The prevailing hypothesis is that the uniformity in their erupted products is related to the flux of mantle-derived magma entering the crust the volcanoes receive enough thermal input to sustain thick, thermochemically steady-state gabbroic mush zones in the mid-crust, which interact with ascending magmas, buffering their temperature and composition 8. Monotonous volcanoes on Fernandina and Isabela are located near the centre of the inferred Galápagos plume 29.
However, historic lavas at each individual centre have pre-eruptive storage temperatures within a range of only ~22–30 ☌ 17, 28. All of the basalts emplaced during monotonous Galápagos eruptions have undergone extensive olivine, clinopyroxene and plagioclase crystallisation 8, 26, 27. These include Fernandina volcano (on the island of the same name) and Wolf, Darwin, Ecuador, and Sierra Negra volcanoes on Isabela Island (Fig. The western Galápagos Archipelago is an ideal location for studying compositionally monotonous volcanism because it hosts several volcanoes that have erupted near-homogeneous basaltic magmas for several millennia 8, 17, 24, 25. In this case, monotonous activity necessitates an exact thermal balance between the heat supplied by ascending magma and the heat lost by advection and eruption.
The second model often includes repeated recharge of upper crustal magma bodies by chemically consistent melts ascending from depth, maintaining the crustal system within a narrow temperature and compositional range 7, 10, 11, 23. Geophysical and geobarometric constraints on the crustal structure beneath monotonous volcanoes are also inconsistent with single large storage regions 9, 19, 20, 21, 22.
#Shield volcanoes diagrams series#
In many cases, however, the products of monotonous eruption series contain multiple macrocryst populations that record complex growth histories. The first model typically requires hot primitive melts to cool and react as they ascend through a super-solidus mush column, buffering their temperature and composition 1, 8, 17, 18. The causes of monotonous volcanism are not currently well understood, but constraining the architecture and dynamics of monotonous systems is essential for determining their longevity and identifying the potential for future changes in eruptive behaviour that may result in more hazardous activity 15.Ĭurrent models for producing chemically monotonous eruption sequences typically involve: (1) uniformity by reactive filtration 8, whereby distinct magma batches interact with surrounding gabbroic material to form chemically homogeneous products or (2) uniformity by processing, whereby successive batches of ascending magma evolve under the same P-T conditions 16. Despite this multitude of sub-surface processes, many magmatic systems exhibit remarkably monotonous volcanic behaviour, erupting chemically homogeneous liquids over long timescales (several decades to millennia) 7, 8, 9, 10, 11, 12, 13, 14. These processes modify magma compositions, creating significant diversity in the chemistry of igneous rocks observed at the Earth’s surface 6. As melts ascend through the lithosphere they undergo diverse processes, including crystallisation, volatile exsolution, assimilation of the surrounding country rock, mixing between different magma batches, and interaction with mush crystallised from previous magmas 3, 4, 5. Volcanoes are underlain by complex and dynamic magmatic systems that often span tens of kilometres of crust from the Moho to the near-surface 1, 2. This process will only maintain monotonous activity while the volume of melt entering the crust is high, raising the possibility of transitions to more silicic activity given a decrease in the crustal melt flux.
Evolved melts are in low abundance and large volumes of basalt flushing through the crust from depth overprint their chemical signatures. In contrast with the uniform basaltic magmas erupted at the surface over long timescales, we find that the sub-volcanic systems contain extreme heterogeneity, with melts extending to rhyolitic compositions. We combine petrological analyses of erupted crystals with new thermodynamic models to characterise the diversity of melts in magmatic systems beneath monotonous shield volcanoes in the Galápagos Archipelago (Wolf and Fernandina). This monotonous activity is thought to reflect a high degree of chemical homogeneity in their magmatic systems, leading to predictable eruptive behaviour. Many volcanoes erupt compositionally homogeneous magmas over timescales ranging from decades to millennia.
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