Ksudach (Voniuchi Khrebet) is a volcanic system on the volcanic front in Kamchatka. It has produced multiple caldera-forming eruptions, with at least five nested calderas currently identified. The only historic eruption was in 1907, when the Shtyubel (Stubel) volcano in the most recent caldera ejected nearly 2 km3 of airfall.
The volcano itself is a large, low angled volcano truncated by multiple calderas. Basaltic and andesitic lavas, dacitic pyroclastics are interlayered in the cone. The top of the volcano holds a pair of lakes, Klyuchevoye (Bolshoe) and Shtyubel (Kraternoe, Shtyubelya). Shtyubel occupies the 1907 Shtyubel crater and is the headwaters of the Khodutka River. The other lake has no current outlet. Klyuchevoye has an area of 8 km2, is 96 m deep. It is also thermally heated, lightly alkaline and mineralized.
The system is located 54 km NE of our previously described Diky Greben – Pauzhetka caldera system, between neighboring Zheltovsky volcano 30 km to the SW and Khodutka volcano 31 km to the NE. Eruptive products from the neighboring volcanoes have interlayered over the lifetime of the systems from the Pleistocene to Holocene.
The 1,079 m volcano is located in the Volcanoes of Kamchatka UNESCO World Heritage Property.
The area is sparsely populated with just under 3,900 within 100 km. The closest large city is Petropavlosk some 154 km to the NNE.
Kamchatka generally has a subarctic climate, with cold arctic winds from Siberia combining with moisture laden winds from the cold Oyashio current to provide plenty of snow Oct – May. Weather is generally much wetter and milder than eastern Siberia. The eastern part of the peninsula is rain-drenched and heavily glaciated. The interior valley is much drier. The northern part of the peninsula has a polar climate. Summers can get as warm as 19C, winters as cold as -41C.
While most tourists visit in the summer, there is a growing winter sports activity. The lakes and hot springs in the surrounding wilderness make Ksudach a popular trekking destination.
Ksudach is monitored by KVERT. It does not appear to have either an active webicorder or webcam.
The volcano itself is a shield built from lavas and tephras of various ages. There are at least five calderas of various ages present. The two larger calderas (I and II) were formed in the late Pleistocene. The three smaller ones (III, IV and V) were formed in the Holocene. The active stratovolcano, Shtyubel was built shortly after the formation of Caldera V. Its most recent eruption was in 1907.
Pyroclastic deposits of the largest Holocene eruptions are tens to hundreds of meters thick near the vent. Their layers can be traced over much of the Kamchatka Peninsula, forming markers for stratigraphy. Caldera forming tephras are generally andesites to rhyodacites with volumes 1.5 – 19 km3. Most Ksudach tephras contain silicic and mafic pumice with alternative bands, suggesting intensive mixing of melts in the course of an eruption.
Initial volcanic activity at the location started before 162 ka, with a VEI 6.0 eruption that ejected over 10 km3 of material. The activity built a broad andesite shield volcano. Subsequent activity truncated the volcano and created multiple nested caldera craters. The diameter of the largest one is 10 – 11 km.
VOGRIPA lists the Caldera I eruption at 40,000 years ago. It was a VEI 6.6 that ejected some 40 km3 of material. The Caldera II eruption took place 5,000 years later, a VEI 6.4 that ejected some 25 km3 of material.
The first Holocene event creating Caldera III took place 8,800 years ago. Deposits from this eruption include multiple tephra fall units, alternating pyroclastic surge beds, explosion breccias, topped with a pyroclastic flow unit. Total tephra volume is estimated at 1.5 – 2.0 km3. Early products during the eruption were mostly silicic. End products are mostly andesites and dacites. Black andesitic bombs near the vent are occasionally welded. It was a VEI 5.2.
The next caldera forming eruption (Caldera IV) took place around 6,000 years ago. There were two closely spaced eruptions producing explosion breccias, pyroclastic flows and airfalls. Eruptive products varied within and between the two eruptions, with the final products being uniformly andesitic. Total volume of tephras for the two eruptions is estimated 10 – 11 km3, with 80% from the second (most recent) eruption. Multiple domes were extruded following this eruption with a total volume around 0.5 km3. The two eruptions were separated by some 300 years, with the oldest being a VEI 5.5 and the youngest being a VEI 5.9.
The youngest caldera-forming eruption took place AD 240, about 1,760 years ago. It was a VEI 6.3. Caldera V was formed with the second largest Holocene eruption in Kamchatka. It was similar in size and violence to the 1883 Krakatau eruption, producing 18 – 19 km3 of tephras. Eruptive column is estimated at 22 – 30 km, creating a 4 x 6.5 km caldera. Pyroclastic flows extended to 20 km from the volcano. Tephras were deposited as far as 1,000 km N of the eruption. As with the previous caldera-forming eruptions, eruptive products varied in chemistry and type over the course of the eruption. This eruption was likely an ecological disaster in Kamchatka, with the minimum area of total devastation (40 cm isopach) 400 – 500 km2. Vegetation was also damaged (5 – 40 cm tephra depth) over an additional 12,000 km2 surrounding the volcano and north of the eruption.
The Shtyubel Cone began to grow within Caldera V about 100 years following the caldera collapse. All subsequent eruptions from Ksudach have come from this vent. They are generally separated by a quiet period less than a few hundred years. Formation of this cone began with a moderate explosive eruption and lava extrusion. There were three more large recent eruptions with individual volumes up to 2 km3. Products from Shtyubel are andesites to dacites. Fall deposits from the larger eruptions generally have at least two units, with the initial products being andesites.
The Shtyubel Cone lies between the two lakes, has a diameter of 3 km. There is a dome currently being extruded beneath Shtyubel lake. Extrusion was not observed in 1991 but was obvious by 2020. This extrusion also drives an active hydrothermal system within the lake and nested caldera structure.
The most significant hydrothermal discharge is the NW part of Klyuchevoye Lake, the Goryachy Plyazh (Hot Beach) and Shtyubelevsky thermal springs. The crater lake filling Shtyubel shows activity due to the extruding dome. Gas emissions are rising from the bottom of the lake, visible via sonar. Surface waters have SO4 and Ca several times normal. There are also high concentrations of N2, CO2, He, CH4 and other hydrocarbons measured. Fluid flows from the bottom of the crater lake is responsible for these measured emissions. There are multiple hot springs on and surrounding the flanks of the Ksudach system.
To put in perspective how dangerous this system is, over the last 10,000 years, there have been five catastrophic explosive caldera-forming eruptions in Kamchatka. Three of these were associated with Ksudach. The other two with Karymsky and Kuril Lake – Iliinsky (Diky Greben – Pauzhetka). Four of these took place between 9,000 – 6,000 years ago.
Since then, only one eruption of that size took place, creation of Caldera V at Ksudach 240 AD. The eruption was similar in type and size to the 1883 Krakatau eruption and produced a large acidity peak in the Greenland ice sheet. Its tephras cover almost all of Kamchatka and provide one of the most important marker horizons for Holocene deposits. This eruption took place some 4,000 years after the Caldera IV eruption.
Eruption deposits from the 240 AD eruption fill river valleys in the N, W, and eastern slopes of the volcano. They are more extensive to the north. They are also present within older caldera depressions of the central massif. These are poorly consolidated white-, fine- or coarse-grained tuffs. They are generally non-stratified and include mostly juvenile material. There are four pyroclastic flow units separated by pumice fall deposits. As with other eruptions from Ksudach, there is significant color variation between layers of the eruption.
Collapse Caldera V was formed by the eruption. The southern part of the caldera coincides with previous Calderas I – III and the Shtyubel cone. The northern part of this caldera corresponds to Caldera IV which surrounds it. The rim of Caldera V is defined by scarps E and W of the lake, and in the SW it bounds the Paryashchiy Utes extrusion. This extrusion was formed following the Caldera IV eruption and is overlain with Caldera V eruptive products.
Caldera V measures 4 x 6.5 km with an area of 19 km2 within the rim. The original unfilled volume of the caldera is estimated at 6.5 – 7.0 km3. Volcanic products and lakes have filled at least 2 km3 of the original caldera volume.
While most of the ash went north along the Kamchatka Peninsula, there was a significantly easterly component which put a lot of it in the neighboring Pacific Ocean. Total volume of ashfall is at least 14 – 15 km3, covering 2 -3 million km2. Pyroclastic flows from the eruption covered roughly 200 km2 with a volume of at least 3 km3. Include at least 1 km3 of pyroclastic fill in the newly formed caldera, and the minimum erupted volume is 19 km3. As it was similar in size to both Krakatau and Katmai – Novarupta, it may have had the same environmental impact.
The eruption began as phreatomagmatic with formation of pyroclastic base surges, likely interacting with an intercaldera lake. This eruption began with ejection of juvenile material and proceeded with at least four phases – initial, main, lithic and gray. Each cycle began with an ash column in the 22 – 30 km range which eventually collapsed and finished with pyroclastic flow emplacement. The percentage of juvenile material erupted was very high through the course of the eruption. While the ashfall axis was generally to the NNE into the Pacific Ocean, shifting wind deposited a significant portion of it north along the peninsula. Pyroclastic flows moved down all flanks of the volcano except to the south, where the high rims of older calderas screened them.
The initial phase of the eruption began with a small phreatomagmatic explosion and quickly evolved into a magmatic Plinian eruption with a 25 – 30 km plume. The main phase supported a 30 – 36 km plume. Two thirds of the volume erupted as ash falls. The other third as pyroclastic flows. This was the most productive phase of the eruption. The lithic phase has the smallest volume, but size and intensity of the plume was similar to that of the main phase, though volume was smallest. The gray phase flux was similar to the lithic phase, but column height decreased to that of the initial phase. More material was deposited in the grey phase by pyroclastic flows than ash falls from the plume.
Caldera collapse likely took place during the lithic phase after about 66% of the magma had been erupted, about 7 km3 DRE. The caldera collapse also corresponds to a change in pumice texture and color from white to gray pumice. Similar abrupt changes in pumice colors during caldera-forming eruptions are not uncommon. They usually reflect a substantial change in composition and vent geometry. Larger vents typically are unable to support large plumes and tend to collapse sooner.
Immediately following caldera formation, a small extrusive dome grew inside it. It seems to exist at the bottom of Kliuchevoe Lake, which filled the caldera depression. The Shtyubel Cone started growing within 100 years following the Caldera V eruption. Three large explosive eruptions from this took place around 900 – 1000 AD (VEI 5.0), 1700 AD (VEI 4.7), and the 1907 VEI 5.3 eruption. Erupted volumes were 1.0, 0.5, and 1.5 – 2.0 km3 respectively. Eruptive columns were once again as high as 22 km. Like the other eruptions, these produced banded pumices, and marked shifts in the color of materials during the eruption.
Variations in erupted magmas during the Caldera V eruption may have been triggered by injection of hot mafic magma into a cooler silicic magma reservoir. More mafic magmas were erupted later during the formation of the Shtyubel Cone.
The March 1907 sequence of explosive eruptions from the Shtyubel Cone partly destroyed it, leaving a 1.5 km diameter horseshoe shaped crater filled with a deep lake. Total erupted products is estimated at 2.4 km3, with 1.9 km3 being juvenile andesitic to rhyolitic material. This was classified as a VEI 5.3.
The eruption took place in four phases, starting with initial vent opening that distributed 0.02 km3 of pyroclastic fallout within the caldera complex. The second phase produced a Plinian column that distributed 1.5 km3 of juvenile pyroclastic ashfall to the NE and SE. The third phase was a powerful blast toward the NNW. The blast cloud overran local barriers 500 – 600 m high. Other explosions produced pyroclastic flows, surges, and breccias that cover 115 km3 with a volume of 0.15 km3. The fourth phase was a series of shallow hydromagmatic eruptions that emplaced five pyroclastic surge deposits with ash cloud surge beds to the WNW. Two or three of these clouds topped the Ksudach Caldera walls. They cover 160 km2 with a volume of 0.2 km3. The last two phases were increasingly hydromagmatic. The eruption took place from a zoned magma chamber below Shtyubel Cone and produced basaltic andesites to rhyolites.
Air fall deposits from the 1907 eruption impacted over 2,000 km2 of neighboring forests. This destruction and recovery of the greenery has been studied over the course of the last century. Zone I closest to the volcano destroyed all vegetation under 100 cm of ashfall. As of 1996, this was a lichen-dominated desert.
Zone II is 30 – 100 cm ashfall. Deposits of 70 – 100 cm destroyed all vegetation but left scattered snags which provided dead wood for replanting. 30 – 70 cm left isolated trees, bushes, tall shrubs, all of which permitted colonizing species to reach buried soils a bit easier.
Zone III received less than 30 cm of ashfall, which permitted substantial survival. This zone has well-developed forests similar to undisturbed forests. Deposits less than 10 cm damaged herbs, moss and lichen, but did not eliminated any species. Estimated recovery of the surrounding forest system will take longer than 2,000 years.
We covered Kamchatka tectonics in a 2015 post. That post would be a good starting point for further research.
The short form of Kamchatkan tectonics is the subduction of the Pacific Plate under the Okhotsk Plate (or block). The volcanic front consists of volcanic arc subduction volcanoes. The Kuril Kamchatkan Trench offshore to the east is the subduction zone. The subducted Pacific Plate provides mantle fluids powering volcanic activity in Kamchatka and south along the Kuril Islands.
As a subduction zone, the entire region is subject to massive megathrust earthquakes. There was a M 8.3 in 1737, a M 8.5 in 1923, a M 8.2 inn 1952, a M 8.2 in 1959, and a M 7.6 in 2006.
The Ksudach system is capable of massive though infrequent eruptions. Given the ongoing subduction of the Pacific Plate beneath the Okhotsk Plate, there is no shortage of eruptible magma throughout Kamchatka. It has an active hydrothermal system with hot springs in and around the volcano. Given the recently discovered dome extrusion beneath the Shtyubel Lake, it is only a matter of time before the next round of violent activity from this system takes place.