Friday, July 18, 2014

Friday Rocks #20: Boudinaged Obsidian

Obsidian is a volcanic glass, lava that has cooled so quickly that crystals don't form. In Panama Crater, Mono Lake, California this lava was partially cooled. Stresses pulled the partially solidified obsidian apart, like taffy, forming what Structural Geologists call "boudinage". The less cohesive rock surrounding the boudining obsidian filled the gap. Other obsidian layers above and below the boudined layer have "bent" and deformed towards the boudin.

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Friday, July 4, 2014

Friday Rocks #19: Ammonite

Here is a Ammonite fossil in cretaceous sandstone outside of Manitou Springs, CO

Photo by Tim Sherry 

Friday, June 27, 2014

Friday Rocks #18: Pencil Cleavage

Pencil Cleavage is developed through the intersection of cleavage and bedding planes. The fractures form long, skinny pieces of rock.

Friday, June 20, 2014

Friday Rocks #17: Fault Core Shale

This vertically dipping fault shows deformed shales in the core. On the right side of the photo slickenlines on the carbonates are seen. Bedding has been rotated to vertical. The shales are folded and fault by motion on the larger fault.

Friday, June 13, 2014

Friday Rocks #16: Dacite textures

Here are some of the lava textures on Bishops Peak Mt., San Luis Obispo. The chain of dacitic volcano known as the "Nine Sisters" is associated with the shut down of subduction along the California Coast.

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