Earth Science & Geophysics

When Neighbouring Volcanoes Share a Plumbing, They Listen to Each Other

Some volcanoes sit close enough that a single underground magma reservoir feeds both of them. When one stirs, the other often answers — and researchers are learning to read that exchange.

• Katla and Eyjafjallajökull in Iceland share magma plumbing; activity in one has a measurable effect on the other.
• Hawaiian volcanoes tend to erupt in turn, as if taking shifts at a common feed.
• Researchers watch tiny pressure changes and seismic signals between vents to build better eruption forecasts.

A single eruption does not tell the whole story of a volcano. On Kodiak Island, the aftermath of the 1912 Katmai eruption left the ground still venting steam and gas for decades — what botanist Robert Fiske Griggs called "tens of thousands of smokes curling up." That event collapsed two of Katmai's three peaks into a pit a kilometre deep and cooled the entire Northern Hemisphere by about a degree for more than a year. Dramatic as any one event is, the bigger scientific question is how magma moves before and between eruptions, and whether neighbouring volcanoes can predict each other's behaviour.

The key idea is coupling. Two volcanoes are "coupled" when they draw magma from a shared underground reservoir or an interconnected network of dikes and sills. Because they are not independent systems, a change in one is physically transmitted to the other. In southern Iceland, Katla and its smaller neighbour Eyjafjallajökull are the textbook example: they sit so close and share such intimate plumbing that tremor, inflation, and gas release at one system reliably show up at the other. Scientists describe this connection as "mercurial" — it shifts over time rather than staying fixed.

The Hawaiian Volcanoes Observatory sees a different flavour of the same phenomenon. The chain of active vents there tends to erupt in sequence, one after another, as magma migrates along a shared shallow reservoir rather than all bursting at once. In Iceland, the Fagradalsfjall fissure system erupted repeatedly from 2021 to 2023, after which the nearby Svartsengi fissure system appeared to take its place.

Listening for the coupling is a matter of very small signals. Seismometers and GPS stations record faint tremor bursts, slow ground deformations, and sudden pressure steps that travel between vents. Machine-learning models are beginning to separate those true coupling signals from ordinary tectonic noise, turning a jumble of tiny quakes into a coherent picture of where magma is flowing beneath the surface. The practical payoff is forecast quality: if the behaviour of one vent carries information about the next, volcanologists gain warning time and a more honest estimate of hazard before an eruption.

The lesson, counterintuitive as it sounds, is that a volcano is rarely an island. Its behaviour is shaped by its neighbours, the shared plumbing between them, and the slow conversation of pressure that they exchange long before any vent opens.

Knowledge takeaway: coupled volcanoes share a magma reservoir, so activity in one drives signals into the other; Katla and Eyjafjallajökull in Iceland and the Hawaiian fissure systems are real-world examples; reading the tiny seismic and pressure exchange between them is a route to improved eruption forecasts.