In 1901, the world’s tallest geyser was discovered by Dr Humphrey Haines on the North Island of New Zealand. He was following up reports of great clouds of steam, and found the Waimangu Geyser near Rotorua. It appeared after an enormous eruption of Mt. Tarawera in 1886. The Waimangu Geyser was the largest geyser in the world and erupted on a 36 hour cycle for four years, hurling black mud and rocks in the air. Waimangu is Maori for “black water.” It stopped in 1904 when a landslide changed the local water table. Eruptions would typically reach 600 feet. Some superbursts are known to have reached 1,600 feet (10 times as high as Yellowstone’s famous Old Faithful, and which would be higher than the Empire State Building.)
Geyser activity, like all hot spring activity, is caused by surface water gradually seeping down through the ground until it meets rock heated by magma. The geothermally heated water then rises back toward the surface by convection through porous and fractured rocks. Geysers differ from non-eruptive hot springs in their subterranean structure; many consist of a small vent at the surface connected to one or more narrow tubes that lead to underground reservoirs of water and pressure tight rock.
|Steamboat Geyser in Yellowstone. By Brocken Inaglory (Own work) [CC BY-SA 3.0 (http://ift.tt/HKkdTz) or GFDL (http://ift.tt/KbUOlc)], via Wikimedia Commons
As the geyser fills, the water at the top of the column cools off, but because of the narrowness of the channel, convective cooling of the water in the reservoir is impossible. The cooler water above presses down on the hotter water beneath, not unlike the lid of a pressure cooker, allowing the water in the reservoir to become superheated, i.e. to remain liquid at temperatures well above the standard-pressure boiling point.
Ultimately, the temperatures near the bottom of the geyser rise to a point where boiling begins which forces steam bubbles to rise to the top of the column. As they burst through the geyser’s vent, some water overflows or splashes out, reducing the weight of the column and thus the pressure on the water below. With this release of pressure, the superheated water flashes into steam, boiling violently throughout the column. The resulting froth of expanding steam and hot water then sprays out of the geyser vent.
A key requirement that enables a geyser to erupt is a material called geyserite found in rocks nearby the geyser. Geyserite—mostly silicon dioxide (SiO2), is dissolved from the rocks and gets deposited on the walls of the geyser’s plumbing system and on the surface. The deposits make the channels carrying the water up to the surface pressure-tight. This allows the pressure to be carried all the way to the top and not be leaked out into the loose gravel or soil that are normally under the geyser fields.
Eventually the water remaining in the geyser cools back to below the boiling point and the eruption ends; heated groundwater begins seeping back into the reservoir, and the whole cycle begins again. The duration of eruptions and time between successive eruptions vary greatly from geyser to geyser; Strokkur in Iceland erupts for a few seconds every few minutes, while Grand Geyser in the United States erupts for up to 10 minutes every 8–12 hours.
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