Forms of Igneous Rocks

FORMS OF IGNEOUS ROCKS

The form, i.e., the size and shape of the igneous rocks depends on the following factors.


a) Mode of formation
b) Viscosity of magma (fluidity of magma)
c) Their relation with the surrounding rocks

There are two major categories of forms of igneous rocks. They are,

i) Discordant forms: When the igneous rock has a cross cutting relationship with its surrounding rocks, they are said to be discordant forms.

ii) Concordant forms: In this case the intrusive igneous rock run parallel to the existing rock masses.

Forms of Discordant bodies:

a) Dyke: Dyke is a wall like igneous body that cut across the pre existing rock. Dykes may be vertical or inclined. 

There are different types of dykes.

Ring dyke: A dyke with circular outcrop.
Cone sheet: Inclined dyke like mass

b ) Batholith: Large igneous intrusives are called batholiths. Usually their will be more than 1002 km. Batholiths of smaller dimension are called stocks. The stocks with circular outcrop are called bosses.

c) Ethmolith: Funnel shaped basic igneous bodies

d) Harpolith: Sickle shaped igneous bodies

e) Chonolith: Any irregular intrusive body

Forms of concordant bodies:

     a)      Sills: A sill is a sheet like igneous body which runs parallel to the bedding plane of the pre-existing rocks. 

     b)     Laccolith: These are the igneous bodies with flat base and convex top. 

     c)      Lopolith: These are saucer shaped bodies.

     d)     Phacolith: These are concentric shaped igneous bodies occur at the crest and troughs of a fold

Distinguishing features of 3 types of rocks

                         Distinguishing features of 3 types of rocks

Igneous Rocks	Sedimentary Rocks	Metamorphic Rocks
1. Generally hard, massive, compact with interlocking grains 2.No fossils present 3.No bedding planes present 4.Surrounding rocks are baked 5. Contains much feldspar group of minerals E.g.: Granite, basalt, syenite etc.	1. Generally soft and layered 2.Fossils are common 3.No effect on the surrounding rocks E.g.: Sandstone, Limestone, Conglomerate etc.	1.Generally hard, interlocking grains and bedded (if derived from sedimentary rocks) 2. Rarely preserves fossils in case of sedimentary origin 3.Foliated E.g.: Gneiss, Schist, Quartzite, Marble etc.

Formaton of Sedimentary Rocks

Sedimentary rock formation begins with igneous, metamorphic, or other sedimentary rocks. When these rocks are exposed at the earth’s surface they begin the long slow but relentless process of becoming sedimentary rock.

Weathering
All rocks are subject to weathering. Weathering is anything that breaks the rocks into smaller pieces or sediments. This can happen by the forces of like wind, rain, and freezing water.

Deposition

The sediments that form from these actions are often carried to other places by the wind, running water, and gravity. As these forces lose energy the sediments settle out of the air or water. As the settling takes place the rock fragments are graded by size. The larger heavier pieces settle out first. The smallest fragments travel farther and settle out last. This process of settling out is called deposition.

Erosion
The combination of weathering and movement of the resulting sediments is called erosion.

Lithification
Lithification is the changing of sediments into rock. There are two processes involved in this change. They are compaction and cementation.

Compaction
Compaction occurs after the sediments have been deposited. The weight of the sediments squeezes the particles together. As more and more sediments are deposited the weight on the sediments below increases. Waterborne sediments become so tightly squeezed together that most of the water is pushed out. Cementation happens as dissolved minerals become deposited in the spaces between the sediments. These minerals act as glue or cement to bind the sediments together.

The process of sedimentary rock formation takes millions of years to complete only to begin a new cycle of rock formation.

Igneous Petrology

Igneous Petrology

Definition of “Rock”: Rock may be defined as aggregate of minerals that makes up part of the earth's crust

Classification of Rocks

Based on the mode of formation, all the rocks are classified into 3 groups.  They are:

1)      Igneous rocks or primary rocks

2)      Sedimentary rocks or secondary rocks

3)      Metamorphic rocks.

General characters of igneous rocks:

i)        Generally hard, massive, compact with interlocking grains

ii)      No fossils present

iii)    No bedding planes present

iv)    Surrounding rocks are baked

v)      Contains much feldspar group of minerals

2. Sedimentary rocks: These rocks will be formed through the process of weathering, transportation and deposition of preexisting rocks.  The deposited loose sediments will undergo a process called lithification and diagnensis  to form sedimentary rocks.

Based on their place of formation, sedimentary rocks are classified into 2 types.

Residual deposits: Formed at the site of preexisting rocks

Transported deposits: Formed at far off place from the preexisting rocks.   

General characters of sedimentary rocks:

i)        Generally soft and layered

ii)      Fossils are common

iii)    No effect on the surrounding rocks

3.                  Metamorphic rocks: These rocks are formed by the alteration of preexisting rocks by the action of Pressure and temperature.  

General characters of metamorphic rocks:

i)        Generally hard, interlocking grains and bedded (if derived from sedimentary rocks)

ii)      Foliated

iii)    Rarely preserves fossils in case of sedimentary origin

Metamorphism

Metamorphism is defined as the mineralogical, chemical and structural adjustments in solid rocks to physical and chemical conditions which have been imposed due to changes in pressure and temperature or both.
Metamorphism occurs below the surface zones of weathering and cementation.
Conditions of metamorphism differ from the conditions under which the rocks in question were originally formed.

Metamorphism produced as a result of the progressive increase in temperature and pressure, i.e. by burial of a rock within the earth, is termed prograde metamorphism and in general terms is characterized by dehydration reactions, which release water. 

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