Stepwise accumulation and ascent of magmas

One of the currently popular theories on magma ascent is that it mainly occ urs by propagating hydrofractures (dykes) and that magma viscosity is the p rimary rate-controlling factor. This theory is based on mathematical models for single hydrofractures under idealised conditions. We simulated magma a scent with air ascending through gelatine and observed that the air ascende d in batches, following paths made by their predecessors. Multiple batches accumulate at obstacles along the path. Although magma viscosity may contro l ascent rate during movement, obstacles ultimately control the size and av erage ascent velocity of ascending batches. We propose that step-wise movem ent of magma batches is the mechanism of primary accumulation and ascent fr om the partially molten source rock of a magma to its first emplacement sit e and therefore the main ascent mechanism for granitic magmas. 'Classical' dyking is the mechanism for secondary ascent from a magma chamber.