1. Geography

2. Geological context

3. Geomorphology

4. Effects of glaciation

5. Composition

6. Chronology

7. References
     Sources  

Azas Plateau is a volcanic field in Russia. It is also known as East Tuva Plateau, Khamsara-Biykhem Plateau and Northeast Tuva Plateau.^[1] It covers a surface area of {{convert|2000|km2}} west of Lake Baikal. Volcanic activity in the area also occurred at the Oka Plateau and the Jom-Bolok volcanic field.

The field is formed by a late Pliocene lava plateau. Several volcanoes formed under the influence of ice and are constructed by lava flows and hyaloclastite, these are so-called tuyas which form by eruptions of volcanoes into ice. Ice meltwater floods may have accompanied this activity. The youngest dated volcanic centre was active 48,000 ± 20,000 years ago, but some lava flows may be even younger.

Geography

The field is located in southern Siberia,{{sfn|Komatsu|Arzhannikov|Arzhannikova|Ershov|2007|p=312}} between the East Sayan Mountains and the Todzha basin.{{sfn|Komatsu|Arzhannikov|Arzhannikova|Ori|2007|p=353}} There it covers a surface area of {{convert|2000|km2}} west of Lake Baikal,^[1] at an altitude of {{convert|2000|m}}.{{sfn|Arzhannikov|Braucher|Jolivet|Arzhannikova|2012|p=22}} Volcanic landforms include cones,{{sfn|Arzhannikov|Braucher|Jolivet|Arzhannikova|2012|p=17}} lava fields, tuyas,{{sfn|Komatsu|Arzhannikov|Arzhannikova|Ori|2007|p=352}} and other volcanic structures including scoria cones, volcanic dykes, volcanic necks and volcanic vents.{{sfn|Komatsu|Gabriele Ori|Ciarcelluti|D. Litasov|2004|p=172}} Other landforms are cirques and U-shaped valleys left by previous glaciations.{{sfn|Komatsu|Arzhannikov|Arzhannikova|Ershov|2007|p=315}} The Bii Khem river, a tributary of the Yenisei River,{{sfn|Komatsu|Baker|Arzhannikov|Gallagher|2015|p=1713}} crosses the field. Politically the field belongs to the Tuva Republic of Russia, and is in a remote area where access is difficult.{{sfn|Komatsu|Arzhannikov|Arzhannikova|Ershov|2007|p=313}} The volcanoes have been described in the 1850–1900 years.{{sfn|Arzhannikov|Braucher|Jolivet|Arzhannikova|2012|p=17}}

Geological context

It is part of the Tuva volcanic province of the Baikal Rift Zone, which is linked to a hotspot that has raised the terrain to up to {{convert|3000|m}}.{{sfn|Komatsu|Gabriele Ori|Ciarcelluti|D. Litasov|2004|p=168}} Activity in the rift zone after starting in the Oligocene reached its maximum during the Miocene.{{sfn|Litasov|Hasenaka|Litasov|Yarmolyuk|2002|p=201}} Together with the Oka Plateau the Azas Plateau forms the East Sayan province of the Baikal Rift Zone.{{sfn|HASENAKA|LITASOV|TANIGUCHI|MIYAMOTO|1999|p=250}} Activity in the Azas Plateau has been influenced by a tectonic graben in the area. The Azas Plateau is the largest lava field in the Tuva volcanic province.{{sfn|Komatsu|Gabriele Ori|Ciarcelluti|D. Litasov|2004|p=169}} The Jom-Bolok volcanic field may be related to the Azas Plateau.{{sfn|Jolivet|Arzhannikov|Arzhannikova|Chauvet|2013|p=121}}

Geomorphology

A large lava field with thicknesses of {{circa}} {{convert|250|m}} formed during the Late Pliocene, when the largest eruptions took place.{{sfn|Komatsu|Arzhannikov|Arzhannikova|Ershov|2007|p=314}} These flows reached a thickness of approximately {{convert|250|m}}.{{sfn|Litasov|Hasenaka|Litasov|Yarmolyuk|2002|p=205}} Nine volcanoes bearing evidence of having formed beneath ice are found in the southeastern part of the field.{{sfn|Komatsu|Arzhannikov|Arzhannikova|Ershov|2007|p=316}} Volcanoes on the plateau are about {{convert|400|-|600|m}} high and have the form of tables. Ash cones formed on the individual edifices.{{sfn|Arzhannikov|Braucher|Jolivet|Arzhannikova|2012|p=23}} The lava flows on the table-shaped volcanoes also display rounded hollows.{{sfn|HASENAKA|LITASOV|TANIGUCHI|MIYAMOTO|1999|p=264}} Many volcanoes appear to be part of a lineament.{{sfn|Litasov|Hasenaka|Litasov|Yarmolyuk|2002|p=217}} Volcanism may be influenced by strike-slip faults associated with the Jom-Bolok fault.{{sfn|Jolivet|Arzhannikov|Arzhannikova|Chauvet|2013|p=122}}

Individual volcanoes in the Azas Plateau volcanic field include the Derbi-Taiga tuya ({{coord|52|22|15|N|98|14|8|E|display=inline|notes=}}), which is formed by several layers of hyaloclastite and lava flows and displays a prominent "winged" topography.{{sfn|Komatsu|Gabriele Ori|Ciarcelluti|D. Litasov|2004|p=171}} The hyaloclastite layers were formerly considered to be tuffs and reach thicknesses of {{convert|8|m}}. Pillow lavas also exist.{{sfn|HASENAKA|LITASOV|TANIGUCHI|MIYAMOTO|1999|pp=264-265}} A system of ancient valleys formed by floods are found near to Derbi-Taiga.{{sfn|Komatsu|Arzhannikov|Arzhannikova|Ori|2007|p=357}} Ulug-Arginskii ({{coord|52|23|18| N|98|23|7|E|display=inline|notes=}}) is a cone that formed exposed in the air.{{sfn|Komatsu|Arzhannikov|Arzhannikova|Ershov|2007|p=316}} The cone is accompanied by a maar with a lake to the south, and a lava flow from the cone reached {{convert|10|km}} in length.{{sfn|HASENAKA|LITASOV|TANIGUCHI|MIYAMOTO|1999|p=265}} Shivit-Taiga with a length of over {{convert|10|km}} and a height of {{convert|500|m}} is among the biggest volcanoes in the world that formed beneath ice, but Derby-Taiga may be even larger depending on how the size is measured.{{sfn|Komatsu|Arzhannikov|Arzhannikova|Ershov|2007|p=317}} Shivit-Taiga covers a surface of {{convert|100|km2}} and is {{convert|500|m}} high.{{sfn|Arzhannikov|Braucher|Jolivet|Arzhannikova|2012|p=23}} This edifice is a stratovolcano and it features two craters with diameters of {{convert|250|m}} and {{convert|100|m}}.{{sfn|HASENAKA|LITASOV|TANIGUCHI|MIYAMOTO|1999|p=265}} Lava lakes formed in two craters on the volcano.^[1] Shivit-Taiga too is accompanied by a topography that indicates that ancient floods passed there.{{sfn|Komatsu|Arzhannikov|Arzhannikova|Ori|2007|p=357}} Kokhemskii is {{convert|2648|m}} and was affected by glacial erosion. Sorug-Chushku-Uzu {{convert|1|km}} north of Shivit-Taiga is {{convert|2525|m}} high.{{sfn|Arzhannikov|Braucher|Jolivet|Arzhannikova|2012|p=23}}

Some volcanoes have been affected by landsliding, gully formation and glaciation after their formation.{{sfn|Komatsu|Arzhannikov|Arzhannikova|Ershov|2007|p=322}} Likewise, the field has been affected by erosion, forming deep valleys with glacier deposits.{{sfn|HASENAKA|LITASOV|TANIGUCHI|MIYAMOTO|1999|p=264}}

Effects of glaciation

The Azas Plateau was affected by an ice sheet which formed on the East Sayan mountains during the Pleistocene.{{sfn|Arzhannikov|Braucher|Jolivet|Arzhannikova|2015|p=1510}} When volcanoes erupt into ice, the ice melts and volcanic structures named tuyas can form in meltwater lakes.{{sfn|Komatsu|Gabriele Ori|Ciarcelluti|D. Litasov|2004|p=169}} Runny lavas can form such edifices, which tend to have steep slopes, only when trapped within ice.{{sfn|Komatsu|Gabriele Ori|Ciarcelluti|D. Litasov|2004|p=170}} During tuya activity, the subaqueous activity forms hyaloclastite and when the activity occurs in the air lava flows are formed,{{sfn|Komatsu|Gabriele Ori|Ciarcelluti|D. Litasov|2004|p=185}} including lava deltas.{{sfn|Komatsu|Arzhannikov|Arzhannikova|Ershov|2007|p=318}} Some of the tuyas in the Azas Plateau may have formed through complex eruptive processes.{{sfn|Komatsu|Arzhannikov|Arzhannikova|Ershov|2007|p=317}} Based on the tuyas of the Tuvan field, it has been determined that ice thicknesses in the area exceeded {{convert|700|m}} and that at least three different ice ages occurred in the field.{{sfn|Komatsu|Gabriele Ori|Ciarcelluti|D. Litasov|2004|p=171}} The tuyas in the Azas plateau have been compared to volcanic edifices in the Valles Marineris on Mars.{{sfn|Komatsu|Gabriele Ori|Ciarcelluti|D. Litasov|2004|p=177}} Non-tuya volcanic ice interaction landforms are also found, such as Albine-Bondok, Charash-Dag and Sagan which are subglacial mounds.{{sfn|Komatsu|Arzhannikov|Arzhannikova|Ershov|2007|p=318}} Glaciation was already in progress under marine isotope stage 5 (127,000 - 71,000 years ago) and by the stage 4 (71,000 - 57,000 years ago) it was continuous over the Azas Plateau.{{sfn|Arzhannikov|Braucher|Jolivet|Arzhannikova|2015|p=1518}} Glacier retreat was underway by the isotope stage 3 (57,000 - 24,000 years ago), when some volcanoes formed in the air.{{sfn|Arzhannikov|Braucher|Jolivet|Arzhannikova|2015|p=1519}}

Composition

Volcanism is overall basaltic,{{sfn|Komatsu|Arzhannikov|Arzhannikova|Ershov|2007|p=312}} volcanic rocks erupted by the Azas Plateau include basalt and trachybasalt. Other volcanoes in the Baikal Rift Zone have erupted the same rocks.{{sfn|Komatsu|Gabriele Ori|Ciarcelluti|D. Litasov|2004|p=168}} These rocks are alkaline and their texture is aphyric.{{sfn|Litasov|Hasenaka|Litasov|Yarmolyuk|2002|p=207}} Minerals contained in the rocks include augite, clinopyroxene, olivine, plagioclase, {{titanium}}-containing magnetite and nepheline in the more alkaline rocks.{{sfn|Litasov|Hasenaka|Litasov|Yarmolyuk|2002|p=209}} The total volume of volcanic rocks is about {{convert|600|km3}}.{{sfn|Litasov|Hasenaka|Litasov|Yarmolyuk|2002|p=204}} Hematite is present in the rocks and gives many tuyas a red colour.{{sfn|Komatsu|Gabriele Ori|Ciarcelluti|D. Litasov|2004|p=172}} Based on geochemistry, the magma formed at depths of {{convert|80|km}}.{{sfn|Jolivet|Arzhannikov|Arzhannikova|Chauvet|2013|p=120}} Volcaniclastic rocks were originally considered to be tuffs.{{sfn|Arzhannikov|Braucher|Jolivet|Arzhannikova|2012|p=22}} Cumulates and xenoliths from the mantle have been found at some volcanoes.{{sfn|Litasov|Hasenaka|Litasov|Yarmolyuk|2002|p=206}}

Chronology

Volcanic activity in the Azas Plateau commenced during the Late Pliocene. Activity continued into the Holocene.{{sfn|Komatsu|Gabriele Ori|Ciarcelluti|D. Litasov|2004|p=169}} Before volcanism of the Azas Plateau started, volcanism occurred in the Oka plateau. The Azas trough formed in the Miocene-Pliocene.{{sfn|Litasov|Hasenaka|Litasov|Yarmolyuk|2002|p=204}}

The first volcanic activity beginning 2 million years ago formed voluminous lava flows from fissure vents. Later the activity focused on deep seated lineaments.{{sfn|Litasov|Hasenaka|Litasov|Yarmolyuk|2002|p=201}} Volcanic activity between 1.65 - 1.75 mya occurred beneath ice in the ancient valleys of the Bii Khem river. In the middle-late Pleistocene volcanism occurred in the southeastern parts of the field.{{sfn|Komatsu|Arzhannikov|Arzhannikova|Ershov|2007|p=314}} Some dates obtained on edifices in the Azas Plateau are 725,000 ± 50,000 - 760,000 ± 50,000 years for Derbi-Taiga,{{sfn|Komatsu|Arzhannikov|Arzhannikova|Ershov|2007|p=325}} 600,000 - 560,000 years for Kadyr-Sugskii, 600,000 - 290,000 for some volcanoes surrounding Derbi-Taiga, 350,000 - 290,000 for Yurdawa between Derbi-Taiga and Shivit-Taiga,{{sfn|Litasov|Hasenaka|Litasov|Yarmolyuk|2002|p=206}} 225,000 ± 50,000 for Ploskii,{{sfn|Komatsu|Arzhannikov|Arzhannikova|Ershov|2007|p=325}} 150,000 ± 50,000 for Kokhemskii, 130,000 ± 40,000 - 110,000 ± 000 for Shivit-Taiga,{{sfn|Arzhannikov|Braucher|Jolivet|Arzhannikova|2012|p=23}} 75,000 ± 40,000 for Priozernyi, {{sfn|Komatsu|Arzhannikov|Arzhannikova|Ershov|2007|p=325}} 60,000 ± 40,000 for Sorug-Chushku-Uzu and 48,000 ± 20,000 for Ulug-Arginskii.{{sfn|Arzhannikov|Braucher|Jolivet|Arzhannikova|2012|p=23}} Some lava flows in the Bii-Khem valley are even younger than the Ulug-Arginskii volcano.{{sfn|Arzhannikov|Braucher|Jolivet|Arzhannikova|2012|pp=23-24}} It is possible that when eruptions in the Azas Plateau occurred beneath ice, they caused floods of glacial meltwater called Jokulhlaup;{{sfn|Komatsu|Gabriele Ori|Ciarcelluti|D. Litasov|2004|p=184}} corresponding landforms have been found at Derbi-Taiga and Shivit-Taiga volcanoes.{{sfn|Komatsu|Baker|Arzhannikov|Gallagher|2015|p=1712}}

References

Sources

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