Download Rocks

Rocks
• Rocks are composed of two or more
minerals
• Rocks are very helpful in dating the
age of the earth
Classification of Rocks
Igneous rocks
Sedimentary rocks
Metamorphic rocks
Igneous Rocks
 The word ‘igneous’ has been derived from a latin word ‘ignis’
which means fire.
 Igneous rocks are formed due to cooling, solidification and
crystallization of hot and molten materials known as magma and lava.
 These rocks are also known as primary or parent rocks.
Characteristics of igneous rocks
1. Hard rocks
2. Non porous rocks
3. Granular rocks but there are variations in form,
size & texture of grains
4. Do not have any layers
5. Less affected by chemical weathering
6. Do not contain fossils
7. Mainly associated with volcanic activities that’s
why also known as volcanic rocks
Classification of igneous rocks
Based on mode of occurence
Based on amount of silica
Acidic- having more silica
Example- Granites
Intrusive
Batholiths
Laccoliths
Basic- having lower amount of silica
Example- gabbro
Phacoliths
Lapoliths
Sills
Dykes
extrusive
Intrusive igneous rocks
• When the rising magma cooled and solidifies below the surface of the earth, the
resultant rocks are intrusive igneous rocks.
1. Batholiths- long irregular & undulating form of solidified magma
• Dome shaped
• Almost vertical side walls
2. laccoliths- formed due to injection of magma along the bedding planes of
horizontally bedded sedimentary rocks.
• Mushroom shaped
• Convex summital form.
3. Phacoliths- formed due to injection of magma along the anticlines and
synclines.
4. Lapoliths- magma solidified in concave shallow basin whose central part is
sagged downward
5. Sills- syl means ledge
• Formed due to injection & solidification of magmas between the bedding planes
of sedimentry rocks.
6. Dykes- wall like formation of solidified magma.
• Perpendicular to the beds of sedimentary rocks.
Extrusive igneous rocks
• Igneous rocks formed due to cooling and
solidification of hot and molten lavas at the
surface of the earth are extrusive igneous
rocks.
• These rocks are also known as volcanic rocks.
• Example – Basalt
Difference between intrusive and
extrusive igneous rocks
Intrusive igneous rocks
Extrusive igneous rocks
Magma cooled and solidified below Lava cooled and solidified at the
the surface of the earth
surface of the earth
Sedimentary rocks
 These rocks are formed due to aggregation and
compaction of sediments. Sedimentary rocks are also
called stratified or layered rocks.
 The sediments and debris derived through the
disintegration and decomposition of rocks by the
agents of weathering and erosion are gradually
deposited in water bodies. Thus, layer after layer of
sediments are regularly deposited after consolidating
the layers form sedimentary rocks.
Characteristics of sedimentary rocks
1. Contains fossils of plants and
animals
2. 75% of surface area of the globe is
covered with sedimentary rocks
3. Contains several layers
4. They are not found in massive form
like igneous rocks
5. Most of the rocks are permeable
and porous
Classification of sedimentary rocks
Mechanically formed
Sandstones
Clay and shale
conglomerates
Chemically formed
When chemically active water
comes in contact with rocks
soluble materials are removed
from the rocks . Such materials
are chemically derived rocks.
Organically formed
Calcareous
Carbonaceous
Siliceous
Metamorphic rocks
 These rocks include rocks that have been
changed either in form or composition
without disintegration
 They generally form due to changes in
form of sedimentary and igneous rocks.
Types of metamorphism
Based on nature of agents
Thermal (due to heat)
Dynamic (due to pressure)
Hydro (due to hydro static ressure)
Hydro thermal (due to water & heat)
Based on place
Contact
Regional
Composite classification
Contact or thermal
Dynamic or regional
Hydro
Hydro- thermal
Types of metamorphism
• Contact- It takes place when the mineral composition of the
surrounding rocks known as aureoles is changed due to intense
heat of the intruding magmas.
• Regional- when the rocks are altered in their forms in extensive
area the process is called regional metamorphism.
• Hydro- When chemically active water passes through the
countryside rocks, there occur several chemical changes in the rocks
due to varied chemical reactions.
• Hydro- Thermo- The minor alteration in the physical & chemical
composition of the rocks caused by the weight and pressure of
water mass & chemically active hot gases and water vapour.