I was fortunate to visit the southern part of the East African Rift recently, South of Mt Suswa, beautiful flood trachytes (lava) along the N-S fissures of the rift floor. Reported to have erupted approximately 1 Million years ago (Pleistocene). Subsequent erosion has left these elongated volcanic features.
On the ground examining these rocks, you can see gas bubbles (vesicles) and large crystals (feldspar phenocrysts) in a fine matrix. This indicates there were likely two stages of cooling for the magma, firstly below the surface in the magma chamber, forming the large crystals, then during eruption it cooled quickly forming the finer matrix.
Added some references and a geological map of the area for those interested.
A Geodyssey - Geoscience Text Analytics and Enterprise Search Research 
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