Continents bear testimony to long-term differentiation of the planet. On the other hand, there is short-term practicality in assessing the timing of volcanic eruptions. I here focus on timescales from a few million to a few tens of millions of years to probe magmatic processes in the crust. Which processes act, when and where? 

The overall pattern is onset of bimodal mafic and silicic volcanism with crustal melting and magma mingling. A pulse of voluminous hybridized dacite (AFC, mixing) marks climax and the episode closes with a crystal fractionated suite. The similarity of the sequence, regardless of volume or of tectonic setting indicates that the driver is fundamentally basalt (thank you Wes), delivering mass and heat to drive the system, making mafic recharge the most important process in crustal magmatism. Two of many examples follow. 

In the central Basin and Range, 40-32 Ma magmatism, related to the southward sweep of Farallon Plate delamination crustal subsequent extension, yielded ~7000 km³ of volcanic rocks (DRE). Early basalts mixed with crustal melts to yield minor xenocryst-bearing mafic volcanics with dominant mingled dacites and sparse rhyolites. Extensive mixing and hybridization yielded the dominant pulse of bi-hb dacites during thermal climax. Late-stage waning of the episode is marked by a basalt to rhyodacite suite with rare hydrous phenocrysts dominated by crystal fractionation. Petrologic mass balance requires at least double the erupted volume in basalt to materially and thermally maintain the volcanic event, neglecting intrusive complements of the felsic suite. Recharge is the dominant driver.

Ten million years of volcanism at the Aucanquilcha volcanic cluster (~300 km³) in northern Chile tells a similar tale. Early crudely bimodal pyroxene andesite-rhyodacite volcanism gave way to the largest pulse in bi-hb dacite volcanism after about 6 my. Late stage volcanism was more felsic and centralized. Quenched mafic inclusions and rare peripheral basalt are the only record of what must have been a vigorously recharged and underplated system. By analogy to the remarkably similar Tuolumne Intrusive suite, the intrusive to extrusive ratio of the felsic suite is about 50. The mafic recharge to sustain this 3-25 km-deep system must be formidable and requires magmatic reworking of the crustal column.