Sir please explain the last 4 lines ( question given in paragraph).

Eduncre Geology (Petrology Partial melting of the asthenosphere takes place, resulting in a mafic magma. In most cases, melting occurs because the subducted oceanic crust releases water into the asthenosphere. The water collected in the oceanic crust when it was beneath the ocean and is driven out as the descending plate is heated. The water lowers the melting temperature of the ultramafic rocks in this part of the mantle. Partial melting produces a mafic magma. But how can we keep producing magma from ultramafic rock ? after those rocks have been depleted of the constituents of the mafic magma ? The answer is that hot asthenospheric rock continues to flow into the zone of partial melting. As shown in figure above asthenospheric ultramafic rock is dragged downward by the descending lithospheric slab. More ultramafic 1 alhaia tonaco the descending material. A continuous flow of hot, "fertile" (containing the