Geyser
A geyser is a
spring characterized by intermittent discharge of water ejected
turbulently and accompanied by a vapour phase (steam). The name
geyser comes from Geysir, the name of an erupting spring at
Haukadalur, Iceland; that name, in turn, comes from the Icelandic
verb gjósa, "to gush".
The formation of
geysers is due to particular hydro geological conditions, which
exist in only a few places on earth, so they are a fairly rare
phenomenon. Generally all geyser field sites are located near
active volcanic areas, and the geyser effect is due to the
proximity of magma. Generally, surface water works its way down to
an average depth of around 2,000 metres (6,600 ft) where it meets
up with hot rocks. The resultant boiling of the pressurized water
results in the geyser effect of hot water and steam spraying out of
the geyser's surface vent.
About a thousand
known geysers exist worldwide, roughly half of which are in
Yellowstone National Park, United States. A geyser's eruptive
activity may change or cease due to ongoing mineral deposition
within the geyser plumbing, exchange of functions with nearby hot
springs, earthquake influences, and human intervention.
Form and function
Geysers are temporary geological features and are generally
associated with volcanic areas. The water boils, the resultant
pressure forces a superheated column of steam and water to the
surface through the geyser's internal plumbing. The formation of
geysers specifically requires the combination of three geologic
conditions that are usually found in volcanic terrain.
1) Intense heat
The heat needed for geyser formation comes from magma that needs to
be near the surface of the earth. The fact that they need heat much
higher than normally found near the earth's surface is the reason
they are associated with volcanoes or volcanic areas. The pressures
encountered at the areas where the water is heated makes the
boiling point of the water much higher than at normal atmospheric
pressures.
2) Water
The water that is ejected from a geyser must travel underground
through deep, pressurized fissures in the earth's crust.
3) A plumbing system
In order for the heated water to form a geyser, a plumbing system
is required. This includes a reservoir to hold the water while it
is being heated. Geysers are generally aligned along faults. The
plumbing system is made up of a system of fractures, fissures,
porous spaces and sometimes cavities. Constrictions in the system
are essential to the building up of pressure before an
eruption.
Ultimately, the temperatures near the bottom of the geyser rise to
a point where boiling begins; steam bubbles 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 underneath. 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 hole.
Eruptions
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.
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.
The rocks in the nearby region produce a material called geyserite.
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.
General categorization
There are two types of geysers: fountain geysers which erupt from
pools of water, typically in a series of intense, even violent,
bursts; and cone geysers which erupt from cones or mounds of
siliceous sinter (also known as geyserite), usually in steady jets
that last anywhere from a few seconds to several minutes. Old
Faithful, perhaps the best-known geyser at Yellowstone National
Park, is an example of a cone geyser. Grand Geyser, the tallest
predictable geyser on earth, (although Geysir in Iceland is taller,
it is not predictable), also at Yellowstone National Park, is an
example of a fountain geyser.
The Taupo Volcanic
Zone
The Taupo Volcanic Zone in the Northern Island of New Zeland is
approximately 350 kilometres long by 50 kilometres wide. Mount
Ruapehu marks its southwestern end, while the submarine Whakatane
volcano (85 kilometres beyond White Island) is considered its
northeastern limit. The Taupo Volcanic Zone can be seen as the
southwestern end of the Pacific Ring of Fire, which marks out the
subduction zones around the Pacific Ocean.
Recent scientific work indicates that the earth's crust below the
Taupo Volcanic Zone may be as little as 16 kilometres thick. A film
of magma 50 kilometres (30 mi) wide and 160 kilometres (100 mi)
long lies 10 kilometres under the surface. The geological record
indicates that some of the volcanoes in the area erupt infrequently
but have large, violent and destructive eruptions when they do.
There is also some possible rifting in the Taupo Volcanic
Zone.
The
Lady Knox Geyser
Many of New Zealand’s geysers have been destroyed by humans
in the last century. Several New Zealand geysers have also become
dormant or extinct by natural means. The main remaining field is
Whakarewarewa at Rotorua. Two thirds of the geysers at Orakei
Korako were flooded by the Ohakuri hydroelectric dam in 1961. The
Wairakei field was lost to a geothermal power plant in 1958. The
Taupo Spa field was lost when the Waikato River level was
deliberately altered in the 1950s.
As there is no geyser related EarthCache in New Zeland and the
North Island has a highly active volcanic area in the Taupo zone, I
decided to add one, the Lady Knox Geyser.
This geyser, of medium size, is induced to erupt daily at 10:15am
by dropping soap into the opening of the vent. Eruptions produce a
jet of water reaching heights of 10 to 20 meters and can last for
up to one hour but that can vary depending on the weather
conditions. Te visible spout is made of rocks placed around the
base of the spring to enhance the eruption; over the years the
eruptions has built up to give a cone-shaped appearance. You will
have the opportunity to learn the history and mechanics of the Lady
Knox Geyser during a presentation by an informed guide in the
natural amphitheatre. It provides a unique opportunity to see a
geyser in eruption.
The geyser has two water chambers, one lower, hot one and one
upper, cold one. The upper chamber cools due to a larger opening to
the outside. The lower one heats up due to the volcanic activity
below. When soap is thrown into the upper water chamber, the
lowered surface tension of the water allows it to mix with the
hotter water below, causing the eruption.
By comparison with the Pohutu Geyser in the Whakarewarewa Thermal
Valley, the main geyser of the area which spurts two to three times
per hour (up to twenty times per day) and can reach heights of up
to 100 metres, the Lady Knox Geyser is much smaller.
In order to log this
EarthCache , you must answer the following questions :
1e : Of the 2 different types of existing geysers, of what type is
this one?
2e : What colour is
the geyser cone and what is the mineral responsible for this
colour?
3e : Geysers are temporary geological features; what is the life
span of a geyser, at the most?
4e : As most of the
answers can be found on internet, with your log, I would appreciate
that you include a picture showing you with your GPSr or your GPSr
and the geyser in the background at pz.
Please e-mail
me your answers, don’t forget to specify the EarthCache name
and GCxxxx code and I will give you the ok to log your find.
At 10.15am each day there is the opportunity to
learn the history and mechanics of the Lady Knox Geyser during a
presentation by an informed guide in the natural ampitheatre. It
provides a unique opportunity to see a geyser in eruption to
heights of 10 to 20 metres.
The Lady Knox Geyser is located in a separate area
from the Visitor Centre (a 3 minute drive in your own vehicle)
where there is also plenty of parking. Directions to her location
are given when tickets are purchased from the Visitor Centre.