Monogenetic Volcanic Fields: Where Volcanoes Erupt Just Once
Most famous volcanoes erupt again and again from the same vent, building a single great mountain over time. But there is another, very different style of volcanism. In a monogenetic volcanic field, each volcano erupts only once, and new eruptions break out at new locations across a wide area. The result is a scattered cluster of small cones, craters, and lava flows, a landscape that poses unusual challenges for hazard planning, especially where such fields lie beneath cities.
What monogenetic means
The word monogenetic means born once. A monogenetic volcano forms in a single eruptive episode, which may last days, months, or a few years, and then never erupts again from that spot. This contrasts with polygenetic volcanoes, the large stratovolcanoes and shields that erupt repeatedly from the same central system over tens of thousands of years.
Fields of small volcanoes
Because each eruption builds a new volcano at a new location, monogenetic volcanism produces fields of many small volcanoes rather than a single large one. These fields can contain dozens or hundreds of cinder cones, maars, and small shields, scattered across a region. The San Francisco Volcanic Field in Arizona and the Auckland Volcanic Field in New Zealand are well-known examples.
How the eruptions work
In a monogenetic field, a batch of magma rises from depth and erupts at the surface, building a small volcano before its supply is exhausted. The next eruption draws on a fresh batch of magma, which may rise at a different point in the field. Over time, this produces a scattering of volcanoes of different ages, recording a long history of intermittent eruptions across the area.
Cones, maars, and shields
The volcanoes of a monogenetic field take several forms depending on the magma and the local conditions. Most common are cinder cones, built from fragments thrown out by mildly explosive eruptions. Where rising magma meets groundwater, explosive interactions can blast out maars, broad low craters. Small shields and lava flows form where fluid lava erupts more quietly. Together these create a varied volcanic landscape.
Volcanic fields beneath cities
One of the most significant hazards of monogenetic fields arises when a city is built on top of one. The Auckland Volcanic Field lies directly beneath New Zealand's largest city, meaning a future eruption could break out almost anywhere within the urban area. This unpredictability, with no single volcano to watch, poses a distinctive and serious challenge for monitoring and emergency planning.
The challenge of forecasting
Forecasting eruptions in a monogenetic field is especially difficult because the next eruption may occur at a location with no previous volcano. Scientists must monitor the whole field for signs of rising magma, rather than focusing on a single vent. This makes understanding the field's structure and history vital for assessing where and when a future eruption might occur.
Reading a volcanic field
The scattered volcanoes of a monogenetic field record a long history of intermittent activity. By dating the cones and mapping their distribution, geologists can reconstruct how volcanism has migrated across the field over time and estimate the likelihood and possible location of future eruptions, informing the planning for communities that live among these small volcanoes.
Explore on the map
From the volcanic field beneath Auckland to the scattered cones of Arizona and volcanic plains worldwide, monogenetic fields are a widespread and distinctive form of volcanism. Explore them on the interactive map — filter by type to see the clusters of small cones and craters that mark these fields around the globe.