Obsidian Rock – Meaning, Uses, Facts, Properties & Color
We know three main classes of rocks, sedimentary, igneous, and metamorphic. In the Limestone article, we learned how sedimentation occurs and the formation of sedimentary rocks takes place. Today, we are going to know Obsidian rock, which falls in the main class of igneous rocks.
We know igneous rocks have two basic types:
1. Intrusive Igneous Rocks
When molten lava/magma is cooling slowly below the surface
of the earth and forms large crystals in the rocks, they are known as
intrusive igneous rocks. For example-
- Diorite
- Gabbro
- Granite
- Pegmatite
- Peridotite
2. Extrusive Igneous Rocks
When molten magma is cooling rapidly onto the surface of the
earth and forms small crystals in the rocks, they are known as extrusive
igneous rocks. For example-
- Obsidian
- Andesite
- Basalt
- Dacite
- Pumice
- Rhyolite
- Scoria
- Tuff
- Fire
Opal
Obsidian is a glassy igneous rock that comes in a wide
range of colours and textures. It forms when felsic lava extruded from a
volcano onto the surface of the earth.
The cooling of lava takes place so rapidly that no chance
remains for proper crystallization (Atomic Arrangements) and finds as an
amorphous glass material. It has a smooth and uniform texture, which breaks
with a conchoidal fracture.
Geological Classification of Obsidian Rocks:
Let’s see the classification of obsidian rocks from the
perspective of a geologist.
- Class: Igneous Rocks
- Sub-Class: Medium Hardness Rocks
- Family-Group:
Volcanic
- Category:
Opaque Rocks
- Alternate
Names: Lava, Glassy Lava, Xaga, Royal Agate, Glass Agate, Volcanic
Glass, Glass Gem, Guardian Gem, and Black Beauty
Formation of Obsidian Rocks in Nature
We have seen in the earlier section that obsidian is a type of extrusive rock. It forms on the surface of the
earth when magma or lava comes out due to volcanic phenomena and rapid cooling
takes place. Obsidian rock can form in a variety of cooling environments such
as-
- Along
the edges of a flow of magma or lava
- Along
the edges of a dome of the volcano
- Along
the edges of a sill or dike, which takes place under the surface
and it is the only intrusive type
- The
point where magma comes into contact with water on the surface
- The
point where magma cools due to airborne cooling effects
Properties of Obsidian
Obsidians have various physical & visual properties,
such as Hardness, Grain Size, Fracture, Streak (Colors), Porosity, Luster, and
Strength. These all property attributes determine the applications of obsidian
rocks. Let’s see some significant properties of obsidians.
Specific Gravity & Density of Obsidian
The density of rock is used to express specific gravity. It
is measured in relevance to the density of water in gram per cubic centimeter
of the mass. The density of obsidian is 2.55 g/cubic cm. It means it is heavier
than water and dense medium as a mass.
Streaks & Colors of Obsidian
The streak of obsidian is white, but various colors occur in
nature.
Frequently found colors of Obsidian rocks are:
The highest occurrence of obsidians is in Black, but it is
also available in Brown, Green, or Tan.
Rarely found colors of Obsidian rocks are:
A very rare occurrence of obsidians is Blue, Orange, Red,
and Yellow.
The occurrence of color is due to the presence of different
trace elements in obsidian rocks.
Compressive Strength of Obsidian
When a compressive pressure (In Newton unit) is applied to
the rocks, a permanent deformation of rocks takes place at a point of pressure.
It is termed as the compressive strength of rocks and obsidians fractured or
permanently deformed at 0.15 N per square millimeter. It means obsidian is easy
to break and chipped by some impacts. It also renders obsidian the unfit stone
for rings and bracelets like jewelry production.
Obsidian Fracture
Obsidians break into the conchoidal fracture. Obsidian has
no natural or defined planes of separation when a pressure exceeding its
physical strength exerts on it. Therefore, smoothly curved fracture takes place
on the surface of obsidian glass rock.
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