Monogenetic means in Latin 'one birth' and refers to volcanos that erupt for only a short period of time and then never erupt again. The volcanic field underlying Auckland is of this nature, and the result has been a large number of volcanic cones and other features. Despite having various forms, the same eruptive process is followed. The difference comes from the stage when the eruption stops. Here are the steps;

1) Magma is forced upward through weak points in the crust.

2) Either the magma contacts ground-water, or reduced pressure near the surface causes gases to bubble out of solution. The result is a phraetic or steam-blast eruption. The heaviest material is thrown out horizontally to form a tuff ring. Lighter material is blasted vertically to form an eruptive column. After a few days, weeks or months, the volcano falls quiet. Several of Auckland's volcanos became extinct at this point. (e.g. Pukaki, Onepoto, Orakei and Panmure Basins, Styak's Swamp)

3) Additional lava myrise in the vent and form a lava lake. If enough lava is produced, fire fountaing starts through one or more vents. Small lava flows may be produced, which do not escape the tuff ring (e.g. Mt. Richmond, Mangere Lagoon, Taylor Hill, Sturges Park) Sometimes the lava lake crusts over, and subsequent eruptions blast through it and build cones. (e.g. Three Kings).

4) If fire fountaining continues beyond this point, the scoria cones can coalesce to rise and bury the tuff ring (e.g. Mt St. John) Lava can also fill the surrounding valleys, as at North Head, Mt. Eden, Mt. Mangere or Mt. Smart. Fire fountaining tends to diminish once a low-level outlet is established.

5) Sometimes the outflow of lava is so great that it undermines the cone, which collapses into the flow and is carried away, leaving a horseshoe-shaped breached crater (such as Mt. Victoria, Mt. Hobson, One Tree Hill). If lava flows for long enough, nearby valleys are totally filled in and the lava floods the entire area with a large sheet (e.g. One Tree Hill, Rangitoto).

Whenever the next stage of eruptive activity starts in the Auckland area, it will follow this pattern.

So what is your question?

Yes, the word "monogenetic" volcano means that it is one that was formed during a relatively short time period and during a single eruption. But! If it erupts again, then you can no longer call it a monogentic volcano. "Mono" means "one": "genesis" means "origin." So then you call it a "poly" genetic volcano. Just a difference in words.

What is your question?

Wasn't really a question-thread, valich, more an FYI thread if people wanted to know about monogenetic fields. As for the mono part, that implies that there's only one series of eruptions from that centre, and when that series of eruptions dies away and the volcano becomes quiet, it never erupts again from that locality. This has been the experience in Auckland, anyway. That's what makes the city so dangerous--you can't look at any of the already-existing peaks and say 'that one will erupt again'. You know that an eruption will come again one day, but we have no idea where on or near the isthmus it will occur. There's no pattern to the events, so we can't even point to a probable location. That's a bit of a problem in a city of just over a million people.

Xylene-
Having never visited the Audkland area, from your description, these sound like mafic volcanics? Do you know the type of lava?

Basalt and scoria cones, shield volcanoes with lava tubes, phraetic maars (shallow, water-filled blast craters) where the magma has met the ground water and exploded violently, but not built a cone.

So you're saying that you really don't want to know more about monogenetic fields, but just want to bring it to our attention that they exist. What some want to call monogenetic fields, underlying volcanic areas, are a collection of separate vents and flows: as in Mid-Oceanic Ridges.

A monogenetic field is the same as a geologically spreading of a single volcano over a wide area: similar like moving a slow-moving magma core underneath the crust. What I fear is that a reader might associate monogenetic with monomagnetic or magnetoshere or geomagnetics, since the spelling is almost identical:

"Monogenetic fields also don't look like "volcanoes", rather they are collections of sometimes hundreds to thousands of separate vents and flows. Monogenetic fields are the result of very low supply rates of magma. In fact, the supply rate is so spread out both temporally and spatially that no preferred "plumbing" ever gets established; the next batch of magma doesn't have a pre-existing pathway to the surface and it makes its own. A monogenetic field is kind of like taking a single volcano and spreading all its separate eruptions over a large area. There are numerous monogenetic fields in the American southwest and in México, including Michoacan-Guanajuato, San Martín Tuxtla, Pinacate, and the San Francisco volcanic field."
http://volcano.und.edu/vwdocs/vwlessons/volcano_types/mono.htm

Yes, I thought it would be interesting for those who didn't know about them.

The FYI makes me think that this has a strong relation to what we sometimes call "hot spots."

We?
Who's this "we" you're referring to?

Don't know your education or your job- but neither is geology.

Love to see your sources on a "hot spot" near Auckland.

There is a hotspot under Auckland actually. It first came ashore, if you like, in the western Wakato area, south of the estuary of the Waikato River, about three million years ago. It's been moving roughly north ever since, and now it's under Auckland. In a few million years it'll be up around Whangarei, and in about ten million years it'll be up equal to North Cape, but further to the east (because Northland shoots off on such an odd angle).

I hadn't read about the presence of a hot spot in New Zealand- I apologise for speaking too soon.

But now I'm intrigued, Xylene, do tell more. I vaguely remember a reference to rift-related volcanics on the north island- is the Auckland volcanic field part of the rift field? Is the hot spot driving the rift? What is the age range of the Auckland field? (again I vaguely remember less than 150,000 years old)? Any apparent hot spot migration?

You mentioned basalt eruptions- anything more silicic? Are the eruptions just localized to the Auckland area? Or is there a more extensive linear trend of similar volcanics?

I hadn't read about the presence of a hot spot in New Zealand- I apologise for speaking too soon.

But now I'm intrigued, Xylene, do tell more. I vaguely remember a reference to rift-related volcanics on the north island- is the Auckland volcanic field part of the rift field? Is the hot spot driving the rift? What is the age range of the Auckland field? (again I vaguely remember less than 150,000 years old)? Any apparent hot spot migration?

You mentioned basalt eruptions- anything more silicic? Are the eruptions just localized to the Auckland area? Or is there a more extensive linear trend of similar volcanics?

I'm not certain about the age of the oldest volcanoes in the Auckland field, doodah--I'd have to check that. The hotspot presently under Auckland was originally under Waikato, and has been moving north gradually for about three million years. It's continuing on that track, and will be moving up to the north-east of Auckland in future, creating new volcanic islands out at sea (and quite likely some new volcanoes in Auckland metro area as well, which will be interesting :eek: for the people living there). As for the nature of the eruptions, there are all basaltic or scoria. Presently the Auckland field is the only hot-spot related field in the country.

Regarding the rift volcanics, that's because of the nature of New Zealand's relationship to the plate boundaries. It sits astride the junction between the Pacific and Australian plates, and it's literally being torn in half because of the twisting motion of the plates. The Pacific Plate is rotating counter-clockwise, with its axis of rotation is 60 degrees south and on the International Dateline. Northern New Zealand is being twisted counter-clockwise, and the Pacific Plate is diving (subducting) under the North Island. The motion of the plate against the North Island is head-to-head, hence the subduction. However, as you go further south the motion of the Pacific Plate is much more oblique. The results are; in the North Island, as the plate dives beneath it creates tension in the Taupo Volcanic Zone (TVZ). As you'll know, stress is compression and tension is splitting apart, so the TVZ is being stretched, and the crust fractured and thinned, as the plate moves underneath. This is because the diving plate pulls on the leading edge of the plate beneath which it is descending. The plate has been diving under northern New Zealand for at least the last twenty million years (see some of my other Earth Science posts) and therefore it's been creating volcanoes in Northland (20-15 miliion years ago) Coromandel Peninsula (15-10 mya) and in the TVZ for the past roughly 5 million years. When The TVZ eventually pulls away to the northwest, as it will in about 5 million years, the Wairarapa lowlands will be the next zone of tension related volcanic activity. Then in about 10-15 million years from now, at the current rate of movement, the South Island will become active volcanically in the northeast coast area as the descending plate will be adjacent.

I hope this is useful to you.