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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.