I remember, as a teen, I loved rummaging in my mother’s enormous picture postcard collection – what a treasure! There was a photo in it that I never forgot in my life: An old Orthodox church sitting on a ridge high above a village and, behind the church, the humongous cone of a mountain rose to a height that dwarfed church, ridge and village to “Lilliput” size.
The mountain was the steep peak of Kazbek in the Kazbegi volcanic complex. This great volcano is located in the upper Terek (Tergi) basin, in the central part of the Main Caucasus Range. The Greater Caucasus and Lesser Caucasus fold-and-thrust belts are part of the great Alpine–Himalayan continental collision zone – the world’s largest active convergent deformation zone that stretches from western Europe to eastern China.
Kazbek is one of the major mountains of the Caucasus, second in height to Mt. Elbrus. It sits almost on the border between Georgia’s Kazbegi District and the Russian Republic of North Ossetia–Alania. The impressive and photogenic Kazbek (also Kazbegi, or locally called Mkinvartsveri or Bashlam) is the highest peak of the group. In May 2019 the Caucasus Skitouring Network organized an expedition to exactly determine the height of Mt. Kazbek. For the first time, a survey team placed a GPS receiver on its peak. The new height was found to be – with 5 mm accuracy – 5053.927 m.
The Greater Caucasus is that high mountain range which stands out so conspicuously on the elevation map of Eurasia. It runs between the Black Sea and the Caspian Sea, roughly from Sochi in the NW to Baku in the SE. These mountains contain the highest peaks in Europe. For over 850 km they define the leading edge of the second-largest active collisional orogen on Earth (orogeny = mountain-building).
This about 850 by 500 km collision zone is the highest in the whole western Asia. It has an average altitude over 1000 m and the Greater Caucasus is its northern limit. Considerable seismic activity and intense deformations are signs of the ongoing geological processes. Tectonically seen, this region has been compared with the Pamir-Karakorum chain in the Indian-Eurasian collision.
Like volcanoes and mountains in the adjacent areas (Eastern Turkey, Armenia, Azerbaijan), the Caucasus mountain ranges have been shaped by the convergence of the African-Arabian and Eurasian continental plates. The Arabian plate moves roughly north at 25 mm per year, squeezing away the Anatolian Plateau to the SW and the Iranian Block to the E. In the process of this collision, former island arcs, back-arc basins and microcontinents were shoved into and onto one another to form a landscape of accretionary terranes and subterranes.
As a result of that pressure from the south, several great thrust belts have been folded up. One is the Great Caucasus, itself divided into several parallel mountain ranges. To the south of it is the Lesser Caucasus and between the two lies the Transcaucasian intermontane depression. Today, the borders of the old terranes and subterranes are still active faults that cause numerous earthquakes in the area.
Scientists ‘agree to disagree’ about mountain building and volcanism
Various theories about how exactly the Caucasus range is being constructed still remain disputed among geologists. Two ways of how the orogen may have formed have been considered:
1) Delamination: Denser parts of upper mantle lithosphere were detached from the edges of the continental plates. Sinking down, they would allow hot mantle material to rise and settle just below the crust. This, together with other conditions, would be the origin of volcanic centres in the Caucasus and surrounding collisional areas. The image right shows how the delamination mechanism in the Caucasus region could work: The crust is composed of the upper felsic (orange) and lower mafic (green) parts. Blue areas indicate the mantle parts of the lithosphere. Green dots mark the possible locations of “drops” sinking from the lower mafic crust. Red arrows mark the asthenosphere upwelling.
2) Subduction of oceanic crust (the ancient Tethys Sea floor), followed by Arabian continental crust, beneath the Greater Caucasus.
While in 2012 Koulakov et al. pleaded for the delamination model; Mumladze et al. found new evidence for the long debated subduction model in 2015. They show that the central and eastern Greater Caucasus are underlain by a northeast-dipping zone of mantle seismicity which they interpret as a subducted or still subducting slab. In the western part, they suggest that a subducted slab has already broken off and sunken. This detachment of subducted slab would have stopped the subduction here. Mt. Elbrus, Chegem caldera and Kazbegi volcanic complex point to the existence of a volcanic arc in the western Caucasus, developed above the former subduction zone.
Scientists ‘agree to disagree’: Who’s at fault with the Fault?
The Kazbek volcanic center has developed near the intersection of the Main Caucasus Reverse Fault (MCRF) and the major regional Borjomi-Kazbek Fault (or, rather, its upper part Kazbek-Tskhinvali Fault). The MCRF is the frontal thrust zone of the Caucasus. The Borjomi-Kasbek fault cuts NE-SW through the MCRF. It is the northern half or so of the 650 km long North East Anatolian Fault system.
The teaching is that the generally wedge-shaped front of the rigid Arabian block is intensively indenting northward into the relatively mobile Minor-Asia – Caucasus region. The western boundary of this actively indenting wedge is thought to be the large Borjomi-Kazbek fault. This major fault has been known to geologists since the 19th century. However, working with an improved seismic network nowadays, scientists noted an utter lack of seismicity directly along the alleged Borjomi-Kazbek fault. It also did not show up in other investigations with new technologies. So… what if the fault didn’t exist at all?
The researchers questioned the existence of the Borjomi-Kazbek fault, and with it, the tectonic model of an entire region. This fault system is still shown on overview maps in numerous publications. Some scientists might hang on to it as they debate the new findings, others find evidence for at least partial existence. Whether the fault exists in segments, or just very deep in the lower crust, or not at all – remains to be seen in a few years’ further research.
KAZBEGI (KAZBEK) VOLCANIC CENTER
The first pulses of younger magmatism in the Kazbek area show in the geological record around 5-4 Ma. Around the central volcano, arranged along an arch east, south and west, are more than ten satellite volcanoes or vents (e.g. Greater Tkarsheti, Kechuttsveri and Shevandeni).
Eruptions within this center lasted presumably from the first part of Middle Pleistocene (around 450 ka) into the Holocene. By chemical composition of volcanic cones and their lava flows scientists could distinguish four main magmatic phases in the Kazbek volcanic center: phase I: 460–380 ka (mainly northern parts); phase II: 310–200 ka (likely time of caldera collapse: 200–150 ka); phase III: 130–90 ka and phase IV: less than 50 thousand years ago.
In 1958, the geologist N.I. Skhirtladze undertook work to collect, review and evaluate all research on Kazbek that had been done since the beginning of the century. In his resulting monograph, for the first time, the conclusion was drawn that Kazbek is a polygenetic volcano. It would take another 20 or so years until it was agreed that it is also a potentially active volcano which has erupted within the last ten thousand years.
The modern Kazbek volcano has succeeded its predecessors, earlier Proto-K. and later Paleo-K.. Paleo-Kazbek was likely much larger than the present volcano and left a 5 km caldera. Numerous volcanic vents of different age are found along the periphery of Kazbek’s main cone, including mono- and polygenetic cones, extrusion domes, and other relatively small explosion centers.
Unfortunately, most of these are hard to spot on satellite maps – firstly because the upper parts are mostly covered by snow and glaciers. Secondly, these mountains are so heavily eroded and carved into new shapes by water and ice that it is almost impossible to even make out the caldera. Many lava flows have been eroded by rivers from two sides, such that the only remnants today are long, narrow ridges of lava.
The steep modern stratovolcano consists of alternating horizons of dacite lava, lava breccia, and agglomerates. Its eruptive history has seen alternating explosive eruptions and outpourings of lava flows. Kazbek’s intense magmatic activity has produced numerous long (up to 15 km) and thick (up to hundreds of meters) lava flows, the upper ones reaching the river Tergi (Terek). Lava flows are often built up of half a dozen or more layers of different ages.
The rocks of Kazbek show a continuous succession of basaltic andesite‒>andesite‒>dacite. That means that magma in the reservoir was able to evolve into a more silicic rock type during long repose times. Rising new basaltic magma would then mix with the residual more felsic mush in the reservoir. Volcanologists conclude that the present volcano has been fed by the same magma reservoir from its beginning.
Magma temperatures for Kazbek at its heyday were calculated at 1150–1220°C. Such high-temperature magmas have a rather low viscosity and the lavas are highly mobile – which explains the rather long lava flows.
I’m always smitten by the amount of detail geologists can work out about a volcano’s history – they cannot just look inside, after all. For example, Lebedev et al. tell us that Proto-Kazbek volcano likely produced fissure eruptions, related to the Main Caucasus Fault zone. The caldera forming process of Paleo-Kazbek was likely not associated with any intense explosions. They conclude this from the practically absolute absence of tephra sheets in the territory. Along the ancient Terek watercourse between Stepantsminda and the Russian town of Vladikavkaz (40 km north of Kazbek) various lahar deposits have been found. These lahars were likely produced during the catastrophic destruction of the Paleo-Kazbek cone, whose edifice was likely then overlain by a thick ice–snow cover.
The modern cone of Kazbek stratovolcano began to grow at the onset of phase III in the central part of the Paleo-Kazbek somma. Additionally, at the same time, several of the surrounding eruptive centers also emitted lava flows. This phase was also the time, when lava compositions changed from andesites to dacites. However, most recent lavas of phase IV are again of a more mafic nature which speaks for replenishment of fresh magma from the mantle source. Nevertheless, during the last 50k years only small peripheral volcanic vents erupted in various parts of the Kazbek complex, and the magmatic activity was generally of low intensity.
The last magmatic activity probably occurred at the Lesser Tkarsheti (2214 m), a small volcanic cone located in the western wall of the Terek river valley. A single eruption of the volcano issued a 1 km andesitic lava flow. This flow blocked the river Terek for a time, forming a 350 m deep lake. Dating of the Lesser Tkarsheti lava flow came up with an age of ~6-7ka, i.e. it was erupted in the middle of the Holocene. This puts the Kazbek volcano into the category of active dormant volcanoes, although no seismic activity nor any other volcanic rumblings have been recorded in modern times.
Note: The Lesser Tkarsheti is the youngest eruption I have found in recent papers. However, the GVP has one later event listed in Kazbek’s history at 750 BCE – this would mean around the year 1270 AD. Whether this refers to an event from older research, now discarded, or it was merely a phreatic explosion of hydrothermal origin (as it was detected through tephrachronology) – I don’t know. Most probably I have missed something, please let me know if you know more about this.
One thing is for sure, though: All that’s visibly active today is some steam from the geothermal/hot spring system of the volcano.
To get the idea what Kazbek volcano is like, here is a very good 12 minute video of an Kazbek ascent by Grzegorz Gawlik (GrzegorzGawlik.pl)- a pleasure to watch!:
THE KOLKA DISASTER at Kazbek’s northern slopes
More than ten glaciers descend from Kazbek itself and nearby Mount Dzhimara. Starting in the second half of the 20th century, many of these glaciers receded for hundreds of meters. This process was associated with catastrophes such as the ice avalanche of the Kolka glacier in 2002 and mudflows in the Devdoraki Gorge in 2014–2016. Some researchers suggest that one of the driving forces of these processes and phenomena is the “volcanogenic factor”, namely, the warming of the rocks beneath glaciers due to changes in the heat flow from the magma reservoir.
The 2002 collapse of the Kolka Glacier, located in a valley between Mt. Dzhimarai and Kazbegi, was attributed by some to hydrothermal activity along the northern slope of the mountain:
On the evening of September 20, in a cirque just west of Mt. Kazbek, chunks of rock and hanging glacier on the north face of Mt. Dzhimarai-Khokh tumbled onto the Kolka glacier below. The glacier shattered, setting off a massive avalanche of ice, snow, and rocks that poured into the Genaldon River valley. Hurtling downriver for nearly 12 km at speeds up to 250 km/h, the avalanche exploded into a small bowl of land between two mountain ridges – and swallowed the village of Nizhniy Karmadon and several other settlements.
At the northern end of the bowl, the churning mass of debris reached a choke point: the Gates of Karmadon, the narrow entrance to a steep-walled gorge. Gigantic blocks of ice and rock jammed into the narrow slot, and water and mud sluiced through. Trapped by the blockage, avalanche debris finally cemented into a towering dam of dirty ice and rock. 20 million tons of ice, snow and rock poured over the Genaldon and Kamardon area along a total length of 18 km, which killed 125 people. Only 20 bodies were recovered. (after NASA earth observatory, 2004; read more about this event in a detailed post by Albert in Volcanocafe, link below).
BESIDES… The Dariali Gorge
On its southern and eastern sides the Kazbek volcanic center is surrounded by the course of the Terek river. This river did a great job of carving out gorges hundreds of meters deep, notably so on the eastern side. There the Dariali Gorge is an impossibly steep cleft in the mountains, running for 13 km from Stepantsminda to the Russian border and beyond to Zemo Larsi (Upper Lars). The actual border station is near the pass of Darial at 2,378 m. The gorge was cut through various volcanic products and in places through Paleozoic granite intrusions. The cliffs of thick lava flows can be seen along the western side of the road, among them the latest flows from the Greater and Lesser Tkarsheti vent.
For millennia, this mountain passage has been strategically crucial, and has been fortified since at least 150 BC. In places, the cliff faces are more than 1,000 m high. Through the gorge runs the main international road connecting Georgia and Russia, or Asia and Europe, if you will. This road has been for hundreds of years the Georgian Military Road. Before that it was important for the peoples of all ages in history to watch the narrow entrance to their territory. Medieval watchtowers and castles, waterfalls and wildlife make this one of the most incredible roads in the world.
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
– GVP, Kazbek
– Catalogue of Quaternary volcanoes of the Greater Caucasus […] (2014, paywalled)
– Geology of the Caucasus: A Review (2011, PDF)
– Geological Structure of Georgia and […] the Caucasus (2015)
– Major Events in Evolution of the Kazbek Neovolcanic Center (2014, PDF)
– Magnetic dating of the Holocene monogenetic Tkarsheti (2019)
– Chronology of Magmatic Activity and Petrologic […] Kazbek (2017, paywalled)
– […] slabs beneath the Greater Caucasus (2015)
– Deep Structure of the Racha Earthquake […] (2004, PDF)
– Nature of orogenesis and volcanism in the Caucasus region (2012, PDF)
– Borjomi-Kazbegi […] Reality or Fiction (2017, PDF, p. 181 ff)
– The Kazbek disaster: a cryoclastic debris flow (VC)