Obsidian Flow Fabrics

3D Model

This outcrop in Panum Crater shows beautiful flow features that developed as hot obsidian lava extruded from a source vent. The layers of darker obsidian flowed more easily than the layers of lighter obsidian, causing the more resistant rock to neck and finally separate into boudins. However, the lighter obsidian was still somewhat ductile, and edges of the coherent blocks have been drawn out into the black obsidian. The color of the obsidian layers isn’t really related to the chemical composition - frothier layers with abundant tiny gas bubbles appear white, and the black layers had this magmatic gas escape due to shearing. The expulsion of gas from silica melt during decompression drives Earth’s most violent eruptions.