The Azas Plateau is a large volcanic field located west of the SW tip of Lake Baikal, north of the Mongolian border in central Asia. It is one of multiple intraplate volcanic fields associated with the Lake Baikal rift, one of the largest active continental rifts in the world.
The plateau goes by multiple names including the East Tuva Plateau, the Northeast Tuva Plateau, or the Khamsara-Biykhem Plateau. The plateau is remote, located in southern Siberia just north of the Mongolian border. Elevation is quite high, at 2,765 m.
The closest large city is Irkutsk, some 400 km E, with nearly 620,000 people.
The political designation of this part of Siberia is the Tuva Oblast, a republic currently defined in the Constitution of the Russian Federation as a state. It is the geographical center of Asia. Tuva was constituted as a sovereign 1921 – 1944 after which it became part of the USSR. Total population was 307,000 in 2010. It is also claimed by one of the major Taiwanese political parties as part of China.
Climate is generally subarctic, with snow cover diminishing in recent decades. Average winter temperature is -32° C. Average summer temperature is 18° C. Average yearly precipitation ranges between 15 – 100 cm. Much of the region has permafrost. There are few roads, lots of rivers, lakes, mosquitoes, and greenery.
The region is sparsely populated with around 4,500 within 100 km of the plateau. There are no active local monitoring or webcams.
Region
Volcanism around the Baikal Rift Zone is located in three regions of the rift zone. Udokan volcanic field is NE of the rift. Vitim is 200 km SW and off the rift center. The third zone is SW of the end of the rift, a broad zone of volcanism. Eruptive products in the Tunka Basin, Oka Plateau and Tuva volcanic fields to the SW and Vitim to the SE are quite similar and unevolved.
The Udokan volcanic field partly overlaps the Siberian shield on the eastern end of the Baikal Rift. Activity here started at least 30 Ma with massive fissure eruptions that formed a 400 – 500 m thick lava plateau. As with the other regions associated with the rift, magmatism switched from primarily fissure fed to central volcanoes that erupted onto the eroded basalt plateaus. The most recent eruption from all the fields was a Udokan trachytic pyroclastic flow some 2,000 years ago. Udokan magmas are the most variable in the entire rift. Some of them are the most evolved. The location of the field along a continental suture may explain variations in magmas.
The Udokan field was active in three phases. The oldest was over 30 Ma which erupted the massive lava flows from fissures. It went quiet until some 14 Ma, coincident with the subsidence of the Chara basin. This volcanism also stopped for at least 5 Ma, resuming 9.9 – 7.5 Ma, 4.0 – 2.6 Ma, and 1.8 Ma – present, coincident with another round of subsidence on the flanks of the Chara basin and reactivation of the Chukchudu tectonic zone.
The Vitim Plateau is somewhat of an outlier, covering 3,500 km2 200 km SE of Lake Baikal. It is located outside the boundaries of the Sayan-Baikal uplift. Activity here also produced massive sheets of lava, followed by cinder cones, explosion craters and lava domes. It is the largest area of volcanism in the Baikal Rift Zone.
Rift basin formation appears to be independent of volcanic activity. And it has a relatively low volume of erupted volcanic products for a rift of its size (around 6,000 km3). In contrast, it is an order of magnitude less than products from the Rio Grande Rift. One of the oddities about Vitim is that mantle depth is estimated to be 80 – 100 km below the surface and the position of the field 100 from any rift depression. This raises questions about the source of the magma.
The largest volume of volcanic rocks were erupted in the early stages of rifting in the Miocene and has tapered off (except for Udokan) in more recent times. Location of older rocks is highly dependent on local topography at the time of the eruption, generally filling river valleys. Interestingly enough, no volcanic rocks have been found in Lake Baikal.
The western portion of recent volcanism associated with the Baikal Rift Zone extends about 500 km W from the SW tip of Lake Baikal, north to the boundary of the Siberian shield, and S to the Mongolian border. There are additional fields in Mongolia which are part of the broad dome uplift region, but they are considered to be outside the Baikal Rift Zone. The immediate region is referred to as the Tuva Volcanic Province and locus of volcanic activity has shifted over time. Main volcanic activity shifted from the Oka basin to the Azas trough over time. The most recent activity was on the Azas Plateau.
The Tunka Basin is an asymmetric graben that extends 200 km W from the SW end of Lake Baikal. It is bounded by the Tunka fault to the N. The field has a significant number of lavas from fissure eruptions layered with sediments. 65 basalt layers are interbedded with coal bearing sediments. There appears to be a larger volume of lavas on the N end, the most deformed part of the graben. A series of explosive eruptions early Pleistocene deposited 200 – 240 m of pyroclastics in the basin. Basalts in the final phase layered with sediments in a 150 m thick stack in the basin itself. Final phase of volcanism created at least 20 small cinder cones. Activity here is very old, starting as long as 30 Ma and ending as recently as 11 ka.
The Oka Plateau / East Sayan region is a dispersed area of volcanism W of the Tunka Basin. It extends W of Tunka basin and is bounded N by the Main Sayan Fault. The East Sayan Mountain Range separates Oka from the Tuva volcanic field to the W. Similar to other volcanism associated with the rift, volcanism was most prolific in the Miocene and Pliocene, tailing off and becoming more diffuse in recent years. The oldest basalt plateaus are 300 – 500 m thick, 1,440 – 2,500 m above sea level. The youngest known basalt volcanism is an 85 km long Holocene basalt flow in a river valley. The flow has three cinder cones on its surface.
The Tuva volcanic field, of which the Azas Plateau is the most prominent part, is the westernmost piece of Baikal Rift Zone volcanism. It is west of the uplifted East Sayan Mountain Range, and E of the Ulug-Arga Ridge, N of the Mongolian border. The Azas Plateau, also known as the East Tuva Lava Field, covers some 2,400 km2 and consists of multiple lava flows and post-plateau recent volcanoes. There are a number of smaller basalt outliers spread out across the remainder of the rest of the 45,000 km2 volcanic area.
Volcanism proceeded as pulses, with at least 9 stages over the last 2.5 Ma. Two of these stages coincided with widespread glaciation, producing subglacial eruptions of Icelandic-style table mountains (tuyas) 400 – 500 m thick hyaloclastites capped with massive lava flows.
Volcanic Field
The field covers some 2,000 km2 with a total volume up to 600 km3. Older volcanics are generally massive fissure-fed lava flows. Newer volcanics are thick hyaloclastites capped with lavas erupted under glacial cover. Composition of lavas is similar over time. The earliest and most voluminous eruptions were trachy-basalts. Smaller and later volcanoes erupted tephrites / basanites.
The oldest volcanoes of the Tuva volcanic province are located outside the Azas Plateau, with discrete volcano centers W and SW of the plateau. The Uyuk volcano is 28 Ma. The Ulug-Oo volcano is 23 Ma. There are terraces of basalt lavas 200 – 300 m above the local riverbed that likely belong to this stage.
Major activity started 16 – 14 Ma. There are fragments of lava flows that suggest the Azas Trough formed in the middle Miocene. These lava flows are over 100m thick. There are no volcanic products 14 – 2.1 Ma. Alignment of volcanic centers seem to reflect presence of a deep tectonic fissures. In some cases, eruptions took place a intersections of fissures and deep river valleys. More than half of the volcanoes are along a single SW-NE line.
Late Pleistocene volcanic activity dates around 2.1 Ma. It is the most voluminous eruptions that created the lava plateau within the Azas Trough. The lava flows smoothed mountainous relief with a variable thickness around 250 m. The plateau is inclined between 2,200 – 1,200 m, higher to the SE, and over 70 km long.
Early Pleistocene activity started with another round of lava flows 1.75 – 1.65 Ma. These flows filled eroded valleys in the surface of the earlier plateau lava flows and are 100 – 200 m thick between the Shivit-Taiga and Derbi-Taiga volcanoes. The lavas date around 1.2 Ma. The extent of these lavas to the NW is unknown.
Subglacial volcanoes are generally located in the SE part of the field. The Derbi-Taiaga volcano erupted 0.76 – 0.73 Ma. Its middle segment is a huge pile of hyaloclastite fragments and pillow lavas. It appears the volcano started as a shield. As the glaciation proceeded, there was a small layer of hyaloclastites capped with lavas, then built a 400 – 550 m later of hyaloclastites. As the volcano reached the surface of the glacier lake, it was capped with 30 – 70 m of thick lava flows. Dikes have been found in the body of the pile.
Three small volcanoes formed about Derbi-Taiga 0.6 – 0.3 Ma. Their shields are less than 100 high. These are Kadyr-Sugskii, Yrddawa and Shivit-Tiaga. All are generally to the S and SE of Derbi-Taiga. Formation history of these volcanoes is complex.
Several smaller shields formed at the eastern end of the Azas Plateau 225 – 75 ka. These have varying sizes and are generally tuyas, with thick hyaloclastite layers (250 – 500 m) capped with 50 – 100 m thick lava flows. All were built during glaciation and have pillow lavas topped with lava flows.
The final activity some 50 ka appears to be the Ulug-Arginskii scoria cone and lava flows on the flank of the Ulug-Arga volcano. Its lava flows are up to 50 m thick. There is what appears to be recent, well-preserved lava flows near the bed of the Bii-Hem River that have not as yet been dated. Ula-Arginsky is dotted with large glacial erratics.
Average ice sheet thickness from tuya analysis is estimated 300 – 600 m and as thick as 700 m in places. It takes ice that thick to contain a meltwater lake necessary for tuyas. Thinner glaciers (less than 150 m) will not contain these lakes and allow a more conical cone to build.
There are at least 9 prominent volcanic tuyas formed under glaciers (Sorung-Chushku, Priozernyi, Shivit-Taiga, Kok-Hemskii, Ploskii and Derbi-Taiga). This part of the field also has at least one volcanic cone (Ulug-Arginskii) erupted above the ice. These are of various sizes and ages. There are another 5 prominent volcanic structures that have yet to be closely studied. The tuyas are generally flat topped with steep sides. While their horizontal dimensions are variable, they are similar in height. Some of these have minor volcanic cones on the top. Shape is generally circular to semi-circular. Some have extensions of the pile from their sides. Most also have minor volcanic landforms like scoria cones, vents, dikes and necks.
Two of the tuyas stand out – Shivit-Taiga and Derbi-Taiga. Shivit-Taiga is one the largest subglacial volcanoes in the world. It is 10 km long and 500 m high. Derbi-Taiga is smaller than Shivit-Taiga. However, it appears to be surrounded by an enormous pile of thick hyaloclastites and lava flows, only exposed on two sides. This sequence of alternating lava and hyaloclastites is an argument for construction during increasing glaciation. The actual size of Derbi-Taiga may be as large as 22 x 3 km, making it the largest tuya on the plateau.
Tops of these tuyas are not always horizontal or uniform. Shivit-Taiga and Sorung-Chushku-Uzu are large tuyas with sloping tops. Shivit-Taiga has a fractured top surface except for smooth salients on the N. Salients are thought to be formed by erupting lavas through a tunnel in the ice. There is some layering of volcanic materials, with individual layers usually a few to over 10 m thick. The top layers are generally horizontal (with a few exceptions) and mostly lava flows erupted above the ice and water.
There are four types of erosion of the tuya structures. These include failure of steep slopes, gully formation by stream runoff and debris flows, valley glaciers, and modification by rock glaciers. Tuyas formed below a glacial surface have steep slopes. This leads to slope failures before and after the glaciers leave. Gully formation is present in most of the tuyas. Debris flows are observed on the upper slopes of Ploskii. Glacial erosion is visible on the older tuyas.
There are landforms around these volcanoes (generally small hills aligned with the volcano flanks) that one group of researchers (Komatsu, et al, 2007) believe are formed by erosion and deposition of high-energy water streams (jokulhlaup) under the ice or after the ice has retreated. As with most investigation in this part of the world, there is a lot more unknown than known. For instance, there is not any data on the interior of the hills. Blocks? Sediment? Bedrock? There is also no clear source of the massive amounts of water required to form the hills.
Either way, there is a close spatial relationship between subglacial volcanoes and the depositional landforms on the Azas Plateau. The floods do not necessarily require active volcanic activity, as failure of ice dams or emptying of subglacial reservoirs is sufficient to supply water for an outbreak. The floods are also undated, though the ages of lava units capping the tuyas are known. The landforms appear to be younger than the volcanic units they are associated with. Most of them are found in areas surrounding the tuyas. Both the surrounding land and tuyas themselves appear to be modified by the jokulhlaups.
The age of ice coverage is also estimated based on ages of the hyaloclastites of the tuyas. There is a lack of widely-accepted glaciation history for the neighboring Sayan – Tuva uplands. Based on the ages of volcanics on the plateau, there were at least three periods of glaciation: 1.75 – 1.65 Ma, 0.760 – 0.565 Ma, and 0.225 – 0.060 Ma. This probably a minimum number, as glaciation could have occurred when volcanic activity was absent, or glaciation may have been too limited with it confined to valleys to interact with the active volcanic centers.
Some researchers are applying what is currently known about jokulhlaups on the Azas Plateau as a model for similar landforms associated with glaciers on Mars.
Tectonics
The Lake Baikal rift is located along a suture between the Siberian shield and younger accreted continental cratons. There is a running argument whether the rift is caused by an upwelling of mantle fluids not unlike the Kenya Rise or the mantle fluids are simply filling newly opened spaces in the crust. For our purposes, it really doesn’t matter. The rift is located thousands of kilometers from any active plate boundary. It has relatively high heat flow, some 1.5 times background levels. Erupted magmas due to uplift and rifting in the region appear to be relatively uniform over a wide region. Erupted basalts W and SE of the lake appear not to be significantly contaminated by melted lithosphere. One of the oddities about the Baikal Rift is that erupted materials are relatively sparse, in the neighborhood of one tenth of erupted magmas from the Rio Grande Rift since it formed. The most significant activity in the region took place as the rift and depressions were first forming. However, rifting is not necessarily associated with all past volcanism.
The Baikal rift started opening 70 – 30 Ma. There at two segments with different tectonic history. The Northern rift developed with depressions and volcanic fields that shifted S – SE from the rift axis over time. The Southern Baikal Rift includes graben depressions, moderately developed volcanism and lava fields to the S and W. These include Tunka Valley, Hamar-Daban Ridge, Dzhida and Bartoi River Basins, Khovsgol Lake Basin, Oka Plateau and Azas Plateau. There is a triple junction of rift related valleys that has been interpreted by some as a mantle plume head (note that argument is still ongoing) that forced regional dome uplift as high as 3,000 m. This is located at the junction of the Khovsgol Basin, Tunka Depression and Oka-Azas Troughs. One of the arguments against a plume head is the relative lack of magma for the fields. The region remains tectonically active with earthquakes.
Conclusions
The Azas Plateau is an example of volcanic activity from an active mid-continental rift zone, the Baikal Rift Zone. Interestingly enough, this rifting has not been particularly productive in allowing magma to reach the surface. Most of the activity creating Azas took place 16 – 2 Ma from basalt fissure eruptions. Subsequent activity under a glacial variable ice cap created multiple tuyas. There has been little to no actual activity over the last 50,000 years.
Additional information
Smithsonian GVP – Azas Plateau
Late Cenozoic volcanic province in central and east Asia, Yarmolyuk, et al, Jul 2011
Intraplate magmatism in central Asia and China and associated metallogeny, Pirajno, et al, Apr 2009
agimarc
Spica
Granyia
volcanohotspot.wordpress.com 
Pretty good pyroclastic flow out of Merapi Saturday. Not a lot of vertical on the plume. Most of the action was the pf downslope. Twitter has video of the vertical plume from another perspective. Cheers –
LikeLike