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
What Is Obsidian Rock?
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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