Bolshoy (large) geyser is one of the largest, most studied and accessible in the Valley. Eruptions happen approximately hourly, hot water shoots 10 meters high, steam rises up to 200 meters. Observation deck is located next to the geyser, providing an excellent view. Of the larger geysers, Bolshoy is the closest to the visitors centre.
Bolshoy's vent is 1,5x3,5 meters. It's located on the left side of the lake, at water level. Before the 2007 slide, the vent was about 11 meters above river's water level and about 40 meters away from the river. After the slide, geyser was flooded by the lake and for almost 3 months was under water, 2 meters deep.
"Freeing" Bolshoy was one of the main tasks for volunteers; who in 2007 cleared stone debris in the new channel of Geyzernaya river by hand. By September 2007 they managed to bring down water level in the lake, and Bolshoy's vent showed up on the surface again. First eruption after flooding was witnessed on September 12th.
Bolshoy's eruption lasts for a few minutes with maximum intensity, then calms down and gradually fades away in 20—30 minutes, when water stops splashing outside the vent.
T.I. Ustinova gave an excellent description of Bolshoy and its cycle in her book "Geysers of Kamchatka" (1955):
"Bolshoy's eruption is a magnificent site. For 3—4 minutes pulsating water fountain, accompanied by huge steam clouds, shoots up 12 meters; it has the same diameter as the vent. During that time, waves are shooting over the vent and the whole geyser platform is covered with boiling streams. During next 2—3 minutes, intensity of the eruption goes down, less steam gets out and while water fountain still shoots up, water stops spilling over the vent's edges. Streams rolling down the platform get smaller; water shoots up no more than 5 meters. Splashes get lower. 25 minutes from the start, when water stops going over the vent's edges, and fountain barely going up more than half a meter, you can look down the vent. Its bottom is covered with boiling water; the higher water level rises in the vent, the lower and more seldom splashes get above it, and you can see less steam. After 40 minutes there's almost no steam, you can hear muffled bangs from the bottom of the vent - water shooting from the channel. After 10 minutes the sounds are silenced by thick layer of water filling the vent. Water arrives jerkily. Occasionally, large bubbles are slowly rising to the surface. Water temperature in the vent at that point is about 95—98° Celsius. Once the vent is full, water starts to boil in its southern part (apparently the channel is located here); hot stream goes over the edge; geyser is rumbling quietly. In 1945, water temperature during the boiling in the vent's southern part was 99,25°. The steam does not arrive from the channel, only the surface of hot water is steaming as well as a hot creek, which is steadily increasing. The water comes and overflows from the vent in more and more quantity, the amount of steam increases, water boils more often and for 1—2 minutes before the eruption it boils continuously, shooting two meters up in thick clouds of steam and overflows jerkily over the edge of the vent".
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