<P> Flow banding is a geological term to describe bands or layers that can sometimes be seen in rock that formed from the substance molten rock or magma . </P> <P> Flow banding is caused by friction of the viscous magma that is in contact with a solid rock interface, usually the wall rock to an intrusive chamber or, if the magma is erupted, the surface of the earth across which the lava is flowing . </P> <P> The friction and viscosity of the magma causes phenocrysts and xenoliths within the magma or lava to slow down near the interface and become trapped in a viscous layer . This forms laminar flow, which manifests as a banded, streaky appearance . </P> <P> Flow banding also results from the process of fractional crystallization that occurs by convection if the crystals that are caught in the flow - banded margins are removed from the melt . This can change the composition of the melt in large intrusions, leading to differentiation . </P>

What type of eruption formed this flow-banded obsidian