Clasts entrained in obsidian flows from Rock Mesa, Oregon

In fact, in some places you can find layers of entrained chunks of other rocks -- possibly picked up off the ground surface as the flow was moving -- that become incorporated into the flow itself. This layer within a large block of Rock Mesa rhyodacite lava is clearly different material than the dark, dense and glassy lava itself. The taffy-like flow may have flowed over these chunks and they stuck, getting worked into the layer as the flow continued to move.

Image: Erik Klemetti