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1. Structure of Igneous Rocks
• The term structure is very much different from texture. Structure is
used for larger features of a rock, observed in the field on large
outcrops like flow banding, layering, vesicles, etc.
• Structure are included certain large scale features, such as, blocky or
ropy structure of lava, pillow structure, flow- banding, joint structures
etc..
• Structure also denotes some small scales features which are due to
the juxtaposition of more than one kind of textural aggregate within a
rock, such as amygdaloidal and spherulitic structure.

2. Vesicular and Amygdaloidal Structures
Vesicular structure Amygdaloidal structure
• When lavas heavily charged with gases and other
volatiles have erupted on the surface, the gaseous
constituents escape from the magma as there is a
decrease in the pressure. Thus, near the top of flows,
empty cavities of variable dimensions are formed.
The individual openings are known as vesicles and the
structure as a whole is known as vesicular structure.
• 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’ lavas containing
amygdales are said to have
amygdaloidal structure. These are
the typical character of a basic lava
flow.

3. Block lava
• Because lava of acidic composition, due to their high viscosity, does
not flow to greater distances, they are found after solidification to
offer a very rough surface. The surface is covered with the mass of
rough jagged, angular blocks of all dimensions. Such lava flow is called
block lava. It is also called the “aa” structure.
Ropy lava
• The lava of basic writing is highly mobile due to its low volume and can flow to greater
distances and after solidification provides a very smooth surface often highly glazed
which exhibit wrinkled, ropy or corded forms that often resemble with the twisted braids of
ropes. Such lava flows are called lava and are also called ‘pahoehoe’ structure.

4. Flow Structure
• After lava debris has erupted on the surface the species are
slow, flowing from one place to another with great difficulty and
in an attempt to do so, the uneven patches within the lava are
drawn. out in the form of long lenticels.
• Sometimes the crystalline particles already inside the magma
are arranged parallel to the direction of the lava flow. They
naturally indicate the direction of flow of the mass, before it is
fixed. These are more commonly known as steering structures
or flow structures.

5. Pillow Structures
• It is made up of the shape of a remote pillow. Mass was piled on top
of each other. These are made by taking lava into wet air, under ice
sheets, under water-filled sediments, or in seawater. Spilite, albite-
rich lava (sodium-rich) usually exhibits a pillow structure. Pillows
generally have a vesicular crust or glassy skin. Chilled margins develop
at the peripheral portion of pillow due to sudden cooling.

6. Orbicular structure-
• These spheres differ in that they are concentrated shells of different mineral compositions and
textures, which are occasionally found in granite rocks.
• It is an intrusive igneous rock (usually granitic in composition) that features concentrically layered,
spheroidal structures, known as orbicules.
Spherulite structure-
• Its essential feature is the simultaneous crystallization of fibrous diseases with radiating
arrangements about a common center. The large sphere is known as ‘lithophytes’. In lava and
infiltrates, they are called ‘ovarioles’ and contain variolates in the rocks.

7. Platy structure-
• This is also due to the development of different sets of joints, which
cause only blades of rock mass, to hit the rock. Such a feature is
called a platy structure.
Columnar structure-
• As a result of shortening due to cooling, a few sets of vertical joints
develop. Such joints result in the formation of columns, which may be
square, rectangular, rhombic, or hexagonal in outline.
Sheets Structure-
• The development of a single set of joints is well defined, sometimes
having a chipping effect on the body of igneous rocks. If all these
slices are flat, the structure is said to be a leaf structure.

8. Jointing structure
• In igneous rocks, jointing is generally quite irregular; but in granite, two vertical
sets forming right angles to one another on the top surface and another set of
cross joints approximately horizontal occur frequently. (These cross joints are the
effect of weathering.) Intrusions of molten rock, when cooled, form sills and dikes,
which, in many places, show columnar jointing.