When we think of a volcanic eruption, rivers of glowing lava or giant columns of ash come to mind. However, beneath the Earth’s surface, a much more complex and fascinating story is unfolding. The magma that is found in the depths of volcanoes is more than just boiling liquid. It is a thick mixture of melted rock and solid mineral crystals known as “magma mush”. To picture this magma mush, imagine a semi-frozen granita: it is a slushy blend where solid ice crystals are suspended in a sweet liquid syrup.In our recent study, published in Nature Communications, we investigate crystals from the island of La Palma in the Canary Islands. We find that the solid crystals inside the “granita” mush act as true black boxes for eruptions. Thanks to the crystals, we can track how a volcano prepares before it erupts and better understand the risks of volcanic eruptions.This is particularly relevant for La Palma, where the 2021 Tajogaite eruption marked the first volcanic activity on the island in 50 years, lasting almost three months, destroying thousands of buildings and forcing more than 7,000 people to leave their homes.Minerals that hold memoriesJust as we read the rings of a tree to understand past climate conditions, we can read volcanic crystals to understand volcanoes. Out of all the minerals found in lava, clinopyroxene is a particularly good recorder of volcanic history.Clinopyroxene grows slowly as magma cools deep within the Earth, adding layers over time. As a tree records climate conditions years in its rings, clinopyroxene crystal records the specific temperature, depth, and chemical conditions of the magma it was growing from, allowing researchers to reconstruct the pre-eruptive story of an eruption. To uncover this history, we collected lava samples from the sites of La Palma’s 1712, 1971, and 2021 eruptions. We then examined clinopyroxene crystals inside these rocks using high-resolution chemical imaging, allowing us to read their tree-ring-like growth records. The analyses revealed a recurring pattern that persisted over more than 300 years of volcanic activity.The centre of the crystals reveals that, at 18 to 25 kilometres deep, in the upper Earth’s mantle, there was an ancient, cooler reservoir of magma mush. This was a very thick granita, packed with older crystals that had been sitting undisturbed for a long period.Moving outward, the middle crystal rings capture a sudden shift in the magma environment. They show that days or weeks before each eruption, hot magma rose from the mantle. This hot liquid injected energy into the dormant granita, melting parts of the solid crystals, altering their outer chemistry, and vigorously stirring the mixture.Finally, the outermost rings record the fast journey of the magma towards the surface as it broke through the rock to fuel the eruption.Why this matters for volcano monitoringVolcanic monitoring networks detect earthquakes, ground deformation, and gas emissions as magma moves and pressure changes beneath a volcano. These signals are fundamental for anticipating an eruption, but magma that moves deep down underground is much harder to detect than magma approaching the surface.Our results show that much of the magma accumulation beneath La Palma occurs at great depth in the upper mantle. This means that unrest processes can develop far from the surface and be difficult to detect with volcano monitoring networks.While our research does not forecast when the next eruption will occur, it does help us interpret future episodes of volcanic activity. Understanding that eruptions are triggered at great depth allows us to read monitoring signals more accurately. This is because when warning signs are detected, we can optimally assess how quickly an eruption might develop.La Palma and beyondWhat happened beneath La Palma may also happen beneath other volcanic islands.Clinopyroxene crystals erupted in the Azores, Cape Verde, the Galápagos, and other Canary Islands show similar chemical patterns in their growth zones. These chemical records reveal magma histories comparable to those observed at La Palma and are consistent with the presence of deep magma mushes that may be widespread under volcanic islands around the world.These mushes can remain hidden below the surface partly molten until fresh, hot magma rises from the mantle. The new magma supplies heat, introduces new material and stirs the crystal-rich granita, remobilising old crystals and carrying them upwards. The erupted magma can therefore contain materials formed at different times, under different conditions and at different depths.Deep, cold, crystal mushes are therefore not limited to La Palma. Evidence from other volcanic islands suggests that they may be particularly common during the early growth stages of an island, when magma supply is relatively high and deep magma reservoirs can be maintained. Frequent magma injections can repeatedly replenish these reservoirs and prevent them from cooling and solidifying completely. As volcanic islands mature and magma supply declines, these mushes may receive less heat and fresh magma. The mushes may then gradually cool and become less likely to be reactivated by new magma injections. By studying the chemical records preserved in volcanic crystals, scientists can track these changes through time and build a clear picture of how magma evolves beneath volcanic islands. A weekly e-mail in English featuring expertise from scholars and researchers. It provides an introduction to the diversity of research coming out of the continent and considers some of the key issues facing European countries. Get the newsletter!Alberto Caracciolo acknowledges the VOLC-LAPSE cascading fund within the RETURN Project (CUP H93C22000610002, funded by the European Union)Maurizio Petrelli works for the University of Perugia (Italy). He acknowledges the VOLC-LAPSE cascading fund within the RETURN Project (CUP H93C22000610002, funded by the European Union)Teresa Ubide works for The University of Queensland. She receives funding from the Australian Research Council and infrastructure funding from AuScope NCRIS.