Skip Navigation

Intrusive and Extrusive Igneous Rocks

Intrusive rocks cool slowly and extrusive igneous rocks cool rapidly; this leads to different crystal sizes.

Atoms Practice
Estimated4 minsto complete
Practice Intrusive and Extrusive Igneous Rocks
This indicates how strong in your memory this concept is
Estimated4 minsto complete
Practice Now
Turn In
Stone Tools

Stone Tools

Credit: Darron Birgenheier
Source: http://www.flickr.com/photos/darronb/10373219344/
License: CC BY-NC 3.0

Before humans figured out to make tools from metal, they made stone tools. Obsidian could be made into sharp and effective blades and arrowheads.

Why It Matters

  • In North America, obsidian is found in some volcanic regions of the West.
  • Obsidian comes from molten magma that cools quickly. It contains no crystals. (Obsidian is not a liquid; it is a non-crystalline solid.)
  • The composition of obsidian is about 70% silica so it is glassy and hard.
  • Obsidian is so viscous that it often forms domes.
  • Obsidian breaks with conchoidal fracture, smooth and curved.
  • The edges of the stone can be really sharp. This is beneficial for stone tools.
  • Credit: Unknown
    Source: http://commons.wikimedia.org/wiki/File:Arrowhead.jpg
    License: CC BY-NC 3.0

    Arrowhead made out of Obsidian [Figure2]

Explore More

With the links below, learn more about obsidian tools. Then answer the following questions.

  1. How do you get a chip of obsidian from a rock?
  2. What is the technique for pressure flaking?
  3. Why would ancient people go to the effort of getting rock to make tools from 1800 miles away?
  4. Were the obsidian blades made at Yellowstone where the rock was found or at Hopewell Mountain where the blades were used? What is the evidence?
  5. What do you think the enormous blades might have been used for?
  6. Why is obsidian a rock but not a mineral?

Notes/Highlights Having trouble? Report an issue.

Color Highlighted Text Notes
Show More

Image Attributions

  1. [1]^ Credit: Darron Birgenheier; Source: http://www.flickr.com/photos/darronb/10373219344/; License: CC BY-NC 3.0
  2. [2]^ Credit: Unknown; Source: http://commons.wikimedia.org/wiki/File:Arrowhead.jpg; License: CC BY-NC 3.0

Explore More

Sign in to explore more, including practice questions and solutions for Intrusive and Extrusive Igneous Rocks.
Please wait...
Please wait...