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Abstract Maunaloa—the largest active volcano on Earth—erupted in 2022 after its longest known repose period (~38 years) and two decades of volcanic unrest. This eruptive hiatus at Maunaloa encompasses most of the ~35-year-long Puʻuʻōʻō eruption of neighboring Kīlauea, which ended in 2018 with a collapse of the summit caldera and an unusually voluminous (~1 km3) rift eruption. A long-term pattern of such anticorrelated eruptive behavior suggests that a magmatic connection exists between these volcanoes within the asthenospheric mantle source and melting region, the lithospheric mantle, and/or the volcanic edifice. The exact nature of this connection is enigmatic. In the past, the distinct compositions of lavas from Kīlauea and Maunaloa were thought to require completely separate magma pathways from the mantle source of each volcano to the surface. Here, we use a nearly 200-yr record of lava chemistry from both volcanoes to demonstrate that melt from a shared mantle source within the Hawaiian plume may be transported alternately to Kīlauea or Maunaloa on a timescale of decades. This process led to a correlated temporal variation in 206Pb/204Pb and 87Sr/86Sr at these volcanoes since the early 19th century with each becoming more active when it received melt from the shared source. Ratios of highly over moderately incompatible trace elements (e.g. Nb/Y) at Kīlauea reached a minimum from ~2000 to 2010, which coincides with an increase in seismicity and inflation at the summit of Maunaloa. Thereafter, a reversal in Nb/Y at Kīlauea signals a decline in the degree of mantle partial melting at this volcano and suggests that melt from the shared source is now being diverted from Kīlauea to Maunaloa for the first time since the early to mid-20th century. These observations link a mantle-related shift in melt generation and transport at Kīlauea to the awakening of Maunaloa in 2002 and its eruption in 2022. Monitoring of lava chemistry is a potential tool that may be used to forecast the behavior (e.g. eruption rate and frequency) of these adjacent volcanoes on a timescale of decades. A future increase in eruptive activity at Maunaloa is likely if the temporal increase in Nb/Y continues at Kīlauea. 

Chemical weathering influences many aspects of the Earth system, including biogeochemical cycling, climate, and ecosystem function. Physical erosion influences chemical weathering rates by setting the supply of fresh minerals to the critical zone. Vegetation also influences chemical weathering rates, both by physical processes that expose mineral surfaces and via production of acids that contribute to mineral dissolution. However, the role of vegetation in setting surface process rates in different landscapes is unclear. Here we use 10Be and geochemical mass balance to quantify soil production, physical erosion, and chemical weathering rates in a landscape where a migrating drainage divide separates catchments with an order-of magnitude contrast in erosion rates and where vegetation spans temperate rainforest, tussock grassland, and unvegetated alpine ecosystems in the western Southern Alps of New Zealand. Soil production, physical erosion, and chemical weathering rates are significantly higher on the rapidly eroding versus the slowly eroding side of the drainage divide. However, chemical weathering intensity does not vary significantly across the divide or as a function of vegetation type. Soil production rates are correlated with ridgetop curvature, and ridgetops are more convex on the rapidly eroding side of the divide, where soil mineral residence times are lowest. Hence our findings suggest fluvially-driven erosion rates control soil production and soil chemical weathering rates by influencing the relationship between hillslope topography and mineral residence times. In the western Southern Alps, soil production and chemical weathering rates are more strongly mediated by physical rock breakdown driven by landscape response to tectonics, than by vegetation.