User:Volcán bobcat/BezyDraft

Bezymianny (Безымянный — lit. unnamed) is an active stratovolcano in Kamchatka, Russia. Before its noted 1955-56 eruption, Bezymianny volcano had been considered extinct. The modern Bezymianny volcano, much smaller than its massive neighbors Kamen and Kliuchevskoi, is part of the Klyuchevskaya Volcanic Group and was formed about 5,500 years ago. There have been three periods of intensified activity in the past 3000 years. The latest period, which was preceded by a 1000-year quiescence, began with the dramatic 1955-56 eruption. This eruption, similar to that of Mount St. Helens in 1980, produced a large horseshoe-shaped crater that was formed by collapse of the summit and an associated lateral blast. This eruption style is commonly referred to as a Bezymianny-type eruption. Subsequent episodic but ongoing lava-dome growth, accompanied by intermittent explosive activity and pyroclastic flows, has largely filled the 1956 crater.

Tectonic Setting


Laying in the center of the Kamchatka Peninsula of Russia, Bezymianny is part of a volcanic arc created by the subduction of the Pacific Plate under the Okhotsk Plate at the Kuril–Kamchatka Trench, 100 km off the east coast of the peninsula. Bezymianny is a Holocene volcano, forming about 5,500 years ago over a late-Pleistocene lava-dome complex and an ancestral volcano, Pra-Bezymianny, that was built between about 11,000–7,000 years ago.

1955-56 Eruption
In 1955, Bezymianny left a 1000 year quiescence by erupting from October 1955 to April 1956, producing a VEI 5 eruption. This event was its largest historical eruption, ejecting about 1 cu. km of juvenile material over seven months. The event is notable for its paroxysmal phase where a lateral blast and subsequent debris avalanche destroyed the crater and sent large blocks up to 18 km away.

Pre-eruptive Phase
A three week swarm of volcanic earthquakes occurred between 29 September to 21 October 1955.

Vulcanian Phase
Gas and ash erupted from Bezymianny beginning 22 October after fewer than 1300 seismic events. The eruption style was vulcanian. Ash up to 25 mm was deposited in the town of Klyuchi. By the end of November the summit vent had widened to 800 m.

Dome Inflation
From December 1955 to March 1956 the old dome of Bezymianny inflated, raising the surface of the volcano by 100 m. This may have been because the previous phase sealed the vent, closing the system.

Paroxysmal Phase
The major eruptive phase of Bezymianny occurred on 30 March 1956. This included a lateral blast and a debris avalanche deposit, which contained >50% juvenile material.

Post-1956 Activity
Since the 1955-56 eruption, Bezymianny has been in a phase of periodic lava dome growth in its horseshoe-shaped crater. Unlike St Helens, this lava dome has nearly filled the crater. This dome is composed of medium-potassium calc-alkaline andesite, which is consistent with the overall composition of the stratovolcano.

Bezymianny-type Debris Avalanches
Bezymianny-type debris avalanches are characterized as highly mobile sector collapses that remove juvenile material from the volcanic edifice. These debris avalanches excavate a bowl-shaped amphitheater at the summit of the volcano and deposit large blocks of the volcano down slope, producing poorly-sorted hummocky terrain. Bezymianny-type collapses came to prominence after the analogous debris avalanche of Mount St. Helens in May 1980 that killed 57 people. Since this eruption several stratovolcanoes have been identified as exhibiting Bezymianny-style collapses in the past, including Koma-ga-take, Japan, Mount St. Helens, United States, Socompa, Chile, Soufrière Hills, Montserrat.