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Extinct vs. Dormant

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Author Topic: Extinct vs. Dormant  (Read 774 times)
« on: January 05, 2011, 03:28:27 pm »

The following is a paid presentation...


It's a pdf from Civil Defence/ Ecan


On page 2, the second to last bullet point says:

Banks Peninsula is an extinct volcano and the earthquake activity is not related to it. Volcanic earthquakes are
very different in style to the 4 September earthquake and subsequent aftershocks. Measurement of warm springs
in the Lyttelton Harbour basin show an increase in flow, but no significant increase in temperature after the

How do they define it as extinct when in the same paragraph they admit to an increased flow in the hot pools?  Surely if it is truly extinct, then there is no residual heating.  I would expect a dormant volcano to have hot pools, but I'm not a scientist and overall have little interest in volcanology.  I just like to know that officialdom are presenting the facts and they are true facts.
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« Reply #1 on: January 11, 2011, 07:01:19 pm »

I am curious about that too so I went searching. This is the best explanation I found regarding hot springs and extinct volcanoes.
How do geothermal systems work?
Geothermal water starts life as rainwater, which seeps down though cracks in the rock towards a heat source deep within the earth. Hot water is less dense than cold water, so it rises and emerges at the earth’s surface, sometimes as steam or mixed with steam. The hot water reacts with the rock it comes into contact with, and becomes enriched with dissolved minerals.

New Zealand’s geothermal systems
Scientists divide New Zealand’s geothermal systems into three main groups:

•Large, intense systems associated with young volcanoes
•Cooler systems associated with older or extinct volcanoes
•Isolated warm springs associated with faultlines.
The Taupō volcanic zone
The major geothermal fields in the Taupō region give rise to the spectacular geysers, boiling springs, mud pools and fumaroles (steam or gas vents) throughout the region. These features are closely associated with active volcanoes. In volcanic zones such as Taupō the ground is heated by magma (molten rock) close to the surface.

Because of this intense heat source, water temperatures in the deeper parts of a geothermal system may be greater than 300ºC. The waters are under great pressure, so can become superheated well above their normal boiling temperature (100ºC at sea level).

Outside the Taupō region, the only other high-temperature geothermal system is at Ngāwhā, in Northland, which has been volcanically active in the last few thousand years.

Cooler systems in volcanic zones
Numerous other hot springs are associated with remnants of volcanic activity, particularly in Northland, Coromandel, the Hauraki Plains and the Bay of Plenty. Heat sources for these fields are diffuse rather than intense, producing fluid temperatures of less than 100ºC.

These smaller systems are confined to the North Island. Banks Peninsula and Otago Peninsula in the South Island are ancient volcanoes, but there is no longer any volcanic heat flow underneath them.

Springs associated with faultlines
Warm springs (less than 70ºC) are found in non-volcanic areas of New Zealand. Faults – deep fractures in the rock – provide channels for warm water to rise rapidly from depths where it has been heated. Striking examples are the hot springs aligned along the Hope Fault, in North Canterbury, and the Alpine Fault, in the Southern Alps. At Hanmer, on the Alpine Fault, a range of thermal pools attract thousands every year.

the diffuse heat source explains why the hot springs around Auckland (Waiwera, Parakai and Miranda) are a long way from the volcanoes
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« Reply #2 on: January 11, 2011, 08:44:16 pm »

Thanks for that sp.  I hadn't found that one.  The ones I was finding all sort of went along with the dormant rather than extinct theory. 

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« Reply #3 on: January 14, 2012, 11:15:46 pm »

According to the popular definition the answer is no. Volcanoes that erupt regularly are called active, those that have erupted in historical times but are now quiet called dormant, and those that have not erupted in historical times called extinct. As the last Yellowstone eruption was 600,000 years ago Yellowstone seems extinct.

However, volcanologists call a volcano extinct only if the volcano does no longer have a magma chamber filled with a lava supply. But the Yellowstone does have an active magma chamber.

The Yellowstone Caldera is at least 2 million years old and hasn't erupted violently for approximately 640,000 years, although there has been some lesser activity more recently, with hydrothermal eruptions less than 10,000 years ago and lava flows about 70,000 years ago. For this reason, scientists do not consider the Yellowstone Caldera extinct, but only dormant.
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« Reply #4 on: February 09, 2012, 09:22:15 am »

Sounds a bit like the Auckland volcanoes.

They are all - with the possible exception of Rangitoto Island - considered extinct.

Rangitoto last erupted 300-600 years ago depending  on which expert opinion you are listening to,there may have been more than one eruption which is unusual for an Auckland volcano. Oddly there is very little in local Maori mythology about it although there is clear evidence that the eruption and formation of the island was witnessed by the Maori, there are footprints preserved in the rocks on neighbouring Motutapu Island.

As for rest, the Auckland volcanoes are considered to be one hit wonders but the hot spot of magma that fed them is still there.

This is why Auckanders get a bit twitchy whenever we feel a tremor. For some reason we don't feel earthquakes centred in other parts of the country at all but when one happens in the region everybody is on tenderhooks waiting to see if there are more and if they are shallowing.
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