Structure of igneous rocks

1. Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)
Md. Yousuf Gazi
Lecturer
Department of Geology
University of Dhaka

2. The structures of igneous rocks are large scale
features, which are dependent on several factors
like:
(a) Composition of magma.
(b) Viscosity of magma.
(c) Temperature and pressure at which cooling and
consolidation takes place.
(d) Presence of gases and other volatiles.
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)

3. 1. Vesicular and Amygdaloidal Structure
2. Block Lava and Ropy Lava
3. Pillow Lava
4. Jointing, Sheet Structure
5. Columnar Structure
6. Pegmatites
7. Spherulitic
8. Orbicular
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)
Seven common igneous rocks structures:

4.  When lavas heavily charged with gases and other volatiles
are erupted on the surface, the gaseous constituent's escapes
from the magma as there is a decrease in the pressure.
 Near the top of the flows in the material , empty cavities,
bubbles, vesicles are formed, which may elliptical, spherical,
cylindrical or irregular in shape.
 The individual openings are known as vesicles and the
structure as a whole is known as vesicular structure.
 If, however, the vesicles thus formed are subsequently filled
in with some low-temperature secondary minerals, such as
calcite, zeolite, chalcedony etc., these infillings are called
“amygdales”.
1. Vesicular and Amygdaloidal Structure
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)

5. 2. Block Lava and Ropy Lava
 Since lavas of acidic composition, due to their
high viscosity, do not flow to greater distances,
after solidification they are found to offer a very
rough surface. Such lava flows are known as
block lava. It is also known as 'aa' structure.
 Lavas of basic composition are quite mobile
because of their low viscosity and they can flow
to greater distances and after solidification offers
very smooth surface. Such lava flows are known
as ropy lava and are also known as 'pahoehoe'
structure.
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)

6.  A distinctive form of lava flow in which the solidified rock
forms rounded masses with a glassy exterior.
 Pillow lava is found where lava flows into the ocean or a
lake and quickly cools.
 The liquid lava forms a sphere when it enters the cold water
as a result of surface tension. These hot balls of quickly
solidifying lava settle to the ground quickly.
 As they settle, they flatten and become pillow like in form. As
more and more pillows settle to the seafloor, the weight
flattens them even more. Pillow lavas therefore form piles
around the vent or entry point of a lava flow. They make up
much of the oceanic crust.
3. Pillow Lava
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)

7. • Joints are divisional planes which are found
in all kinds of igneous rock impressed upon
them mainly by force acting from outside.
Joints plains may be vertical or horizontal.
• The horizontal joints planes are sometimes so
closely spaced as to produce a sheet
structure. The sheets are commonly thinner
as the surface of the ground is approached
and they usually show some degree of
parallelism.
4. Jointing, Sheet Structure
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)

8. With uniform cooling and contraction in a
homogenous magma, the parting planes tend
to take on a regular or prismatic form,
Characterized by the development of four,
five or six sided prisms, which may be
intersected by cross-joints.
5. Columnar Structure
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)

9.  Pegmatites are extreme igneous rock that form
during the final stage of a magma’s
crystallization. They are extreme because they
contain exceptionally large crystals and they
sometimes contain minerals that are rarely
found in other types of rocks.
 To be called a “pegmatite,” a rock should be
composed almost entirely of crystals that are at
least one centimeter in diameter. Granite and
Syenite more often show this type of structure.
6. Pegmatites
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)

10. Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)
• Spherulitic aggregate (Spherulites) are radiating
arrays of fibrous (needle-like, acicular) crystals that
are common in glassy felsic volcanic rocks.
• Spherulites are typically two-mineral aggregates (mainly
quartz and feldspar), formed by initial Spherulitic growth of
one mineral and later crystallization of a second mineral
from the liquid or glass between the fibres.
7. Spherulitic

11. Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)
• Orbicular granite (also known
as orbicular rock or orbiculite) is an
uncommon plutonic rock type which is
usually granitic in composition.
• These rocks have a unique appearance
due to orbicules - concentrically layered,
spheroidal structures, probably formed
through nucleation around a grain in a
cooling magma chamber
8. Orbicular