One of the difficulties in maintaining a blog like Volcano Hotspot is finding new volcanoes to explore.  The Smithsonian GVP and Alaska Volcano Observatory are favorite sources for new systems.  Over the years, I find volcano topics in rather odd corners.  This is one of them. 

My Lovely Lady and I watch murder mysteries in the evenings.  Having gone through most of them here in the US, we branched out to various British, European and Korean series.  The basic stories don’t change a lot from country to country, and the subtitles mostly work, but the way different locales deal with investigations and lawlessness is fascinating.  We know it is all entertainment and as disconnected from reality as the US shows are (which is a lot) but is still worth our entertainment time.  As usual, your actual mileage may vary.

The differences in series in the English-speaking world is particularly interesting to us, driving home the observation that Great Britain and the US are two nations divided by a common language.  There are three other English-speaking nations with the same problem, Canada, Australia (Oz) and New Zealand. 

We ran across a three-season series featuring New Zealand actress Lucy Lawless named My Life is Murder.  The first year was shot in Australia.  During COVID, they moved the production to Auckland, her hometown.  We watched the first episode of the second season (series), shot as love note to her home town of Auckland and saw volcanoes everywhere.  An intentional tourist video could not have done a better job selling the city.  I vaguely remembered Auckland was built on a monogenetic volcanic field.  This visual reminder gave me an excuse to take a deeper look into it.  Please come along.  

The Auckland volcanic field is located at the S end of the Northland Peninsula on the North Island of New Zealand.  Auckland, with a population of 1.6 million is built on top of the 600 km2 monogenetic field.  The field is elliptical, 30 x 20 km, with the long axis generally N-S.  This is the northernmost recently active volcanic field of the Auckland Intraplate Province.  It is dominated by intraplate basalts.  There are at least 53 volcanic centers including maars, tuff rings, small shields, and scoria cones. 

Activity began some 193 ka.  Half the vents were active the last 60 ka.  There are 19 known eruptions the last 20 ka, with a single vent, Rangitoto active in the last 11 ka.  Rangitoto island is the largest vent, a 6 km wide shield capped by multiple scoria cones and a broad apron of lava flows. 

The most comprehensive description of the field is Bruce Hayward’s 2011 Volcanoes of Auckland. a Field Guide was published.  It is available commercially and can be found on Amazon.  There is an expanded version entitled Volcanoes of Auckland:  The Essential Guide by Hayward, et al, 2011 available in pdf form. 

Auckland is by far the largest city in New Zealand, 5x larger than Christchurch on South Island.  While the population of city is listed at over 1.2 million, its larger metropolitan population is over 1.7 million.  The city is relatively flat, with most vertical development due to cones, tuff rings and shields. 

The region was initially settled by Maori around 1350.  Tribal battles took place early and often, continuing through the time Europeans arrived in 1642.  From then through the mid-19^th Century warfare between European settlers and Maori was common.  This mostly died down by the end of the 1860s when there were too many Europeans to defeat in battle.  Many of the cones (maunga) had substantial Maori fortifications, terraces and other archaeological remnants.  Many of the cones were leveled or strongly altered by scoria quarrying over the last few centuries.  Basalt from lava flows was used for building materials. 

The central part of the city is located on the Auckland Isthmus, less than 2 km wide.  The city has multiple harbors surrounded by smaller islands.  The city itself is the major economic and financial center of New Zealand with a full array of major corporations, banking, professional, scientific and technical services.  Retail trade is also strong along with education and technical training.  Auckland produces nearly 40% of the national GDP.  There is a strong and growing tourist sector.

Climate is classified as a subtropical oceanic climate with warm humid summers and mild damp winter.  Average highs are around 28° C.  Average lows are around 2° C.  Average rainfall is over 120 cm/year. 

New Zealand has a strong environmental movement that here expresses itself in a call to preserve the various volcanic features.  The concern is over a century old.  Early settlers actively mined the cinder cones like we have seen in many other parts of the world for road building and other materials.  This has mostly stopped in favor of preservation of the volcanic features. 

Volcanoes in New Zealand are monitored by GeoNet, which also provides information about earthquakes, landslides and tsunamis.  There are no vents specifically monitored in Auckland, though there is a local seismic net with 6 seismometers.  There are a variety of webcams in and around Auckland.  Once again, none of them are dedicated on volcanic vents.  Rangitoto is ringed with active webcams aimed at beaches and highways.  Meteoblue is as good a place as any to start.

Region

Auckland is located at the base of what is known as the Northland Peninsula, a finger of land that stretches some 330 km NW from the Auckland (Tamaki) Isthmus on the North Island of New Zealand.  There are multiple monogenetic volcanic fields on the peninsula, most of them intraplate basaltic fields, The Northland Volcanic Province is a grouping of many of those fields.  Over the years, we have published multiple posts on New Zealand volcanoes.  These include White Island in 2020, Ruapehu in 2020, Taranaki in 2021, and Hatepe (Taupo) in 2022.   

The Northland Volcanic Province includes at least four volcanic fields some 100 – 250 NW from Auckland.  While the various names for the fields and province are widely used, specific field location maps tend to vary a bit from paper to paper.  For purposes of this post, we will briefly review the list of volcanic fields listed in South Auckland Volcanic Field Geoheritage.  From the N, these are in order, the Kaikohe – Kerikeri, Kaihoke – Bay of Islands, Puhipuhi, and Whangarei volcanic fields.  S of Auckland, we have the South Auckland,  Ngatultura and Okete volcanic fields.  There are five small outliers near Auckland: Tara, Ti Point, Rakitu Island, Stony Batter, and the Mercury Basalts. 

Kaikohe – Kerikeri volcanic field

The Whangaroa volcanic complex and the Wairakau volcanic center are part of the Kaikohe – Kerikeri volcanic field, located some 228 km NNW from Auckland.  Whangaroa is an 80 km long NW trending belt on the E coast of Northland.  These are eroded remnants of several large stratovolcanoes 1.5 – 3.0 km high.  The belt is divided into two halves.  The N part is deeply eroded exposing volcanic plutons and dikes.  The S part is a ring plain of lahar debris and subvolcanic intrusions.  The Wairakau volcanic center is an older volcanic region was active 20 – 18 Ma.  Activity here shifted S over time.

The Kaikohe – Kerikeri (Whangaroa) volcanic field is located some 191 km NNW from Auckland.  It has been active for the last 10 – 1.0 Ma, producing some 2,500 km2 of volcanic rocks.  Recent activity erupted intraplate basalts producing lava flows, shields, cinder cones, scoria falls, tuff rings and domes.  These are generally inland, centered around Lake Omapere, though there is an eruptive center on the coast NE of the lake. 

Kaikohe – Bay of Islands volcanic field

The Kaikohe – Bay of Islands volcanic field is located 187 NNE from Auckland.  It consists of at least 30 eruptive centers including basaltic scoria cones, associated lava flows, small shields, minor rhyolitic lava flows and domes active over the last 500 ka.  It is located at the N end of the Northland Intraplate Province.  Activity shifted SE over time, with younger Taheke Basalts aligned along a NE trend at the S end of the field. 

The most recent eruption produced explosive activity and lava flows from four scoria cones at Te Puke 1,800 – 1,300 years ago.  These eruptions have not yet been precisely dated, though the single dated piece of carbonized wood is associated with an eruption 400 AD.  There are hot springs at three locations including Lake Omapere, created by a lava dam. 

All cones older than 2 Ma have eroded away, leaving plateaus from ancient andesite / dacite stratovolcanoes.  The field is part of the Northland volcanic belt that extends S to the Whangarei volcanic field near Whangarei and the SE volcanics of the Taurikura volcanic complex at the Hen of the Hen and Chicks Island. 

Over the last 12 ka, at least 12 small basaltic vents erupted in the S part of the field forming a cluster of scoria cones around Kaikohe.  Te Puke erupted 100 ka, creating three small cones and lava flows.  The youngest volcano Tauanui, 10 km S of Kaikohe, erupted 60 ka building a high scoria cone and 19 km lava flow down the Taheke Valley toward the Hokianga Harbour.  The field has a small rhyolite dome overlooking Lake Omapere.

Hot springs in the Ngawha geothermal field are the only known high temperature system outside the Taupo Volcanic Zone.  Its temperatures reach 300° C.  Thermal activity is concentrated on a single fault at the center of Ngawha.  The system appears to be powered by the rhyolite intrusion.  The geothermal reservoir is over a kilometer thick.

Puhipuhi – Whangarei volcanic field

Although our source treats the Puhipuhi – Whangarei volcanic fields as two separate fields, they are lumped together in the Smithsonian GVP database entry.  These are located some 132 km NNW from Auckland.  As with other fields, activity here shifted generally S over time, with Puhipuhi being active 10 – 4.0 Ma and Whangarei being active 5.0 Ma – present.

Recent activity were intraplate basaltic eruptions creating scoria cones and lava flows.  These are also considered to be part of the Northland Intraplate Province.  There are young dacite domes and rhyolitic breccias present in Whangarei.  Recent activity built a group of large cones SE of Whangarei, a NE-aligned group of scoria cones N of the city and a group of lava flows E of the city.  They are all aligned along a major local fault.  Recent dating pushed activity back to at least 1.0 Ma.  A set of flows in Whangeri were dated 1.0 Ma – 260 ka.  One cone was dated 310 ka. 

South Auckland volcanic field

The South Auckland (Franklin) volcanic field is located some 43 km SE from Auckland.  It is larger than neighboring Auckland volcanic field, with 84 volcanic vents in a smaller 300 km2 area.  It was active 1.6 – 0.5 Ma and today is considered to be extinct.  Activity migrated N 500 – 200 ka and resumed as the Auckland volcanic field. 

Volcanoes here are more eroded than those in Auckland.  They are also mantled in thick rhyolite and ignimbrite, the Hamilton Ash erupted sometime before 70 – 36 ka from the center of North Island.  The ash covered all basalt exposures.  As this was a monogenetic field in a wet region, there are scoria cones, lava flows, maars, tuff rings and shields.  Most of them are explosion craters and small shields.  The largest maar is 2.7 km in diameter, dated at 880 ka.  Explosion craters have been filled with sediment and no lakes remain.  Tuff rings surrounding the craters are for the most part removed by erosion.  The combination of eroded vents and lava flows covered with a thick layer of ash make it more difficult than normal to identify what happened in the field and when. 

The field is divided into three areas, with eroded lava flows and scoria cones to the NE, the S section has more recent and better-preserved scoria cones, and the central section, which is the lowest lying, wettest part.  Vent location is influenced by local faulting, creating multiple N-S belts of vents.   

Many of the volcanoes in the NE part of the field erupted along fault lines.  These were at a higher elevation and are more eroded.  The largest and youngest of these volcanoes is the Bombay shield, 600 ka.  It is topped with small, twin conical peaks, source of the most recent lava flows. 

Explosion craters and tuff rings are more common in South Auckland than in other northern basalt fields.  Many of them are in the Manukau lowlands.  There is a major aquifer, the Kaawa Shellbeds.  Rising magma interacted with the aquifer creating the 38 recognized craters.  

South Auckland also has more shields than all the other N basalt fields combined, with 40 identified.  Most of these do not appear to have a fire-fountaining eruptive phase.  Some in the S did and are capped by scoria cones. 

Regional environmentalists are vigorously trying to preserve as many volcanic features from further development as possible, creating conflicts with local farmers, ranchers and developers over ownership and transfer of property.  One of the tools used is designation of volcanic features as Outstanding Natural Features via geoheritage assessments by government entities at all levels.  The Waikato District government is quite active in this process.  

Ngatultura

The Ngatultura volcanic field is a smaller, older intraplate basaltic field located some 77 km S from Auckland.  It was most recently active 1.8 – 1.5 Ma and is one of four volcanic fields in a relationship with the still active Hauraki Rift.  These four volcanic fields from N – include the Auckland volcanic field, South Auckland volcanic field, Ngatutura volcanic field, and the Okete volcanic field, in the upper NE corner of the larger and older Alexandra field.  Activity in these fields generally migrated N over time, starting at least 3.8 Ma in the Alexandra volcanics over 105 km S, with the most recent activity at Auckland. 

The field is the smallest of the four, with at least 16 volcanic centers, mostly scoria cones and associated lava flows.  There are some older basalt deposits exposed on the coast and at least one basalt 60 km W offshore dated 3.5 Ma.   Basalts from this vent flowed E and are identified on the coast.  Ongoing uplift exposed these older basalts.  The Ghattura complex shows evidence of at least five major stages of activity, with basalts exposed in bays along the coast.  There were some vigorous phreatomagmatic eruptions from this complex.

Okete

The older Okete volcanic formation is located 105 km S of Auckland.  It is the NW portion of the larger Alexandra volcanic group extending S and E.  Activity at Okete dates 2.7 – 1.8 Ma.  There are multiple heavily eroded volcanic centers covering 75 km2 on the S side of Raglan Harbor and the Tawatahi River.  There are multiple eroded scoria cones, basalt flows and tuff rings.  Scoria is abundant and usually associated with rich, red loam soil.  Most of the lava flows are thin but may reach 30 m thick.  The presence of explosive tuff rings and phreatomagmatic deposits demonstrate that much of the activity took place in the presence of water. 

Small Miscellaneous Volcanics

Tara is located some 95 km N of Auckland.  It was active 800 ka.  There is little published on this volcano.  Ti Point is the remains of a Miocene stratovolcano some 65 km N from Auckland.  The volcano was active 10 – 7.0 Ma.  Erosion exposed much of the underlying plumbing system inside the cone.  Rakitu Island is the eroded remains of two rhyolite domes active 12 – 8 Ma.  It is located 110 km NE from Auckland.  The Mercury Basalts were active 9 – 4 Ma.  They are located some 99 km E of Auckland.  There is little written about this system.  Stony Batter is the remains of a basalt volcano active 8-7-Ma.  Erosion left an extensive boulder field of rounded, fluted basalt boulders.  The volcano is located on Waiheke Island, 48 km E of Auckland.  The island is notable for a secret WWII military installation intended to defend Auckland. 

Auckland volcanic field

The Auckland volcanic field is a monogenetic volcanic field with at least 53 vents underneath Auckland and its 1.6 million people.  It has been active 250 ka – present, with its most recent activity from Rangitoto volcano in 1450.  This eruption was witnessed by Maori, making it the only eruption of the field observed by humans. 

The field erupted in spurts over the last 250 ka.  It is currently dormant with Rangitoto being the most recent eruption around 1450 AD.  The field is considered to be active and likely to erupt again.  Over the lifespan of the field, the world went through two ice age cycles.  For 90% of that time, climate was cooler than today and sea level up to 130 m lower.  During these long periods, the harbors on either side of Auckland were forested river valleys.  All of Auckland’s volcanoes other than Rangitoto erupted through forested land.  After the peak of the last ice age 20 ka, climate warmed and sea level rose to its current level by 7.3 ka.  The only eruption in the field over the last 9.0 ka was Rangitoto through the ocean at the mouth of Waitemata Harbor. 

The field produced large volume lava flows that cover much of the Auckland Isthmus.  One estimate has the field producing around 3.4 km3 of basalt.  More than 50 lava tubes and other caves are known.  The longest two are 290 and 270 m respectively.  There are at least two depressions caused by lava tube collapse that have lakes in them.  Most of the volcanoes are located within 500 m of known or suspected faults.  There is a trend of increasing volume and rate of eruption from the field over the last 20 ka, with half that volume erupted at Rangitoto. 

The field has a variety of eruptive styles from phreatomagmatic to Strombolian to Hawaiian and effusive volcanism.  Phreatomagmatic eruptions created tuff rings with wide craters surrounded by thin crater rim deposits and gently dipping beds.  Tuff rings are usually covered by pyroclastic scoria deposits created as water supply was exhausted before the magma pulse ended.  Many of these eruptions also created small pyroclastic flows.

83% of eruptions begin with a wet phreatomagmatic vent-opening phase.  This is sometimes followed by dry magmatic explosive or effusive phase once magma-water interaction has stopped.  35% of the current field area is covered by less than 30 m of water.  Minimum volume estimate for the combined field is 1.7 km3 (Dense Rock Equivalent – DRE).  78% of this erupted as lava flows, 6% as scoria cones, 6% lava infill of craters, 5% tuff rings and 4% phreatomagmatic crater lava infills.  Most eruptions are small, punctuated by a few large volume eruptions.  Outside an apparent increase in eruption frequency and size over the last 20 ka, there are no clear patterns in eruptive behavior. 

The field is not extinct.  The usual time between events is hundreds to thousands of years.  Eruptions are typically small, though the most recent was relatively large.  Underwater eruptions will be explosive.  For example, the 0.7 km diameter Orakei crater was formed by an explosion that destroyed 3.0 km2 by crater formation and base surge.  It is possible that several vents can erupt simultaneously.  There is evidence that 8 erupted within 3,000 years 31 – 28 ka.  Local disaster planners assume that future eruptions will take place in contact with water, all but guaranteeing an initial explosive eruption.  

There have been extensive hazard assessments and planning scenarios done over the last few decades.  The Auckland Council has a Civil Defense and Emergency Management entity responsible for creating, updating and executing the Auckland Volcanic Field Contingency Plan, Mar 2015.  The worst volcanic hazards considered in the plan are base surges and volcano formation, both of which are expected to have extreme impact to humans and infrastructure.  Neither of these have any mitigation options.  impact from these may extend 3 – 5 km from the vent. 

North Auckland

Eight volcanoes erupted in this part of the field.  Two are in Waitemata Harbour and six on the North Shore.  Rangitoto is the only one known to have erupted in the sea.  Motukorea, now in the harbor, erupted on land when sea level was lower.  Three volcanoes on the North Shore produced scoria cones and lava flows.  Three erupted explosively, creating large craters and tuff rings.  In the N, we have the youngest (Rangitoto) and oldest (Pupuke, Onepoto, 193 ka) vents of the field.  Lake Pupuke is a volcanic explosion crater.  A number of similar craters are open to the sea. 

Rangitoto is the youngest and most unique volcanic center.  Estimates of activity ages significantly tightened over the years, with initial activity estimated 500 – 1500 AD.  Today, there is an argument that the entire sequence erupted 1350 – 1450 AD.  The early phase of the eruption was phreatomagmatic, putting surge deposits across the water.  Once seawater was excluded from the vent, magmatism switched over to effusive shield building.  Physical evolution of the shield is similar to the other three large volcanoes in the field. 

Rangitoto erupted multiple times, with the most recent two separated by 50 years.  Rangitoto is the largest volcano in the field, a shield topping out at 260 m, erupting 1.0 – 0.7 km3 of lava, over 41% of the entire volume of the field.  It is possible that the new shield buried earlier vents in its 1,000-year eruptive history. 

Central Auckland

There are 15 volcanoes in the central isthmus of the field.  Many of the large, iconic scoria cones are located in this part of the field.  Four have been completely quarried away.  Most of the cones of Three Kings are gone.  Four central volcanoes are located here.  They have large explosion craters.  Two of them are breached by the sea.  Many of these volcanoes erupted large volumes of lava from the base of the cones.  All of these cones are partly or fully protected. 

Mt Eden is one of the larger cinder cone centers in the field at 28 ka.  It is a good exposure of scoria and lava flows.  There is an undated second vent now thought to be part of the same system. 

There are four small, circular explosion craters so close to larger volcanoes that they are thought to be satellite vents.  Two of these are well documented.  Two are recently recognized as volcanic craters, demonstrating that analysis of what should be a well understood volcanic field in a major urban area is nowhere near complete.  There is no conclusion on what caused these craters, with speculation ranging between a small phreatomagmatic eruption or a purely phreatic explosion.

Eastern Auckland

There are 14 volcanoes on the E side of the field, located on either side of the Tamaki Estuary.  12 of them began with initial explosive craters and surrounding tuff rings.  11 of them created scoria cones and extensive lava flow fields.  One eruption dammed the former Tamaki River, diverting its flow for a bit.  The remains of 10 of these volcanoes are now in public reserves.

Southern Auckland

There are 13 volcanoes in the S part of the field.  Six are explosion craters and tuff rings.  Three of these have small scoria cones that were quarried away.  The Pukaki Lagoon explosion crater is the best-preserved crater and tuff ring in Auckland.  The other seven volcanoes have scoria cones and lava flows.  All have been quarried, with the loss of four of them.  Several other lost sizeable chunks. 

Otuataua is one of a group of four volcanic centers on the SW part of the field, all within 1.8 km of one another.  Activity took place 88 – 20 ka.  Like Rangitoto, initial eruptions were ashy phreatomagmatic followed by lava flows.  Otuataua appears to be the most recently active vent as its lava flows are not covered by ash. 

Tectonics

New Zealand is the surface expression of a newly identified microplate named Zealandia.  While most of it is below sea level, it has been around for at least 83 Ma when it split from Gondwana.  It is caught up in the ongoing collision between the Indo – Australian and Pacific Plates, being carried along as part of the Indo-European Plate. 

The two big tectonic players in the volcanism of North Island are the Hikurangi Trough – Kermadec Trench, offshore to the E and the penetration of the penetration of the Havre Trough into North Island.  Rate of subduction along this trough is 4.3 cm/year E.  Direction is generally E-W.  There is no volcanic arc on the E coast of North Island. 

The main cause of volcanic activity in North Island is the penetration of the Havre Trough, a spreading center between the Kermadec Ridge and the Colville Ridge.  The trough penetrated North Island in the Bay of Plenty and is the cause of volcanic activity in the Taupo Volcanic Zone (TVZ).  White Island is 230 km SE from Auckland.  TVZ is 240 km SSE from Auckland. 

Volcanic activity along the 330 km Northland Peninsula is unrelated to the tectonic activity associated with subduction and rifting.  Rather, it is currently described as intraplate activity, as basalts from the mantle rise through the crust to the surface.  Activity on the peninsula 20 Ma left a subduction driven volcanic arc, with the trench migrating NE over time.  The volcanic arc went extinct 15 Ma.  Eroded remains of stratovolcanoes built along this arc still remain in the Northland Peninsula mostly along the NE coast.  The peninsula was uplifted from sea bottom over time.  Activity switched over to intraplate, basaltic eruptions by 10 Ma, with all activity since creating multiple monogenetic fields along the peninsula.  Continental crust here is over 25 km thick. 

Conclusions

The Auckland volcanic field is a good example of a heavily populated, active monogenetic field, similar to several we have previously seen in the Trans – Mexican Volcanic Belt.  Its most recent eruption was observed by humans.  The good news is that the field is heavily instrumented with disaster management plans written.  The bad news is that there are currently 1.6 million living on top of the field and eruptions are more frequent and larger over the last 20 ka.  Worse, there is a great deal of water around and future eruptions are expected to begin with an explosive, phreatic phase.  Early warnings will be critical to allow evacuations. 

Additional information

Auckland volcanic field – Wiki

Auckland volcanoes:  the ultimate guide to our volcanic field, New Zealand Travel Tips, Apr 2023

Auckland’s volcanoes, Science Learning Hub

Auckland volcanic field, Science Learning Hub

Small satellite explosion craters in the Auckland volcanic field, BW Hayward, Jan 2015

Auckland volcanic fields, UNESCO

Age of the Auckland volcanic field:  a review of existing data, Lindsay, et al, Nov 2011

How to visit every volcano in Auckland, Smid & Stolberger, Ver 1.1

The Auckland volcanic field, New Zealand:  geophysical evidence for its eruption history, Cassidy, et al, 1999

High-precision 40Ar/39Ar dating of quaternary basalts from Auckland volcanic field, New Zealand, with implications for eruption rates and paleomagnetic correlations, Leonard, et al, Sept 2017

Influences on the variability of eruption sequences and style transitions in the Auckland volcanic field, New Zealand, Kereszturi, et al, Oct 2014

The genesis of volcanic risk assessment for the Auckland engineering lifelines project:  1996 – 2000, Daly & Johnston, Feb 2015

Appendix 2.  South Auckland volcanic field geoheritage, Waikato District

Volcanic risk in Northland, Kear & Thompson, Dec 2011

Open-system magmatic behaviour beneath monogenetic volcanoes revealed by the geochemistry, texture and thermobariometry of clinopyroxene, Kaikohe-Bay of Islands volcanic field (New Zealand), Coote & Shane, Dec 2018

K/Ar ages from the volcanics of Northland, New Zealand, Stipp & Thompson, Jan 2012

Field trip 7.  A taste of Northland geology, Hayward & Smith, Jan 2002

Auckland volcanic field magmatism, volcanism, and hazard:  a review, Taylor & Francis, Mar 2020

Geo junkets:  New Zealand, North Island (Part 1), JG White, Deposits Mag

Field guide monogenetic volcanism of the South Auckland and Auckland volcanic fields, Nemeth, et al, Feb 2012