A maar is a hole in the ground, of volcanic origin… what else?… caused by interaction with water… That was about all I knew about maars when I prepared for this year’s holiday, in the German Eifel maars area (Vulkaneifel, Rheinland-Pfalz district). I did not expect to find a great lot of evidence for former volcanic activity. Probably all covered by sediments or eroded, mined away or grown over – that’s what I thought. Mind you, it was not the famous Laacher See caldera I visited which had produced a VEI 6 eruption some 13 000 years ago. I wanted to see the southern part of this volcanic area, bordered by the river Rhine in the NE and the river Moselle (lovely wines grow there!) to the SE. I was delighted to learn that today this area is not only dotted with maars. All over the landscape lava flows, tuff rings, cinder cones, tephra layers with lava bombs and lapilli still tell of quite recent turbulent times here. The site of the last eruption in Germany is the Ulmen Maar – 10 800 years ago.

 

WHAT ACTUALLY IS A MAAR-DIATREME VOLCANO?

A maar is the surface expression of this (second most common) volcano type. It is a shallow, bowl-shaped depression in the land surface. – A diatreme is the subsurface feeder, an inverted cone‐ or carrot‐shaped, structure below the maar. The “carrot” consists of fractured material that subsided after deep phreato-magmatic explosions at its root. Small and large maars are associated with small and large diatremes, respectively.

Maar-diatreme volcanoes are the phreato-magmatic equivalent of magmatic scoria cones and their associated lava flows. Maar craters in the Eifel are less than 100 m to over 2 km in diameter and several tens of metres to 300 m deep (measurements from the crest of the tuff rings around them). Their ejecta rings or tuff rings are several meters to perhaps over 100 m high. These are characterized by sequences of successive alternating and bedded pyroclastic deposits. Deposit sequences commonly contain large amounts of lithic material (country rock) that is entrained from the rock basement, and in some cases lapilli and lava bombs.

A maar-diatreme volcano generally consists of
– the maar crater at the top, which is cut into the pre-eruptive land surface
– the ejecta ring, surrounding the maar
– the more or less cone-shaped diatreme, which underlies the maar crater
– the irregular-shaped root zone surrounding and underlying the lower end of the diatreme
– the narrow feeder dyke at depth.

An eruption begins, like in every volcano, with a dike of rising magma melting and cracking its way to the surface. At a shallow depth it encounters layers of impermeable rocks that keep groundwater running at that level. The result is an underground phreatic explosion when the magma meets the water. Such thermohydraulic explosions in confined space are very powerful and cause a great crater in the landscape. The sudden decompression shatters the overlying country rock, and together with the water vapour it creates a massive eruption column. Most of the fragmented country rock falls back into the crater by gravity. Base surges, ballistic transport and some minor tephra fall distribute the rest of it on the surrounding surface.

Down where the magma met the water (at the root zone) a near-spherical fragmented cavity has been created, the so-called explosion chamber. It is in the bottom of that chamber where newly arriving magma meets more water for another explosion, creating another chamber just below the first. Thus a series of interconnected explosion chambers is formed, one below the other, sometimes also laterally next to each other (following mostly the trend of the feeder dyke).

Much of the material of that irregularly shaped root zone has been blown out. The resulting mass deficit causes increasing mechanical instability of the overlying country rocks and finally their collapse down into the cavities. As the eruption goes on, the diatreme gets ever deeper, much like in a sinkhole. The upper materials sag down into the space below while on the surface the maar widens. Tephra beds, country rock debris of various kind, and reworked older tephras now form alternating layers on the crater floor and jointly subside towards depth in the growing diatreme.

So, what we are seeing on the surface, the maar, is not just the initial explosion crater. It is the result of the entire period of eruptive activity, with downward penetrating explosions and the following subsidence of fragmented rocks. These eruptions lasted for periods of days to years, the longer the activity the larger grew the maar. In some cases, a cinder cone was formed at the bottom of the maar. Some maars have also produced short lava flows or lava lakes in their craters. This happens when there is no longer groundwater available while the magma is still rising. In very few long-lasting events a small stratovolcano developed from the cone

Despite being so frequent in number, very few maar-diatremes formed during historic time. The most recent ones are
– the Nilahue maar, resp. Carran, Chile 1955
– Iwo Jima, Japan, 1957
– the Ukinrek Maars, Alaska 1977
– the Westdahl maar, Aleutian Islands 1978.

For an example, the Ukinrek West Maar formed within 3 days; its crater was finally 175 m wide and 35 m deep. Ukinrek East Maar formed in the following 8 days and reached a diameter of 340 m and a depth of 70 m. They are located on the south shore of Becharof Lake on the Alaska Peninsula, in an area without previous volcanic activity. Both maars are now filled by crater lakes; the eastern lake encircles a 49-m-high lava dome that was emplaced at the end of the eruption. Base surges were directed primarily to the NW. Juvenile material from the Ukinrek eruptions was of mantle-derived olivine basaltic composition.

 

THE EIFEL VOLCANIC FIELD

The Eifel area became volcanically active around 45 to 35 Ma, it was associated with the colliding of the African and Eurasian plates. As the Alps folded up and pressed on northwards the existing rock base of Devonian sandstones and clay slates cracked and buckled. Grabens and low mountain ranges changed the landscape. Whether volcanism was caused by such deep faulting only or by a (now widely accepted) mantle plume hotspot has long been a matter of debate. Fact is that the Eifel has hundreds of eruptive centers. Most of them were monogenetic maar-diatremes, living just for one eruptive period before the next one sprung up somewhere in the vicinity. Over half of the basaltic eruption centers in the Eifel have also produced lava flows, rarely more than 3 km long.

While the geologic history of the Eifel has been detailed in a blog post by Chryphia (former Volcano Cafe), let’s look at the present situation.

So far 350 eruptions have been documented in total of which some 270 occurred during the most recent time. In 77 locations the existence of a maar has been verified, although some of them can hardly be recognised by eye. Some 13 of them are filled with water or in various stages of silting up, the rest are dry and overgrown. Scientists have tried to determine some sort of system in the spatial distribution of all those events but without success. The earths crust was just like randomly punctured by a shotgun. In contrast, the distribution of rock types, which differ in their quartz content, is systematic. It is sort of annular, with the quartz-richest rocks in the center.

So, from what source did the young Eifel volcanoes at the time get their magma, and what kind of magmatic activity is still present below the Eifel? In order to get to the bottom of these questions, geophysicists launched the European “Eifel-Plume Project” in 1997. It is the most extensive seismologic field experiment of its kind in Europe. With the help of stationary and mobile earthquake measuring stations a so-called seismic tomography was created. The bulk of the 250 mainly mobile stations were set in the Eifel, the rest in adjacent areas.

The researchers used the waves of the more powerful earthquakes around the world. The waves of a violent earthquake, say,  an event some 8,500 km away in Mexico, can be recorded less than 10 minutes later by the Eifel stations. As we know, waves that travel through hotter materials slow down considerably. The records of all the stations made it possible to compute the delayed arrival times of earthquake waves into a 3-D image of the earth’s interior (similar to a computer tomography in medicine):

The result was impressive: Long assumed and now proven – there is a hot zone under the Eifel, apparently a large plume out of the mantle which presumably contains about one percent molten particles. This erratic structure was named the Eifel-Plume. Today scientists assume that localized fusions take place a few km above the Eifel plume, at about 40 km of depth. Deep earthquakes may be indicators for that. Samples of gas measurements also point to such magmatic activities in the upper mantle.

Present gas emissions
Gas samples from fumaroles can tell a lot about the processes down below. But how to distinguish the gases of still cooling ancient magmas from degassing fresh magma that may just be moving below the Moho? This is the problem scientists are presently concerned with. In order to find answers, they are mainly looking at the content of the noble gas helium. Carbon dioxide gives no clear information as to where the gas originated from. Helium, on the other hand, is of the most importance in this question.

This rare gas occurs in two stable isotopes, helium-3 and helium-4. The common helium 4 is located throughout the earth’s mantle and crust and is formed by the radioactive decay of uranium and thorium. The lighter and rare helium-3 resides in the deeper mantle of the earth. It does not form naturally on this planet but has been brought in from space – mostly from meteorites – in Earth’s very early times (some of the He-3 resources can be as old as 4.5 billion years).

From the ratio of the two helium isotopes in the ascending gas mixture, one can ascertain whether it originates from the earth’s crust or from the earth’s mantle. The ratio in old-magma gases is ~130.000 parts (Helium-4) : 1 part helium-3. Coming from fresh magma, the ratio of the gas mixture shifts in favor of the mantle helium: 114 000 (He-4) : 1(He-3).

Present seismicity
Seismographic measurements have shown that there is a hot zone (plume) of 1000°C to 1400°C below the Eifel, which is 200°C hotter than its immediate environment. As melting processes are associated with a rise in volume, this should be noticeable as land elevations. In fact, the Eifel has long been known as a rising area. For 800 000 years, the rise of the Rhenish shield has accelerated to an average of 0.12 mm per year. However, there are areas in the central Eifel which during that same time have domed up to 300 m, at an average rate of 0.35 mm per year. All these movements cause lots of small shallower tectonic quakes in the area, nothing unusual in that. More interesting to us are the deep quakes at 25 to 35 km depth. There were two of them in 2013 with >40 km depth, and just recently (June 2017) another swarm at 25-30 km. Whether such deep swarms are something common or rather unusual cannot be said at the moment as the net of stations has just been set up recently.

 

OTHER VOLCANOES – THE MOSENBERG GROUP

Not a maar but one interesting volcano near my holiday destination was the Mosenberg volcanic group. The Mosenberg range rises above the surface as a two-hill landmark that can be seen from a distance. The volcanic history of the Mosenberg series began 80,000 years ago with the first eruption. Further outbreaks followed along a NNW/SSE line, just like a pearl necklace.

Six eruption points have been determined. In lava quarries, the most southerly and oldest center of the first eruption was excavated revealing several vents. The actual, 517 m high Mosenberg consists of two overlapping cinder cones (2nd and 3rd eruption). From the southern part of the two cones a 17 m thick basaltic lava flow ran down a valley for 1,600 m. These lavas have been quarried for a time, leaving a narrow dark valley (the “Wolfsschlucht”, wolves’ gorge) through the length of the last few hundred meters of it. A path leads through it where one can walk “through the lava flow” and admire basalt columns and other lava formations to both sides.

The Windsborn cinder cone is the 4th eruption site. Its crater lake is surrounded by a ring wall of welded spatter. This was built up when the eruption had somewhat calmed down after its initial violent explosions. From the bubbling depth of the crater flying soft lava fragments landed on the rim and stuck together. The 5th eruption also built a cinder cone, which is now eroded. Instead, the site now hosts a maar that marks the 6th eruption of the Mosenberg.

ANY NEWS AS TO THE EIFEL VOLCANO’S FUTURE?

No, not really… but yes, geologists get nearer to understanding the structures and processes below. They can confirm that this volcanic area is not extinct. There is magma below, there are deep earthquakes, and there are gases coming up to the surface. There may be eruptions again but, so far, there is nothing on the instruments that indicate any stirring in the near future. Having said that, instruments haven’t been around for an awfully long time. Now that scientists can start to compare records on a daily, monthly, yearly level we may expect to learn some exciting new results about the Eifel volcanism.

 

Enjoy!  – Granyia

 

SOURCES AND FURTHER READING

– […] East and West Eifel (2007, Schmincke)
– A mantle plume below the Eifel volcanic fields (2001)
– Eifel Volcanic Field I (VolcanoCafe 2013)
– Eifel Volcanic Field II (VolcanoCafe 2013)
– Episoden des Eifelvulkanismus (2012, German, Blog)
– […] magmatic systems in the West Eifel volcanic field (2013, paywalled)
– Maare und Vulkane (Engl., blog post)
– On the growth of maars […] formation of tuff rings (1986)
– How Polygenetic are Monogenetic Volcanoes: […] Maar‐Diatremes (2016)
– Mosenberg (Bettenfeld website)
– Eifel Area as location for repository for radioactive waste? (2001)