Back in the 1980s I have travelled this part of Romania extensively, we hiked and hitch-hiked through villages and landscapes in 4-week holidays for several years – but never did we know there was a volcano – and such an interesting one at that! The travel guide books didn’t mention volcanoes and the locals didn’t care for geology much. I guess this has changed now; a volcano in the neighborhood has always proved a recipe for visitor attraction!
I admit, Ciomadul has last erupted a long time ago – some 30 ka. The GVP lists volcanoes as active/dormant that have not erupted for 10k years. But that’s an arbitrary red line; none of us has been around long enough to tell whether or not a volcano could become active again after such a long period of dormancy. For some volcanoes it is crystal clear, they are old, cold and eroded. Plugged up to never awaken again. Others, though, have still some signs of life in them – fumaroles, microseismicity, some deforming etc. Volcanoes have been known to erupt out of the blue – had they done this long before and nobody knew? Perhaps they had been dormant for tens or even hundreds of thousands of years?
Researchers on new ground – or rather, deep below and within
The Hungarian volcanologist Dr. S. Harangi writes on his blog that, at a conference in 2002, the geologist-volcanologist Sándor Szakács stated that the Ciomadul volcano could still get active again. That was certainly a bold statement at the time, but who was to contradict? If volcanoes are just stamped “extinct” after a set time, without researching the facts, mankind may be in for some unpleasant surprises.
The Volcanology Group of the Eötvös-Loránd-Universität (ELTE) Budapest, took the challenge. The questions for this new path of volcanological research are not: Where, when, or how big will a volcano blow? They are: Could it still erupt, and if Yes, what exactly would be the conditions that make it active again? To find out the state of a volcano, one has to look not only at its eruptive history but also at the nature of the magma beneath it. How much partially molten rock (crystal mush) is there, if any? Has it reached a state of no return? How did the magma get remobilized in past eruptions?
This is new ground, using modern technologies and digging far, far back in Earth’s history. Naturally, at first researchers would come up with more or less differing results – which are now disputed between them. This explains some differences in figures when reading the various papers. Eventually, more studies and more disputing will clarify matters.
So, the first task for the scientists was to figure out how it all began. When did volcanism begin in southeastern central Europe, and what were the driving forces for it?
We are here in the Carpathian mountains, this conspicuously arm-and-elbow shaped mountain chain stretching mainly through Poland, Slovakia, Hungary and Romania. The area was once part of a subduction zone and had been covered by the Pannonian (ocean) Basin. The sea created the underlying sediments of the Carpathian Flysch Belt. A near-vertical slab of ocean crust is still descending into the upper mantle* here, creating persistent regional seismic unrest with strong earthquakes at times.
During the Middle Miocene this zone was affected by intensive arc volcanism that developed mountain ranges over the subduction zone and the flysch basins. At the same time, the internal zones of the orogenic belt were affected by large extensional structuring of the back-arc Pannonian Basin. The Pannonian Basin has a thin continental crust and lithosphere and is surrounded by the mountain belts of the Alps, Carpathians and Dinaric Alps.
*The reason for this lithospheric slab descending is strongly debated. There are theories that a) it represents the latest stage of the subduction along the Carpathians, or b) lithospheric delamination in the absence of any subduction, or c) that a combination of lithospheric delamination and subduction roll-back could play a role. Volcanic activity of the South Harghita and the nearby Perşani alkali basalt volcanic ﬁeld could be related to this active tectonic setting, although the link between the vertical slab movement, magma generation, and volcanism is still unclear.)
The Călimani-Gurghiu-Harghita andesitic-dacitic volcanic chain
This is the monster term geologists use for this part of the Carpathian mountains. Arc volcanism began from ca. 10.2 Ma onward and gradually shifted southeast with waning intensity. Volcanic activity in the South Harghita started at 5.3 Ma. Ciomadul volcano sits at the SE end of this chain and was the last to erupt.
When geologists of the early 20th century first surveyed the area they were already convinced the Ciomadul was a quite “young” volcanic edifice. However, its actual age and especially its chronological evolution remained poorly understood. One of the main difficulties was to discern between Ciomadul and the products of its nearest neighbor volcano, Pilişca, which is much older, though.
A lot of different methods and materials have been used to achieve an understanding of the eruptive time line of Ciomadul:
– Organic materials such as charcoal pieces, found in pyroclastic flow deposits were radiocarbon dated;
– crater lake bottom sediments and peat-bog fillings were also radiocarbon dated;
– ages of volcanic rocks and single crystals were determined with the methods of K-Ar, Ar-Ar, U-Pb and (U-Th)/He dating;
– tephrostratigraphic investigations, petrological and magnetotelluric surveys were carried out;
– also, a biostratigraphy of the crater lake sediments was established, mainly based on diatom flora and pollen analysis.
Ciomadul (or Ciomatu, and Csomád or Csoma-hegy in Hungarian) is a smaller dome-complex volcano that forms a massif rather than a central volcano. Its principal rock type is dacite, rich in potassium. The volcano cross cuts the fold-and-thrust orogenic belt of the East Carpathians that consists mostly of Cretaceous flysch* nappes**. Its main edifice is made up of two larger dome clusters and a smaller isolated dome, surrounded by volcanic deposits from its two craters Mohoş and Sf. Ana. The floor of Mohoş, the older crater, is a protected peat-bog conservation area today while the lake in Sf. Ana crater invites tourists to enjoy summer outdoor activities. Four older satellite domes are located at the periphery around the main volcano.
*Flysch is a sequence of sedimentary rock layers. It is usually deposited when a deep basin forms rapidly on the continental side of a mountain-building episode.
**A nappe (or thrust sheet) is a large sheet-like body of rock that has been forced a few km over a thrust fault from its original position. Nappes form in compressional tectonic settings like continental collision zones or on the overriding plate in active subduction zones.
Eruptions of small volumes of lava in the area began ~1 million years ago. The central dome clusters were generated during a shorter but intense phase of activity between 0.4 and 0.6 Ma. The fact that bread-crust lava bombs and blocks have been found among dome fragments indicate that explosive events might have taken place between dome formation periods.
After a long repose time (to account for the development of a layer of paleo-soil, likely several 10 k to 120 kyr), two late-phase explosive episodes ended the volcanic activity: one phreatomagmatic from the Mohoş crater and another, Plinian or sub-Plinian phase, from the Sf. Ana crater. These eruptions resulted in a thick succession of pyroclastic-fall deposits around the volcano and farther afield. They are of highly silicic (rhyolitic) glass compositions.
The age of the most recent eruption of Ciomadul (30 ka +/-) is loosely determined by radiocarbon dating of (1) charcoal found in the upper depositional unit of the pyroclastic flow deposits, (2) organic matter in the paleo-soil underlying the Plinian pumice fall deposit, and (3) organic material of the Mohoş peat and Sf. Ana lake bottom sediments.
A total of ca. 9 km³ of volcanic products make up the current volcano complex. This, emplaced over ca. 1 Myr, is an extremely low output rate, but it corresponds to the general decline of volcanic activity in the South Harghita chain.
Is there still hot magma below?
This, of course, is the million dollar question. So, for how long has the magma been down there since the last eruption? To never erupt again a magma has to cool down until its various molecules become immobile, i.e. a locked system is formed. This is a point below a temperature where isotopes no longer can leave a particular mineral. This occurs for all isotopes and minerals at a different temperature.
For example, volcanologists measure helium and uranium isotopes in zirconium crystals, based on the radioactive decay times and other parameters. The results tell them at what time this system locked up. And with that they have quite accurately the time when an eruption happened, namely when the molten lava all of a sudden cooled down from above 700°C to less than 150°C.
New studies also looked into the process of how the volcano got ready for eruption in the past. Its last eruptive period occurred after a significant time of inactivity. A near-solidus silicic crystal mush body existed at 7-14 km depth. Researchers calculated that the temperature of this crystalline nucleus of near 720-740°C (already near solidification) increased by more than 200° before an eruption. An intrusion of hot basaltic magma into the silicic magma reservoir had quickly melted and mobilized the magmatic material, making it ready for eruption. From the examination of amphibole crystals it is also known that this could happen in a few decades, and the mobilized magma could break the surface within a week.
Could a melt-bearing magma body still reside beneath the volcano today?
The results of a magnetotelluric survey can be interpreted as implying a partially melted zone, somewhere between 5 and 25 km of depth. These results have been confirmed by other methods and models – a crystal mush body containing about 5-15% melt fraction does exist directly below the seemingly inactive volcano.
How can a magma reservoir be kept hot over tens of millennia?
From the zoning of zircon crystals and textural features of olivine and clinopyroxene crystals researchers conclude: the shallow crustal magma storage developed by alternating pulses of silicic magma ascent and intrusions of basaltic magmas. This sequence of events could keep the magma reservoir at a temperature above the point of becoming solid (solidus) for very long times with no volcanic eruption.
So, most probably, the magma chamber is still warm, containing some incompletely solidified magma. However, the ability of a volcanic system of producing further eruptions is more dependent on the state of the deeper regions of magma generation. At Ciomadul, sub-crustal seismic activity has been detected down to a depth of ca. 70 km, in the vicinity of a “soft” lithosphere column. This might suggest that even the lower region of magma generation, and its connection to the crustal magma chamber, are not completely frozen and inactive.
PAMS – ???
According to the presently available data, Ciomadul’s status as “active” cannot be completely ruled out. Although no definite conclusion can be made just yet, reactivation of the magma generating system into the still incompletely frozen magma chamber beneath Ciomadul is still a possibility.
The results of these studies – as to the nature of seemingly inactive, long sleeping volcanoes – might change our view of possible future hazards: in 100k-year-old volcanoes, or even older, volcanic activity could be renewed if all conditions are right. A new term proposed by S. Harangi et al., “Volcanoes with Potentially Active Magma Storage” (PAMS) describes the potential of volcanic rejuvenation.
Btw., if you find all this interesting, Dr. Szabolcs Harangi maintains a great volcanology blog where he writes about his and his collegues’ work in plain language… Hungarian language, yes, but Google does a more than half-decent job of translating! 🙂 Link is in the list below.
Băile Tușnad – “The Pearl of Transylvania” at the foot of the volcano
Băile Tușnad, or Tusnádfürdő in Hungarian, is a small but nice spa town in the Olt valley in Transylvania. After the area had changed hands between Hungary and Romania several times in its history, the residents of Tusnádfürdő are up to 95% Székely people, the Hungarian ethnic group in Romania. The town is built around the foot of Ciomadul mountain and lives and thrives with the help of volcanic hydrothermal systems: 25 mineral water wells, pools and springs (mofetas) provide the base for all the wellness sold here in hotels and pensions. But eco tourism is also offered now, along with guided walks through protected nature preserves. Not to forget – the bearwatching!
Disclaimer: I am not a scientist, all information in this (and any of my other posts) is gleaned from the www and/or from books I have read, so hopefully from people who do get things right! 🙂 If you find something not quite right, or if you can add some more interesting stuff, please leave a comment.
Enjoy! – GRANYIA
SOURCES & FURTHER READING
– Ciomadul (Wikipedia)
– Morphometrical and geochronological constraints on the youngest eruptive activity in East-Central Europe at the Ciomadul (Csomád) lava dome complex, East Carpathians (2013, paywalled)
– The latest explosive eruptions of Ciomadul (Csomád) volcano – A tephrostratigraphic approach for the 51–29 ka BP time interval (2016, paywalled)
– Eruptive history of a low-frequency and low-output rate Pleistocene volcano, Ciomadul (2015, paywalled)
– Early to Mid-Miocene syn-extensional massive silicic volcanism in the Pannonian Basin (East-Central Europe): Eruption chronology, correlation potential and geodynamic implications (2018, paywalled)
– The onset of volcanism in the Ciomadul […] Eruption chronology […] (2018)
– Dr. Szabolcs Harangi’s Volcanology Blog
– MTA-ELTE Vulkanológiai Kutatócsoport (Research group)
– Origo.hu Article on Ciomadul volcano
– Combined magnetotelluric and petrologic constrains for the nature of the magma storage system beneath the Late Pleistocene Ciomadul volcano (SE Carpathians) (2015, paywalled)