The Hauraki Rift is an active NeS-to NWeSE-striking

rift valley A rift valley is a linear shaped lowland between several highlands or mountain ranges produced by the action of a geologic rift. Rifts are formed as a result of the pulling apart of the lithosphere due to extensional tectonics. The linear ...

system in the

North Island The North Island ( , 'the fish of Māui', historically New Ulster) is one of the two main islands of New Zealand, islands of New Zealand, separated from the larger but less populous South Island by Cook Strait. With an area of , it is the List ...

New Zealand New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...

that has produced the

Firth of Thames The Firth of Thames () is a large bay located in the north of the North Island of New Zealand. It is the firth of the rivers Waihou and Piako, the former of which was formerly named the Thames River, and the town of Thames lies on its south ...

and the

Hauraki Plains The Hauraki Plains are a geographical area located in the northern North Island of New Zealand, at the lower (northern) end of the Thames Valley, New Zealand, Thames Valley. They are located 75 kilometres south-east of Auckland, at the foot of ...

. It is approximately wide and long.

Geology

The rift valley in the north appears to be delimited in the west by the line of the Firth of Thames Fault and to the east by the Hauraki Fault. The mountains of the

Coromandel Coromandel may refer to: Places India *Coromandel Coast, India ** Presidency of Coromandel and Bengal Settlements **Dutch Coromandel * Coromandel, KGF, Karnataka, India New Zealand *Coromandel, New Zealand, a town on the Coromandel Peninsula *Cor ...

(which include

andesitic Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomina ...

and

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

ic back arc volcanoes) and

Kaimai Range The Kaimai Range (sometimes referred to as the ''Kaimai Ranges'') is a mountain range in the North Island of New Zealand. It is part of a series of ranges, with the Coromandel Range to the north and the Mamaku Ranges to the south. The Kaimai ...

to the east and the Hunua Range to the west give it the appearance of a full graben although it has also been described as a simple structure of 2 to 3

half-graben A half-graben is a geological structure bounded by a fault along one side of its boundaries, unlike a full graben where a depressed block of land is bordered by parallel faults. Rift and fault structure A rift is a region where the lithosphere ...

s so the term Hauraki Graben for the low lying portions would be incorrect. However the apparent line in the south of the Firth of Thames Fault no longer follows the apparent horst structure to the west and some believe the line of the

Waikato River The Waikato River is the longest river in New Zealand, running for through the North Island. It rises on the eastern slopes of Mount Ruapehu, joining the Tongariro River system and flowing through Lake Taupō, New Zealand's largest lake. It th ...

after it leaves the old

Taupō Rift The Taupō Rift is the seismically active rift valley containing the Taupō Volcanic Zone, central North Island of New Zealand. Geology The Taupō Rift (Taupo Rift) is a intra-arc continental rift resulting from an oblique convergence in the H ...

gives a better guide to the western line of the rift as structures have been heavily modified in the last 26,000 years in the south, and the Waikato River historically used the graben in its route to the sea. The active intra-rift Kerepehi Fault in the southern portion of the Hauraki Rift is complex and has the potential for large earthquakes.

Structure

Initially between 10 and 5 million years ago the rift developed in a back arc region to the now extinct Coromandel volcanoes. It has a basement of mesozoic

greywacke Greywacke or graywacke ( ) is a variety of sandstone generally characterized by its hardness (6–7 on Mohs scale), dark color, and Sorting (sediment), poorly sorted angular grains of quartz, feldspar, and small rock fragments or sand-size Lith ...

and

argillite Argillite () is a fine-grained sedimentary rock composed predominantly of Friability, indurated clay particles. Argillaceous rocks are basically lithified muds and Pelagic sediment, oozes. They contain variable amounts of silt-sized particles. T ...

and at least two

volcano A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most oft ...

es erupted within it. To the north the rift is filled with thick Tertiary and Quaternary terrestrial sediments and beyond

Waiheke Island Waiheke Island is the second-largest island (after Great Barrier Island) in the Hauraki Gulf of New Zealand. Its ferry terminal in Matiatia Bay at the western end is from the central-city terminal in Auckland. It is the most populated island ...

it opens up into an oceanic basin. Early work suggested the northern boundary might come ashore in Northland, near

Whangārei Whangārei () is the northernmost city in New Zealand and the largest settlement of the Northland Region. It is part of the Whangarei District, created in 1989 from the former Whangarei City, Whangarei County and Hikurangi Town councils to admi ...

and that the southern boundary while modified by events after the formation of the Mangakino caldera complex continued to be active. Work using geolocation on the Hauraki Rift reveals a widening rate of /year although some of the raw data suggests that at the Te Poi end it might be up to /year which would be consistent with this. However, in this region it is now dominated by the much more recent

(2 million years old at most) associated with the

Taupō Volcanic Zone The Taupō Volcanic Zone (TVZ) is a volcano, volcanic area in the North Island of New Zealand. It has been active for at least the past two million years and is still highly active. Mount Ruapehu marks its south-western end and the zone runs n ...

(TVZ) to which it is off-set by 60 degrees. TVZ eruptive output of 240,000 years ago, being Mamaku ignimbrite from the

Rotorua caldera The Rotorua Caldera is a large rhyolitic caldera that is filled by Lake Rotorua. It was formed by an eruption 240,000 years ago that produced extensive Pyroclastic rock, pyroclastic deposits. Smaller eruptions have occurred in the caldera since ...

has filled in the far southern extremes.

Geothermal activity

Thermal springs along the rift edges and center are found throughout the region with a heat output of the order of 6.5MW in total.

Other Context

At the far side of the

Coromandel Peninsula The Coromandel Peninsula () on the North Island of New Zealand extends north from the western end of the Bay of Plenty, forming a natural barrier protecting the Hauraki Gulf and the Firth of Thames in the west from the Pacific Ocean ...

there is a parallel structure of Late

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

/Quaternary origin extending north, that has been called the East Coromandel Rift. To the south east is the

Tauranga Volcanic Centre The Tauranga Volcanic Centre is a geologic region in New Zealand's Bay of Plenty. It extends from the southern end of Waihi Beach and from the old volcanoes of the Coromandel Peninsula that make up the northern part of the Kaimai Range, towards ...

which is likely related to activity in the old

and the postulated Omanawa Caldera would have been at the intersection of the two rifts about 2 million years ago.

References

{{Seismic faults of New Zealand + Thames-Coromandel District Firth of Thames Hauraki District Geology of New Zealand Geologic formations of New Zealand Seismic zones of New Zealand