By examining the tree-ring-like growth patterns found in volcanic crystals, geologists can track volcanic activity and understand the events that lead up to eruptions. Magma, the thick mixture of melted rock and solid mineral crystals found deep inside volcanoes, is often referred to as "magma mush." A recent study published in Nature Communications focused on crystals from La Palma, one of the Canary Islands, where the 2021 Tajogaite eruption marked the first volcanic activity in 50 years. The eruption lasted nearly three months, destroying thousands of buildings and displacing over 7,000 people. This research highlights how crystals can act as "black boxes" that preserve a record of volcanic processes. Minerals like clinopyroxene are especially useful for understanding volcanic history. These crystals grow slowly as magma cools deep within the Earth, forming layers that can be analyzed to reconstruct the events before an eruption. The core of these crystals revealed an ancient, cooler reservoir of magma mush located 18 to 25 kilometers deep in the upper Earth's mantle. This reservoir was thick and packed with older crystals that had remained undisturbed for a long time. As the crystals grow outward, their middle layers show a sudden change in the magma environment—days or weeks before an eruption—when hot magma rises from the mantle. This rising magma adds energy to the dormant mush, melting parts of the crystals and changing their chemical composition. The outermost layers then record the rapid movement of magma toward the surface as it breaks through the rock to trigger an eruption. Volcanic monitoring systems typically detect signs of an eruption by tracking earthquakes, ground deformation, and gas emissions caused by magma movement. However, magma moving deep underground is more challenging to detect than magma near the surface. The study shows that much of the magma accumulation beneath La Palma occurs at great depths in the upper mantle, meaning unrest can develop far from the surface and be difficult to notice with current monitoring techniques. While the research does not predict when the next eruption will occur, it helps scientists interpret future volcanic activity by revealing the processes that lead up to it. The findings from La Palma may apply to other volcanic islands as well. Clinopyroxene crystals from the Azores, Cape Verde, the Galápagos, and other Canary Islands show similar chemical patterns in their growth zones. These patterns suggest that magma histories in these regions are comparable to those observed at La Palma, and they support the idea of deep magma mushes being widespread under volcanic islands worldwide. These mushes can remain partially molten beneath the surface until fresh, hot magma rises from the mantle, stirring the mixture and remobilizing older crystals. As a result, erupted magma can contain materials formed at different times, under different conditions, and at different depths. Evidence suggests that such deep, cold, crystal-rich mushes are especially common during the early stages of an island’s development, when magma supply is high and deep reservoirs can be sustained. As volcanic islands mature and magma supply decreases, these mushes may gradually cool and become less likely to be reactivated. By studying the chemical records preserved in volcanic crystals, scientists can track these changes over time and better understand how magma evolves beneath volcanic islands.