The plume surrounding a source region for magma may either compact by elastic or viscous deformation when magma leaves the source region. It is shown that the compaction is elastic. The elastic deformation implies that only a small volume fraction of the melt can leave the source region. On the other hand, the isotopic secular disequilibria show that the fraction melt must be much larger. This discrepancy is solved if the source region is veined and consists of intercalated veins and residuum, with the veins forming about 1% volume of the source region. The approximate width of the source region is estimated to be 10-20 km and its height is 1-2 km. An eruption starts when a dike is formed above the source region because of overpressure of the melt within the source region. During the repose time, the overpressure of the melt in the source region increases because of the replenishment of melt from the plume situated below.