Obsidian is extrusive.
The article that follows explains the difference between intrusive and extrusive, and how it is that obsidian qualifies.
Is Obsidian Intrusive or Extrusive? (EXPLAINED)
Obsidian has long been associated with ancient cultures.
It was treated as both a valuable asset and as an instrument.
It’s rarity even in early times before modern mining and excavation made the glassy rock a high prize to chase after and treat as a rarity.
However, naturally, the rock is no more special than any other element or mineral created by volcanic processes.
What makes Obsidian unique, however, is the very nature of the rock type in how it cooled and formed.
As a product of volcanic activity, in particular lava and magma processes, obsidian is a type of volcanic glass.
To get to this state, it needed to have changed from lava form, molten rock, to a crystal form in a very short amount of time.
That typically involves fast cooling, such as making contact with water, a natural heat sink.
Technically, however, obsidian is no different from granite in that they are both igneous rock.
Intrusive or Extrusive?
In technical terms, Obsidian falls into the category of extrusive stone.
This labeling is applied because Obsidian occurs when lava comes out of a volcano, and is a product of that volcanic process.
If Obsidian stayed inside the volcano, then it would be known as intrusive instead.
The creation of Obsidian also happens with a specific type of lava as well.
Magma, based on the type of rock it consists of, can be heavy or light.
Lighter magma, known as felsic lava, occurs when a combination of air, aluminum, potassium, silicon, and sodium combine.
In fact, the lava type is so specific, its regularly referred to as rhyolitic lava or a more layman terms, an obsidian flow.
As the lighter lava exudes out, it moves with a very high viscosity.
This blocks the typical rocky clumping of heavier lava when it cools, and instead the volcanic material forms as a crystal, ergo glassy obsidian.
The structural form, while it hardens in crystal formation, is not as strong as heavier rock.
Instead, Obsidian is extremely brittle, prone to warping with impact, and fracturing.
Many folks find this out when they crack open a lava bomb, a very roundish hurled blob of Obsidian that cools fast, usually because it landed in water or cold material after being shot out of a volcano.
When impacted with a sharp point, the Obsidian rock will fracture and split open.
But on close inspection, one will actually see the ripple of impact through the Obsidian rock in the form of emanating pressure waves in the rock.
It’s one of the few rocks that a human can create this effect.
A Natural Tool
The glassy nature of Obsidian has been extremely useful to people.
From its very early generations, humans have realized Obsidian, as an extrusive byproduct of volcanoes, has been incredibly valuable.
The earliest form of Obsidian was used as broken flakes, either accidentally or intentionally broken off larger forms of the rock.
When fragmented down to smaller pieces, Obsidian make the perfect cutting tool.
Like other forms of glass, Obsidian flakes cut fine and quickly, which made it a functional tool for sizing down food material and other organic matter into useful components.
Whether it was cutting and trimming meat or fashioning wood tools, Obsidian has left a very long fingerprint in history.
In later centuries, the stone also became useful for instruments.
Everything from weapon material to intricate surgical tools were made from Obsidian.
While the rock was not useful as an impact material or penetrating hard targets (or for making obsidian swords), it was very useful for separating and piercing smaller tools.
Modern Times, Unchanged Conditions
Today, Obsidian still occurs naturally in and around volcanoes.
It can be found in new magma issuances in Honduras, the South Pacific, Africa, Iceland, under the ocean and similar.
Where it’s practical, Obsidian is still used for tools, but it’s also used extensively for jewelry and accessories as well.
The rock is most often found in black, glassy form.
However, there are other colors of Obsidian as well. For example, a mahogany Obsidian can easily be found in the American Northwest, and it’s often collected and sold for rockhound collections because it is so different from the standard black form.
The history of Obsidian isn’t over by any means.
However, the glassy rock has taken a backseat to modern tool design and materials that are far stronger and can be designed to a higher degree of exactness.
That said, extrusive Obsidian continues to be functional and, when tested, proves it’s worth every time.
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