This Mesa is sitting on what was once a volcanic basalt flow. The large boulders that you find all over the mesa are the result of a very fluid volcanic lava flow. The lava came up through ground fissures (which are basically, cracks in the ground). It spread out and cooled very quickly. Because it came up out of the earth, we call that extrusive. igneous When the igneous (fire born) rock cools quickly, the individual minerals do not have time to gather together and form crystals. An example of a flow where the lava cooled slowly would be granite. Granite forms within the earth and is called intrusive (inside the earth) igneous rock.

For a short video on intrusive and extrusive igneous rocks, visit this K-12 education site.

The USGS site has a really good graphic showing the difference in crystal accumulation of different igneous rocks.

Fun Facts:

• The holes that you see in the Basalt were once bubbles of trapped gas.
• Basalt is the most common type of rock in the Earth’s crust.
  • The ocean floor is made of basalt
  • The Hawaiian Islands are made of basalt