Why volcanic crystals are black boxes for tracking magma’s journey to the Earth’s surface

When we think of a volcanic eruption, rivers of glowing lava or giant columns of ash come to mind. But beneath Earth’s surface, a more complex story is unfolding. Magma deep within volcanoes is more than molten rock. It is a thick mixture of melted rock and solid mineral crystals known as “magma mush”. To picture it, imagine a semi-frozen granita: a slushy blend of solid ice crystals suspended in 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 black boxes for eruptions. They allow us to track how a volcano prepares to erupt and better understand the risks of volcanic eruptions.

This is particularly relevant for La Palma, where the 2021 Tajogaite eruption marked the island’s first volcanic activity in 50 years. It lasted almost three months, destroyed thousands of buildings and forced more than 7,000 people to leave their homes.

Minerals that hold memories

Just as we read a tree’s rings to understand past climate conditions, we can read volcanic crystals to understand volcanoes. Of all the minerals found in lava, clinopyroxene is a particularly good recorder of volcanic history.

Clinopyroxene grows slowly as magma cools deep within Earth, adding layers over time. Just as a tree records years of climate conditions in its rings, a clinopyroxene crystal records the temperature, depth and chemical conditions of the magma from which it grew. This allows researchers to reconstruct the lead-up to 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 centers of the crystals reveal an ancient, cooler reservoir of magma mush 18 to 25 kilometers (11 to 16 miles) deep in the upper part of Earth’s mantle. This was a very thick granita, packed with older crystals that had sat undisturbed for a long time.

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 magma’s fast journey toward the surface as it broke through the rock to fuel the eruption.

Why this matters for volcano monitoring

Volcanic monitoring networks detect earthquakes, ground deformation and gas emissions as magma moves and pressure changes beneath a volcano. These signals are fundamental to anticipating an eruption, but magma moving deep 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 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. When warning signs are detected, we can better assess how quickly an eruption might develop.

La Palma and beyond

What 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 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 beneath 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, remobilizing old crystals and carrying them upward. The erupted magma can therefore contain materials formed at different times, under different conditions and at different depths.

Deep, cold crystal mushes are not limited to La Palma. Evidence from other volcanic islands suggests 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. They 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 clearer picture of how magma evolves beneath volcanic islands.

Who’s behind this story?

Gaby Clark

MA in English, copy editor since 2021 with experience in higher education and health content. Dedicated to trustworthy science news.

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Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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This article is republished from The Conversation under a Creative Commons license. Read the original article.