Volcanoes and Igneous Rocks: Formation, Characteristics, and Classification

VOLCANOES AND IGNEOUS ROCKS
Introduction
The three classification of rock based on their formation include sedimentary rock, igneous rock
and metamorphic rocks, where igneous rocks are formed in volcanic areas through a process of
solidification and cooling of the magma, whereas sedimentary rocks formation is through
lowering of temperature to accumulate sediments in tectonic basins, while metamorphic rock
formation depend on subjection of higher temperature and higher pressure to an original existing
rock.
The paper mainly concentrates on the formation and characteristic of igneous rock where
igneous rock derives its name from the word ignis that means fire, a clear indication that there
formation are traced through process of volcanism, with an association to resources of
geothermal deposits of the hydrothermal example being porphyry.

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Examining Igneous rocks
The formation of igneous rock is through process of cooling and solidification of the magma,
where melting of the original rock from the mantle or crust form magma, the process that will
ensure that the original rocks are melted are when the rocks are subjected to higher temperature,
low pressure and change in composition of the rock. It now clear that the formation of igneous
rocks is banked to volcanic activities and therefore their distribution will be under the control of
plate tectonics.
The plate tectonic accounts for well-established various varieties of igneous rocks and also their
associate distribution (Carlson et al, 2008).
Divergent type of plates in oceanic crust is associated with the formation of basalts and gabbros
type of igneous rocks in places like mid Atlantic ridges, whereas in convergent type of plate is
associated with the formation of granites and andesite type of igneous rocks in places like South
America and Indonesia.
Igneous rocks are classified in two categories, which includes; plutonic rocks and volcanic rocks,
where plutonic are formed as a result of cooling and slow crystallization of the magma on the
Earth’s crust, example of the rock that is associated with this category is the granite.
The second category type of igneous rock which is volcanic rock associates their formation with
magma traversing to the surface in a form of lava or ejecta, example of volcanic type of igneous
rocks are pumice and basalt. During the process of cooling of the magma there are formation of
arrangement type of reaction known as a series of Bowen’s reaction, which helps in describing
the textures and minerals associated with igneous rocks formation.

Bowen’s reaction series and igneous rock formation.
Figure 1: Bowen’s reaction series (Ehlers and Blatt, 1997)
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Characteristic of igneous rocks based on texture
Igneous rocks are characterized to their color, texture and composition which results to formation
of different types of igneous rocks. The three characteristic depend on the chemistry and the
environmental nature of the deposition of the magma.
1. Texture
When the process of cooling magma is slow, it results into formation of large crystals and
therefore the rock will form phaneritic type of texture, on the same point if the process of cooling
the magma is fast then it will result to formation of small crystals and therefore the rock will
form glassy type of texture where no mineral will be formed in such away.
It is from the texture characteristics that the igneous rock can be classified as either intrusive type
of rock or extrusive type of rock, example both the intrusive type of igneous rock and extrusive
type of igneous rock are shown on figure 2 below, where intrusive type of igneous rocks are
formed by solidification of magma before it transverse to the surface and therefore their textures
will be coarse grained, while extrusive type of igneous rock are formed when magma solidifies
on the surface, therefore their texture will be classified as fine grained.
Data