IGNEOUS ROCKS | ORIGIN | STRUCTURES | TEXTURES

In 2023 We learn Igneous rocks origin structures textures also we learn about Types of structures and type of textures.

IGNEOUS ROCKS

● Igneous rocks are formed from the crystallization of molten material called magma/lava
which usually consists of a solution of the Earth’s most abundant elements, oxygen, and
silicon, with smaller amounts of aluminum, calcium, magnesium, iron, sodium, and
potassium. The major elements of common magmas combine, on cooling, to produce
different rock-forming minerals and other minor constituents.

● The history of igneous rock begins with the formation of magma. When sufficient melt
has been created and coalesced, buoyancy causes it to rise. The magma may rise to the
surface and extrude as lava or if it contains volatile, may explode onto the surface to be
widely distributed as volcanic ash. Most field evidence indicates that igneous rocks have
formed from upward-moving bodies of either magma, a mixture of magma and crystals,
magma and gas bubbles, or even solid rock. The rate, at which magma flows, depends
on the pressure gradient, magma viscosity, and the shape of the conduit.

● Magmas usually consist of many components. To study the solidification of
magma, it is necessary to understand the process of simultaneous crystallization of
different minerals. In magmas composed of several constituents, the special properties of
each constituent are modified by the presence of others. Magma may undergo the
process of crystallization at different cooling conditions at different depths. If cooling is
rapid, the process of crystallization is also rapid and thus the final product is in the form
of natural glass. On the other hand, if the cooling condition is slow, proportionally
crystallization is also a slow process. As a result, the product is coarse-grained.

Origin of Igneous Rock

Igneous is a Latin word derived from ignis means fire, in geology, fire means molten material i.e.
lava and magma. Magma can be derived from a partial melt of pre-existing rock in either earth’s crust or mantle. Magma comes out on the earth’s surface through fissures or volcanic eruptions, start
losing gases, and becomes lava. When both magma and lava cooled and solidified give rise to
igneous rock. If rock forms on the surface called extrusive igneous rock e.g. Basalt, while rock
forms below the surface is called intrusive igneous rock.
e.g. granite.

Structures and Textures of Igneous Rocks

Structures of igneous rocks are the megascopic features that can be identified by the naked eye,
include certain field features such as the ropy surface of the lava, pillow, amygdaloidal structures, flow
banding and spherulites. On the other hand, Texture is the arrangement of the mineral
constituents and glassy matter in a rock made up of an aggregate of minerals can be observed
by naked eyes and under the microscope.

1. Structures of igneous rock:

(i) Vesicular and Amygdaloidal Structure:

Most lava contains gases that escape as soon as pressure is reduced by their eruption at the surface. The escape of gases creates the molten material with the development of vesicles, cavities, or pores which may be elliptical, spherical, or cylindrical. The cavities are often isolated or poorly interconnected. Amygdales are the end product of the crystallization of lava flows consisting of infillings of vesicles by silica, carbonate, and zeolitic minerals. Basaltic lava flows commonly display the presencamygdalasdalas and are said to possess amygdaloidal basalt texture. If vesicles are not filled by secondary mineral and rock remain with vesicles then known as vesicular structure Ex vesicular basalt.

(ii) Ropy Lava Flows:

Very mobile lava often solidifies with very smooth surfaces that exhibit
wrinkled or ropy forms so these flows are called ropy / compound/pahoehoe lava. In this,
cavities/vesicles are smaller in size and mostly spherical.

(iii) Blocky Lava Flows:

Sometimes the surface is covered with a mass of rough, jugged
angular blocks of uneven dimensions. In this, the cavities are larger compared to ropy
lava and irregular in shape. The lava form is called blocky or lava.

(iv) Pillow Structure:

Pillow structure is found in basalts that erupted below water. They are
formed when lava is chilled rapidly by water. A chilled ring (outer surface/covering) forms on the
surface which is sufficiently flexible to move with the flow which remains molten inside. As lava
pressure increases, the rind breaks, causing the formation of a tubular lobe. The pillows may
vary from a few centimeters to several meters. They may be spheroidal, ellipsoidal, or even
flattened. The pillows have thin skins composed of tachylite which in some cases has been
altered by oxidation and absorption of water into yellowish-brown waxy-looking substances
called palagonite. The interstices narrow space between the pillows is often filled with fragments
of broken glassy lava skins. Many pillows have a radiating column structure and may show
concentric zonation of vesicular zones. Ancient pillows have been found in many localities of
India e.g. Mumbai – Bengaluru highway cuttings near Chitradurga.

(v) Flow Structure:

Layers and patches in lava differ slightly in composition, gas content,
viscosity and degree of crystallization. In the process of flow, these patches are drawn into
parallel lenticels, bands, and lines, which may be characterized by the development of various
proportions of vesicles, glass, and crystals of the slightly different compositions shown by changes in
color and texture.
In addition to these structures, jointing and columnar structures are seen prominently in many
igneous rocks.

2. Textures of igneous rock:

An accurate description of texture requires consideration of the following points: Degree of crystallization (crystallinity). Grain size (granularity). The shape of crystals and Mutual relations of crystals and glass. The two factors together are considered as fabric. The texture of igneous rocks is a function of crystallinity, granularity, and fabric.

(i) Crystallinity:

Crystallinity is a ratio of crystallized and non-crystallized matter. A rock
composed entirely of crystals is called holocrystalline. When it consists completely of glass, it is
called holohyaline, and when the rock is composed partly of crystals and partly of glass, it is
called Mesocrystalline. The holocrystalline texture is typical of plutonic igneous rocks and the
holohyaline is characteristic of lava flows or volcanic rocks. The mesocrystalline texture is
indicative of intermediate-depth rocks.
Crystallinity is a measure of the degree of cooling. Thus, holocrystalline is a product of slow cooling
and holohyaline occurs due to fast cooling.

(ii) Granularity:

Granularity is the absolute grain size of crystals in igneous rocks. If crystals are
visible in hand specimens, the rock is said to be phaneritic. On the other hand, the term aphanitic
is used when the crystals cannot be recognized by the naked eye.
Aphanitic rocks may be cryptocrystalline i.e. individual crystals are too small to be separately
recognized even under the microscope or they may be microcrystalline i.e. the individual
crystals are distinguishable only under the microscope.

Phaneritic rocks may be coarse-grained when the average grain size is more than 5 mm,
medium-grained when it is between 5 mm and 1 mm, and fine-grained when it is less than 1 mm.
Like crystallinity, granularity also depends on the rate of cooling. Thus, coarse-grained nature is
indicative of slow cooling and fine-grained is suggestive of fast cooling.

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