Coarse grained rocks- average grain size > 5mm
Medium-grained rocks- average grain size between 5 and 1 mm.
Fine grained rocks- average grain size < 1mm.
 SHAPE OF GRAINS- rounded, surrounded, angular and subangular.
 PACKING OF GRAINS- porous or dense depending on the degree of packing
 FABRIC OF GRAINS- the orientation of the particles are defined with reference to
the orientation of their longer axes.
 CRYSTALLISATION TREND - defined by the degree of crystallized grains.
CLASSIFICATION
 RUDACEOUS DEPOSITS- gravels, pebbles, cobbles or boulders and properly
cemented, e.g., conglomerate and breccia.
 ARNACEOUS DEPOSITS- sandy material with quartz and other minerals like
apatite, garnet, zircon, tourmaline and magnetite, e.g., sandstones.
 ARGILACEOUS DEPOSITS-clay and shale are used for making brick and tiles.
 CARBONACEOUS DEPOSITS- formed by the accumulation of organic materials
consist of plant debris e.g., coal, peat, lignite, etc.
 CALCAREOUS DEPOSITS- limestone and dolomite.
Metamorphic Rocks
THE HEAT COMES FROM:
 Mantle
 Pressure of the layers
 Movement of the Plates on Earth
CLASSIFICATION
 CATACLASTIC METAMORPHISM- earth movements such as folding and faulting.
 DYNAMIC METAMORPHISM- it is associated with low temperature and the rocks
are formed partly by mechanical effects and partly by growth of new minerals
e.g., slates.
 CONTACT METAMORPHISM - local heating of rocks by the intrusion of heat
igneous rock bodies and is also known as thermal metamorphism
 PLUTONIC METAMORPHISM- high static pressure and high temperature at great
depths in earth. These rocks have an even grained texture and denser minerals.
 REGIONAL METAMORPHISM- when directed pressure and heat act together in
presence of migrating hydrothermal fluids at great depths, the rocks are
metamorphosed over wide areas. It is also known as dynamothermal
metamorphism.
  METASOMATISM- process of introducing other elements like circulating fluids
derived from igneous magma during recrystallisation.
 RETROGRESSIVE METAMORPHISM- when high temperature metamorphic
mineral assemblages are changed to a low temperature mineral assemblages,
the process is called the retrogressive metamorphism.
TEXTURE
 CRYSTALLOBLASTICTEXTURES- textures that have been newly imposed upon the
rock during the process of metamorphism and are, therefore, essentially the
product of metamorphism.
 PALIMPSET TEXTURES- essential details as in the parent rock with little or no
modifications taking place during metamorphism.

Occurrence, Properties and Distribution of Rocks

Igneous Rocks
 Granite
Occurrence- Found in large plutons on the continents, in areas where the Earth’s crust has
been deeply eroded.
Properties - quartz and feldspar with minor amounts of mica, amphiboles, and other
minerals.
Distribution - tombstones and monuments, flooring
 Syenite
Occurrence- Cordilleran subduction zones.
Properties - Alkali feldspar (e.g. orthoclase) is the major mineral component
Distribution - refractories, glass making, ceramics and, in pigments and fillers.
 Diorite
Occurrence- volcanic arcs, and in mountain building
Properties - vital in determining its Diorite Texture and Diorite Uses.
Distribution - drainage stone and for erosion control.
 Gabbro
Occurrence- batholiths and laccoliths and is often found along mid-ocean ridges or in
ancient mountains
Properties - coarse-grained, dark-colored, usually black or dark green in color and
composed mainly of the minerals plagioclase and augite
 Distribution - proto-rift zones and around ancient rift zone margins, intruding into the rift
flanks.
 Pegmatite
Occurrence- found in all over the world.
Properties - large crystals and they contain rarely minerals than other types of rocks.
Distribution - components for electronic devices, retardation plates, circuit boards, optical
filters, detector windows
 Dolerite
Occurrence- hypabyssal igneous rock, typically within dykes
Properties - Fine grained, ophitic texture. Primary minerals - plagioclase, pyroxene,
hornblende and quartz.
Distribution - distributed in the rocks that surround such bodies, and still others are not
recognizably associated with igneous rocks.
 Basalt
Occurrence- ocean floor
Properties - quite glassy (tachylytes), and many are very fine-grained and compact.
Distribution - crustal portions of oceanic tectonic plates
Sedimentary Rocks
 Siliciclastic
Occurrence- accumulation of mostly silicate mineral fragments.
Properties - relating to or denoting clastic rocks consisting largely of silica or silicates
Distribution - 50 - 60% of the world oil and gas exploration. 
 Sandstone
Occurrence- in water and land
Properties - composed mostly of quartz sand, but it can also contain significant amounts of
feldspar, and sometimes silt and clay.
Distribution - domestic construction and housewares since prehistoric times, and continues
to be used.
 Limestone
Occurrence- Widespread occurrence throughout the country
Properties - solidification of minerals out of solution into rock form.
Distribution - Great Pyramid and its associated complex in Giza, Egypt, were made of
limestone
  Shale
Occurrence- particles in slow or quiet water, such as river deltas, lakes, swamps, or the
ocean floor. 
Properties - compacted mud consisting of clay and tiny particles of quartz, calcite, mica,
pyrite, other minerals, and organic compounds
Distribution - natural gas or oil.
 Conglomerate
Occurrence- interbedded with sandstone and mudstone. 
Properties - presence of readily visible, rounded clasts bound within a matrix.
Distribution - ornamental stone for interior use
 Breccia

Occurrence- broken, angular fragments of rock or mineral debris accumulate.. 

Properties - It can occur in any color, and may be either hard or soft. The rock may be rough
to the touch because of the angular clasts.
Distribution - sculptures, gems, and architectural elements.
Metamorphic Rocks
 Quartzite
Occurrence- during mountain-building events at convergent plate boundaries.
Properties - Resistance to acids.
Distribution - bricks and other strong building materials. It is also growing in popularity as a
decorative stone, and has a limited use as crushed stone.
 Marble
Occurrence- it is derived from limestone or dolomite
Properties - White, pink, medium grained; can see interlocking calcite crystals with the
naked eye.
Distribution - aggregate in highways, railroad beds, constructing foundations, and different
kinds of construction.
 Slate
Occurrence- created by the alteration of shale or mudstone by low-grade regional
metamorphism.
Properties- color properties from the minerals contained within its parent rock, shale
Distribution - produce roofing slates.
  Gneiss
Occurrence- identified by its bands and lenses of varying composition
Properties - alternating light and dark bands differing in mineral composition (coarser
grained than schist).
Distribution - building material such as flooring, ornamental stones, gravestones, facing
stones on buildings and work surfaces.
 Schist
Occurrence- regional metamorphism of existing rock.
Properties - Physical propety of schist Hardness, Grain, size, fracture, luster, streak,
porosity, strength.
Distribution - building houses or walls.

Rock Cycle
1. Melted rock or magma is sent to the earth's surface by a volcano. It cools and forms an igneous rock.

2. Next the weather, or a river, and other events will slowly break up this rock into small pieces of
sediment.

3. As sediment builds up and hardens over years, a sedimentary rock is formed.

4. Slowly this sediment rock will get covered with other rocks and end up deep in the Earth's crust.

5. When the pressure and heat get high enough, the sedimentary rock will metamorphose into a
metamorphic rock and the cycle will start over again.

Importance of Petrology in Civil Engineering

Different Types of Rocks that are using in Constructions

 Granite- Hard, Competent, Durable => Suitable for Foundation

 Limestone - Comparatively Soft => Best for Flooring
 Marble - Soft And Attractive => Flooring/Sculpturing Etc.
 Sandstone - Sculpturing, Wall Etc.

Petrology is very important from the civil engineering point of view, because this provides a proper
concept and logical basis for interpreting physical properties of rocks.