Understanding Textures of Igneous Rocks in Geology

 
TEXTURES OF
IGNEOUS ROCKS
 
Structures 
and
 
Textures
 
Structures 
and 
textures 
are 
physical 
features
associated with the 
rocks
. 
They are 
generally primary
in nature. 
i.e.. 
they occur along with the 
formation 
of
rocks and are important
 
because.
(i) the contribute to the 
strength 
or 
weakness 
of
 
rocks
.
(ii) 
they 
serve 
as 
distinguished 
features 
of 
rock
groups
.
 
(iii)
 
They
 
re
v
eal
 
the
 
mode
 
o
f
 
origin
 
o
f
 
r
ock
 
concerned
.
Textures 
on 
the 
other 
hand refers 
to small 
scale
features 
like porphyritic 
texture 
and  interlocking
texture 
which 
can 
be studied 
in 
 
hand
 
specimen.
 
Common 
Texture 
of 
Igneous
Rocks
 
The term texture mainly refers to the mutual relationships
of the constituents minerals in a rock in addition to
crystalline, granularity, and shapes of minerals in a  rock.
Texture 
based on the 
degree 
of
 
crystallinity
Igneous
 
rocks are formed due to cooling and solidification
of magma and 
crystallization of different minerals takes
place when respective molecules in magma move to
their center of  Crystallization 
and 
arrange themselves
in 
a 
definite  pattern
. If reasonable 
time is available,
crystallization 
of  
different 
minerals take 
place 
from 
the
melt 
giving 
rise to 
a  rock.
 
Common 
Texture 
of 
Igneous
Rocks
 
Common 
Texture 
of 
Igneous
Rocks
 
But if 
sudden chilling 
of 
lava occurs
, then there 
may
not be 
any time for crystallization 
to take place.
Under such conditions, 
lava solidifies 
as 
completely
amorphous 
or 
glassy matter without 
any minerals
.
On 
the 
other hand, if cooling 
time 
is intermediate,
then 
the 
resulting 
rock 
will 
be 
composed 
partly 
of
glassy matter 
and 
partly 
of
 
minerals
.
 
Common 
Texture 
of 
Igneous
Rocks
 
Common 
Texture 
of 
Igneous
Rocks
 
Thus,
 
d
e
pending
 
on
 
the
 
n
ature
 
of
 
cooling
 
the
resulting igneous 
rocks
 
are
(i) Completely
 
crystalline
(ii) Completely
 
glassy
(iii) partly 
crystalline 
and 
partly 
glassy Thus the
preceding 
three different 
types 
of 
crystallization
give rise 
to 
three textures of igneous rocks
namely;
Holocrystalline,holohyaline, and
merocrystalline
.
 
Common 
Texture 
of 
Igneous
Rocks
 
Texture 
Based 
on
 
Granularity
Depending on the 
physical 
condition 
that 
had  
prevailed
during the 
crystallization of 
magma,  
minerals grain occur
in 
different 
sizes. 
The presence  of 
volatiles, 
low 
viscosity,
slow 
cooling and great  pressure help to grow 
large
 
minerals.
The 
following 
textures have been recognized 
based 
on  
the
granularity 
of 
minerals.
 
If 
minerals in 
the rock are  big enough to be seen by the naked
eye, the texture 
is  
described as 
phaneric 
texture
.
I
f  
minerals are 
too 
fine to 
be seen 
separately by 
the 
naked  
eye,
the texture is described 
as 
aphanitic
 
texture.
 
Texture 
Based 
on
Granularity
 
Common 
Texture 
of 
Igneous
Rocks
 
Texture 
based on shape of
 
crystal
These 
textures are of two 
different 
kinds; 
The 
first term 
of
development of crystal faces or boundary outlines 
and 
the
other 
with reference to 
the 
nature 
of 
the 
growth of 
the
minerals.
When 
the mineral 
is 
completely 
bounded 
by 
crystal 
faces
it is 
called 
“euhedral” 
; when 
crystal faces are absent, it 
is
called “anhedral” and when only a part of 
the 
mineral 
is
bounded 
by 
crystal faces it 
is called “subhedral”.
Euhedral minerals 
are rather rare 
in 
nature. 
The c/s of
euhedral minerals 
as 
seen in 
their thin 
sections appear 
with
polygonal boundary 
outline, 
whereas in the 
case of anhedral
minerals, they 
appear 
rounded 
or surrounded in thin
sections.
 
Common 
Texture 
of 
Igneous
Rocks
 
Suitability 
of Igneous 
rocks 
for
 
building  and
foundation
 
G
r
a
n
i
te
 
Pegmatite
s
 
Dolerite
 
Thanks!
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Structures associated with igneous rocks are primary features that contribute to their strength, distinguish rock groups, and reveal origin. Textures, like crystalline patterns, indicate cooling and crystallization processes. Different cooling rates lead to varied textures such as holocrystalline, holohyaline, and merocrystalline, affecting the rock's composition. These features are essential for studying the formation and properties of igneous rocks.


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  1. TEXTURES OF IGNEOUS ROCKS Dr. A.V.Tejankar Professor & Head Dept. of Geology Deogiri College, Aurangabad

  2. Structures and Textures Structures associated with the rocks. They are generally primary in nature. i.e.. they occur along with the formation of rocks and are important because. (i) the contribute to the strength or weakness of rocks. (ii) they serve as distinguished features of rock groups. (iii) They reveal the mode of origin of rock concerned. Textures on the other hand refers to small scale features like porphyritic texture and interlocking texture which can be studied in hand specimen. and textures are physical features

  3. Common Texture of Igneous Rocks The term texture mainly refers to the mutual relationships of the constituents minerals in a rock in addition to crystalline, granularity, and shapes of minerals in a rock. Texture based on the degree of crystallinity Igneous rocks are formed due to cooling and solidification of magma and crystallization of different minerals takes place when respective molecules in magma move to their center of Crystallization and arrange themselves in a definite pattern. If reasonable time is available, crystallization of different minerals take place from the melt giving rise to a rock.

  4. Common Texture of Igneous Rocks

  5. Common Texture of Igneous Rocks But if sudden chilling of lava occurs, then there may not be any time for crystallization to take place. Under such conditions, lava solidifies as completely amorphous or glassy matter without any minerals. On the other hand, if cooling time is intermediate, then the resulting rock will be composed partly of glassy matter and partly of minerals.

  6. Common Texture of Igneous Rocks

  7. Common Texture of Igneous Rocks Thus, depending on the nature of cooling the resulting igneous rocks are (i) Completely crystalline (ii) Completely glassy (iii) partly crystalline and partly glassy Thus the preceding three different types of crystallization give rise to three textures of igneous rocks namely; Holocrystalline,holohyaline, and merocrystalline.

  8. Common Texture of Igneous Rocks Texture Based on Granularity Depending on the physical condition that had during the crystallization of magma, minerals grain occur in different sizes. The presence slow cooling and great pressure help to grow large minerals. The following textures have been recognized based on granularity of minerals. If minerals in the rock are big enough to be seen by the naked eye, the texture is described as phaneric texture. If minerals are too fine to be seen separately by the naked eye, the texture is described as aphanitic texture. prevailed of volatiles, low viscosity, the

  9. Texture Based on Granularity

  10. Common Texture of Igneous Rocks Texture based on shape of crystal These textures are of two different kinds; The first term of development of crystal faces or boundary outlines and the other with reference to the nature of the growth of the minerals. When the mineral is completely bounded by crystal faces it is called euhedral ; when crystal faces are absent, it is called anhedral and when only a part of the mineral is bounded by crystal faces it is called subhedral . Euhedral minerals are rather rare in nature. The c/s of euhedral minerals as seen in their thin sections appear with polygonal boundary outline, whereas in the case of anhedral minerals, they appear rounded or surrounded in thin sections.

  11. Common Texture of Igneous Rocks

  12. Suitability of Igneous rocks for building and foundation

  13. Granite

  14. Pegmatite s

  15. Dolerite

  16. Thanks!

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