Maar

Lore & Background

The term maar originates from a Moselle Franconian dialect word for the circular lakes of the Daun area in Germany. It was first used in German in the modern geological sense in 1819 and later adopted into English and geological sciences to describe the explosion crater itself, even if the lake has since drained. This extension in meaning occurred because the lake may no longer exist, and the name came to represent the crater type as well. The present definition relates to both common and scientific discourse over two centuries, with alternative terms like tuff ring or maar-diatreme volcanoes available in geological contexts. A 2011 clarification defines maars as having craters with floors below the pre-eruptive surface.

Reader's Guide

Maars are significant in volcanology as distinctive volcanic landforms resulting from explosive interactions between magma and groundwater or permafrost. Their formation produces craters that often become lakes, providing valuable records of past volcanic activity and environmental conditions. The largest known maars, found at Espenberg on the Seward Peninsula in Alaska, range from 4 to 8 km in diameter and up to 300 m deep, formed by prolonged explosive eruptions due to permafrost melting slowly and maintaining a low water-to-magma ratio. Maars occur in various regions worldwide, including western North America, Patagonia, the Eifel region of Germany, and other geologically young volcanic areas. They also serve as important fossil sites, such as the Messel pit in Germany, a former maar lake known for well-preserved fossils. The study of maars helps scientists understand hydromagmatic eruption processes and the geological history of volcanic fields.

Did You Know?

• The name maar comes from a Moselle Franconian dialect word for circular lakes in the Daun area of Germany.
• The largest known maars are at Espenberg on the Seward Peninsula in Alaska, ranging from 4 to 8 km in diameter.
• A dry maar results when a maar lake dries out, becomes aggraded, or silted up.
• Arizona's Meteor Crater was once thought to be a maar but is now known to be an impact crater.

Formation and Physical Structure

A maar is born from a violent interaction between water and fire. When groundwater or subsurface moisture encounters an intrusion of hot magma, the resulting phreatomagmatic explosion blasts a broad, shallow bowl into the landscape. Unlike the towering cones associated with many volcanic events, a maar presents as a low-relief depression with a gently sloping rim. That rim is typically a jumble of loose volcanic fragments mixed with chunks of country rock ripped from the walls of the underlying diatreme—the deep conduit through which the eruption traveled. In terms of dimensions, maars span a wide range: diameters from roughly 20 meters up to 3,000 meters, and depths from about 5 to 200 meters. A defining criterion, clarified in a 2011 geological statement, is that the crater floor sits below the pre-eruptive ground surface, which separates maars from other small volcanic features. Because the explosion excavates rather than builds, the resulting landform is inherently a depression, often later filled by rainwater or groundwater to become a shallow, circular lake.

Etymology and the Evolution of a Term

The word "maar" traces its roots to a Moselle Franconian dialect spoken in the Daun region of Germany, where local residents used it to describe the round lakes dotting the landscape. By 1819, German geologists had adopted the term in a formal scientific context, and it gradually spread into English and the broader geological vocabulary. Originally the word referred specifically to the water body itself, but as scientists recognized that many of these craters had long since lost their lakes—either through drainage, silting, or evaporation—the label was extended to encompass the crater structure regardless of whether water still occupied it. This semantic broadening reflects a two-century trajectory in both common and technical discourse. In modern geological literature, related descriptors such as "tuff ring" or "maar-diatreme volcano" may appear depending on context. A maar-diatreme volcano, for instance, describes an explosive system that cuts deeply into surrounding bedrock, with the maar representing the surface crater ringed by its ejecta. The term's journey from a local dialect word to an internationally standardized geological category illustrates how field observation and linguistic convention shape scientific nomenclature.

Global Distribution and Record-Breaking Giants

Maars are scattered across geologically young volcanic regions worldwide, but the largest known examples cluster on the Seward Peninsula in northwestern Alaska. The Espenberg maars—North Killeak, South Killeak, Devil Mountain, and Whitefish—reach diameters of 4 to 8 kilometers and depths up to 300 meters, far exceeding the typical range. These colossal craters formed over roughly 100,000 years, with the youngest, Devil Mountain Maar, dating to approximately 17,500 years ago. Their extraordinary size is attributed to the explosive interaction between intruding magma and permafrost: because frozen ground thaws slowly, it feeds a steady stream of water into the eruption while maintaining a low water-to-magma ratio, which amplifies explosivity and prolongs the event. Beyond Alaska, maars appear in the Eifel region of Germany, the Pali-Aike field in Patagonia, the Bayuda field in Sudan, the Wells Gray-Clearwater area of British Columbia, and the Auckland volcanic field in New Zealand, where Lake Pupuke sits in the suburb of Takapuna. Other notable examples include Kilbourne Hole and Zuñi Salt Lake in New Mexico, Crocodile Lake in the Philippines, and the CO₂-saturated Lake Nyos in Cameroon. Notably, Arizona's Meteor Crater was long misclassified as a maar before its true impact origin was confirmed.

Maar Lakes, Dry Maars, and the Water Cycle

Whether a maar holds water depends on a delicate balance of geology and climate. In many cases, groundwater seeps into the funnel-shaped hollow or precipitation collects after storms, creating a shallow, typically circular lake—the three maars at Daun in Germany's Eifel mountains being a classic example. Yet a significant proportion of maars are dry. A lake may vanish through gradual silting, aggradation of sediment, or simple evaporation. The Eckfelder Maar in the Eifel has become silted over, while the Eichholzmaar near Steffeln, also known as the Gussweiher, lost its water within the last century and is now undergoing renaturalization to restore its lake character. In other locations, the underlying bedrock is so porous that standing water simply percolates away, preventing a lake from ever forming. Seasonal variation adds another layer: after winters of heavy snowfall and rain, many dry maars fill partially and temporarily, and some retain small bogs or artificial ponds occupying only a fraction of the basin. Across the Volcanic Eifel, roughly 75 maars exist, with dry examples outnumbering their water-filled counterparts, and the most recent eruptions date back at least 11,000 years, leaving many craters heavily eroded and their volcanic features less distinct.

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