Geoscientist Online

Some volcanoes seem to be in the wrong place. They lie right in the middle of continents, about as far from a plate boundary as is possible to be. The Quaternary volcanoes of Mongolia present just such an enigma and uncovering the underlying cause for their existence presents an intriguing challenge which, just possibly, could lead on to a step change in understanding the relationship between the structure of the continents and the behaviour of the mantle below them.

The saying goes that exceptions prove – i.e. test – the rule, and in science such tests may lead on to breakthroughs in understanding. For decades scientists have, for the most part, understood why volcanic belts are distributed the way they are. As the Earth ages and cools, it releases most of its energy along the margins of tectonic plates that cover its surface – and here lie the Earth’s great volcanic belts.

Some of the Earth’s heat is also thought to be released by rising hot spurts of material known as mantle plumes, possibly originating as deep as the core-mantle boundary. According to plume theory, chains of volcanic islands in the middle of ocean basins, such as the Hawaiian Islands in the Pacific, form by the movement of the thin oceanic plate over a heat source. But why should volcanoes, sourced from the mantle beneath thick continental crust, be found smack in the heart of the largest continental mass on Earth?

The Hangay Dome of central Mongolia, is mountainous and peppered with volcanic cones and flows of alkali-olivine basalt (Fig. 1). Some of these volcanoes are so young that no glaciation has worn them down and forests have not yet covered their slopes (Fig. 2). Their precise ages are not known; although carbon ages of around 5000 years are reported in the Russian literature (sadly lacking in experimental detail). There remain some signs of continued activity; in 2002 articles appeared in the Mongolian and Russian press that a volcano north of the Hungay Dome was producing ‘white vapours and constant acoustic noise’. Subsequent investigations confirmed both minor seismic activity (magnitude 2.0-3.5) and the presence of tepid fumaroles. Curiously, the reports cited the migration of marmots from the immediate vicinity during the 1990s as marking the resurgence of this activity although marmots have been virtually hunted to extinction throughout the region.

Weathered cones with breached crater walls indicate older volcanism and basalts from such volcanoes have yielded Ar-isotope ages of a million years or more. There appears to be no obvious trend to this volcanism, either in space or time. To the north lies Lake Baikal – the deepest lake on Earth, lying in a major continental rift. To the south stands the Tibetan Plateau, the largest uplifted region on Earth, caused by a head-on collision between Indian and Asian plates. Did either of these regional structures influence the magmatism in Mongolia?

Nick Rogers, Ian Parkinson, Tiff Barry and I planned a two-pronged assault on the problems posed by these volcanoes. We would look at the isotope geochemistry of the magmas, which would tell us about where they came from and the nature of the mantle below. This task fell to Alison Hunt, NERC-funded research student at the Open University. My particular interest was to collect fragments of material torn from the sides of the magma chamber – mantle xenoliths – which could tell us the temperatures and depths at which melting took place.

The key to this work is to find the right mineral assemblage. Garnet and pyroxenes provide estimates of depths, olivine and spinel inform on temperatures. With luck, the magmas would also bring up fragments of crust that would yield a minimum value for crustal thickness.

Getting there

Mongolia is a large country and to reach many geological sites needs a week or more of bone-breaking travel – few roads outside of Ulaan Baatar are more than dirt tracks. Fortunately by Mongolian standards the volcanoes of the Hangay Dome are accessible and can be reached by a mere two days’ hard driving from Ulaan Baatar. Yo Majigsuren, our colleague from Mongolian University of Science and Technology, recommended a Furgon for transport, an ancient but robust Russian van favoured by Mongolians because vehicles need to be fixed wherever a breakdown might occur.

If transportation is the bane of Mongolian fieldwork, the boon lies in its simplicity. Since well before the times of Genghis Khan, the economy of central Asia has centred on roaming herds of sheep, yak, camels or horses, and this has spawned a rich nomadic culture. As a result, there is almost nowhere across the steppes of central Asia where you can not pull off the road and put up a tent. Food is equally uncomplicated. Majigaa arranged for a sheep to be slaughtered on the first day out from Ulaan Baatar. According to tradition, she boiled the intestines almost immediately and offered them around as a breakfast delicacy while the rest of the animal was butchered and thrown into the back of the van. The meat lasted a week without refrigeration. As for drink, the horses provide it. The Mongolian steppe is littered with gers, round white tents, many of which sell fermented horse’s milk from the family herd. I doubt that the alcoholic yoghurt airag would catch on abroad, but it is nutritious and its effects break up the tedium of the long uncomfortable drives.

Following two field seasons, we have now recovered samples from different parts of the dome. In the central, mountainous region of Tariat (Fig. 1), huge olivine-rich mantle bombs were discovered by Ian and Alison (Fig. 3). Towards the edge of the dome in the Togoo area (Fig. 4) we recently discovered much smaller xenoliths (Fig. 5); for the first time garnet-bearing assemblages offer us the chance of tying down the variations in the depths and temperatures of melting across the volcanic province.

It is too early just yet to speculate on the results of our work. Many of the samples are still in transit from Mongolia. When back at the OU they will be sectioned and individual minerals analysed under the electron microprobe. But we can say that some of the material looks exciting. The rare garnet-peridotite xenolith assemblages from the margins of the dome suggest depths of melting in excess of 80km. Within the central dome, xenoliths of garnet granulites and spinel lherzolites have been brought up from shallower depths in the range 40-60km. We need to flesh out these spatial trends and show how they evolved through time.

The mystery of the ages of individual cones will not be easily solved. However, several of the younger craters not only provided perfect camping grounds but also revealed bronze and stone-age burial sites, so we know these volcanoes have been inactive for at least 3000 years. Isotopic dating in the interval of a few thousand years back to a million years or so is notoriously tricky, but we shall apply both radium techniques to pin down volcanism less than 8000 years old, and Ar-Ar isotope methods to date the sanidine phenocrysts from some of the Pleistocene vents.

Once we have a few answers to these questions we shall have solved at least part of the question, why do volcanoes form in the middle of continents?

Acknowledgments

I would like to thank the Geological Society of London’s W G Fearnsides Fund for supporting my fieldwork in Mongolia.