Maars and Phreatic Eruptions: When Water Meets Magma
Some of the most violent eruptions on Earth are driven not by magma alone but by water. When hot magma or hot rock encounters groundwater, the water can flash to steam in an instant, expanding hundreds of times in volume and triggering an explosion. These steam-driven blasts, known as phreatic and phreatomagmatic eruptions, can carve broad craters called maars and pose a particular danger because they often strike with little warning.
The power of flashing steam
Water expands enormously when it turns to steam. When this happens almost instantaneously, deep underground and under pressure, the result is a powerful explosion. In a purely phreatic eruption, the heat of the volcano flashes groundwater to steam without any fresh magma reaching the surface; the blast ejects shattered older rock and ash. In a phreatomagmatic eruption, rising magma itself interacts directly with water, fragmenting violently into fine ash.
What a maar is
A maar is the landform created by these explosive water-magma interactions. Rather than building a cone, the explosion blasts a broad, low-rimmed crater downward into the ground, often surrounded only by a low ring of ejected debris. Because maars typically cut below the water table, they frequently fill with water to become near-circular crater lakes. They are among the most distinctive and serene-looking products of violent volcanism.
The Eifel maars of Germany
The Eifel region of Germany is the classic home of maars — indeed, the word itself comes from this area. Here, a young volcanic field produced numerous maars, many of which now hold beautiful round lakes such as those of the Daun and Gillenfeld areas. These tranquil pools, ringed by forest, belie the explosive origin that created them, and the field is considered geologically young and not entirely extinct.
Maars around the world
Maars are found in volcanic regions across the globe, from the Pinacate field of Mexico and the volcanic plains of the western United States to the maars of Australia, the Massif Central in France, and many other settings. Wherever rising magma encounters abundant groundwater, the conditions exist to produce these broad explosion craters, making them a widespread feature of monogenetic volcanic fields.
The danger of phreatic eruptions
Phreatic eruptions are especially hazardous because they can occur with little or no warning. Since they may not require fresh magma to move toward the surface, they often lack the clear seismic precursors that herald magmatic eruptions. The 2014 eruption of Mount Ontake in Japan, a phreatic blast that killed dozens of hikers, is a tragic illustration of how suddenly such events can strike, even at well-known volcanoes.
Hydrothermal explosions
Closely related are hydrothermal explosions, in which superheated water trapped in a volcanic or geothermal system suddenly flashes to steam, blasting out rock and mud. These occur in geothermal areas such as Yellowstone in the United States and the geothermal fields of New Zealand, where they can blow out craters without any fresh magma involved at all, driven purely by the heat already stored in the ground.
Reading the record
For volcanologists, maars and phreatic deposits are important clues to a region's volcanic and groundwater history. The layered deposits around a maar record the sequence of explosions, while the lake sediments that accumulate in the crater can preserve thousands of years of environmental history, making maar lakes valuable archives for climate and ecological research.
Explore on the map
From the serene lake-filled maars of the German Eifel to the explosion craters of volcanic fields worldwide, water-driven eruptions have left a distinctive mark on the landscape. Explore these features on the interactive map — filter by region to see where the violent meeting of water and magma has shaped the Earth's surface.