EXAMPLE
The photograph below is of a scoriaceous basalt with bands of rhyolite pumice. The sample is the result of two magmas of different composition coming into contact at or before the time of eruption. Cooling after eruption was rapid enough to "freeze" the mingling-action in place. What we see is incomplete mixing of the two magmas due to the density and viscosity contrasts that exist between them.
The two magmas (rhyolite and basalt) that formed the rock shown above may have come into contact via the eruption process or they may have occupied the same chamber prior to the eruption. In other words the pre-eruption situation may have involved two separate, but physically connected chambers, or a single, compositionally stratified magma chamber (shown below).
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