Assuming the mantle of eruption scale analyst

The scale of eruption in the Auckland Volcanic Field (AVF) appears to be controlled by the dynamics of the soft area (asthenosphere) of the upper mantle.

Modelling shows a correlation between melting in the soft area, the degree of melting and incorporation of the chemically altered uppermost solid section (lithosphere) of the mantle source, and the resultant size of the volcanic centre.

These data come from 12 centres in the volcanic field, reflecting the large variations in composition between volcanoes.

Whole-rock compositions range from subalkaline basalt in the larger centres, through to alkali basalt and then nephelinite in the smallest centres.

Modelling reveals that melting in the AVF begins in the garnet-bearing fertile soft area of the upper mantle, with preferential melting of metamorphic rock (eclogite) domains.

It further shows that melts are variably diluted by melts of the lithospheric source.

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