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Obsidian for Tools: Conchoidal Fracture

Obsidian for Tools: Conchoidal Fracture
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Obsidian Landscape by Shari Maria Silverman is licensed under a Creative Commons Attribution 3.0 Unported License.Obsidian Landscape (Newberry Crater's Big Obsidian Flow)

Obsidian Landscape (Newberry Crater's Big Obsidian Flow): Broken rocks from this large flow contain numerous examples of conchoidal fracture. Photo taken July 11, 2011.

For thousands of years, throughout the globe, people have used obsidian for tools and mirrors. Its glassy properties ease its shaping and create smooth, shiny, reflective surfaces with sharp edges:

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Obsidian Conchoidal Fracture with Split by Shari Maria Silverman is licensed under a Creative Commons Attribution 3.0 Unported License.Obsidian Conchoidal Fracture with Split

Obsidian Conchoidal Fracture with Split: Ripples travel away from the initial point where split began. Photo taken July 11, 2011

  • Obsidian flows in conjunction with pumice
  • Obsidian and pumice separate into layers
    • Obsidian: no gas so no bubbles
    • Pumice: gas so lots of bubbles
  • Both obsidian and pumice cool into rocks before their molecules can organize into crystals
  • The lack of crystals and froth caused by gas form obsidian’s smooth surface with sharp edges when broken (knapped)
  • The glassy texture also allows the knapper to shape the rock

A common break scar seen on cores and other tools is the conchoidal fracture. Although they appear on other rocks, such as fine-grained basalt; they are particularly visible on obsidian due to its glassy texture. They occur both naturally and culturally:

  • They emanate from the break’s starting point of stress or strain
  • They exhibit curved ripples, which grow larger and from the original break (like ripples in a pond, which travel outward from where a raindrop plunked into it)
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Obsidian with Conchoidal Fracture Close Up by Shari Maria Silverman is licensed under a Creative Commons Attribution 3.0 Unported License.Obsidian with Conchoidal Fracture Close Up

Obsidian with Conchoidal Fracture Close Up: This photo shows the concentric rings emanating to the lower left from the initial battering point (upper right): Photo taken July 11, 2011.

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Obsidian for Tools: Conchoidal Fracture by Shari Maria Silverman is licensed under a Creative Commons Attribution 3.0 Unported License.


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