ALBERT

Description: Decompression in the magma chamber due to rapid magma extraction triggers caldera collapse. The roof rock overlying the magma chamber is detached by the development of a ring fault, and then it subsides into the magma chamber by the differential pressure between the lithostatic in the roof and magmatic in the chamber. The subsidence of caldera blocks into the magma chamber causes rapid magma extrusion from the magma chamber resulting the eruption of voluminous ignimbrite. Therefore, magma chamber decompression is the key to understanding the evolution of caldera-forming eruptions (CFE). Here, we examine the decompression process of the magma chamber during CFE based on the sequential variation of the volatile contents in the glass inclusions in the products of several CFEs. We find that some CFEs experienced large magma chamber decompression toward the caldera collapse, whereas some start to collapse with minor decompression in the magma chamber. This variation may represent differences in the fracture strength of the roof rock of the magma chamber. Shallow magma chambers with thin roof rock will collapse with small decompression, whereas deep magma chambers with thick roof rock require larger decompression for caldera collapse. Since the decompression of the magma chamber depends on the volume fraction of magma erupted before the caldera collapse, the structure of the magma chamber, such as the thickness of the roof rock, may be used to predict the variation in the eruption patterns towards the onset of caldera collapse and the eruption of voluminous ignimbrites.