Geology of New Zealand - Volcanic Activity

Volcanic Activity

When a plate is subducted under another plate, it causes melting in the mantle, the layer of the earth below the crust, and produces a volcanic arc, composed of mainly andesitic and rhyolitic volcanoes in New Zealand, but often widely varying in composition depending on the type of island arc. Andesitic eruptions tend to form steep stratovolcanoes, including mountains such as Ruapehu, Tongariro and Taranaki, islands such as Little Barrier, White and Raoul Islands, or submarine seamounts like Monowai Seamount. Rhyolitic eruptions with large amounts of water tend to cause violent eruptions, producing calderas, such as Lake Taupo and Lake Rotorua. Basaltic volcanoes tend to form at intra-plate hot spots. Basaltic eruptions tend to be fairly placid, producing scoria cones and lava flows, such as the volcanic cones in the Auckland Volcanic Field, although Mount Tarawera's violent 1886 eruption was an exception.

The South Island has no currently active volcanoes. However, in the late Cretaceous (100-65 Ma), there was widespread volcanic activity in Marlborough, West Coast, Canterbury and Otago; and in Eocene times (40 Ma), there was volcanic activity in Oamaru. The most well known Miocene volcanic centres are the Otago and Banks Peninsulas. The Otago Peninsula near Dunedin was built up by a series of mainly basaltic intra-plate volcanic eruptions in Miocene times (16-10 Ma). This has resulted in the formation of Port Chalmers. Banks Peninsula near Christchurch was built from two mainly basaltic intra-plate volcanoes in Miocene times (12-6 Ma and 9.5-7.5 Ma), corresponding to the Lyttelton and Akaroa Harbours. Southland's Solander Islands were active around 1 to 2 million years ago.

The North Island is home to both old and currently active volcanoes. Over time, volcanic activity has moved south and east, as the plate boundary moved eastward, and the place where subduction changed from the Pacific Plate to the Australian Plate moved south.

A volcanic arc became active to the west of the current land in Northland in Miocene times (23 Ma), and gradually moved South down to New Plymouth, where Taranaki is still active. It produced mainly andesitic strato-volcanoes. The Northland volcanoes include the volcanoes that produced the Waipoua Plateau (site of Waipoua Forest, with large Kauri trees) and Kaipara volcano.

The Waitakere volcano (22-16 Ma) has mainly been eroded, but conglomerate from the volcano forms the Waitakere Ranges, and produced most of the material that makes up the Waitemata sandstones and mudstones. Lahars produced the coarser Parnell Grit.

Notable visible volcanoes in the Waikato include Karioi and Pirongia (2.5 Ma). Taranaki is a perfectly formed andesitic strato-volcano, that last erupted in 1755.

The volcanoes off the West coast of the North Island, together with Taranaki and the Tongariro Volcanic Centre, are responsible for the black iron sand on many of the beaches between Taranaki and Auckland.

Shortly after (18 Ma), a volcanic arc developed further east to create the Coromandel Ranges and undersea Colville Ridge. The initial activity was andesitic, but later became rhyolytic (12 Ma). In the Kauaeranga Valley, volcanic plugs remain, as does a lava lake that now forms the top of Table Mountain. Active geothermal systems, similar to those that now exist near Rotorua, were present around 6 Ma, and produced the gold and silver deposits that were later mined in the Coromandel gold rush. Later (5-2 Ma), volcanic activity moved further south to form the Kaimai Range.

After this, activity shifted further East to the Taupo Volcanic Zone, which runs from the Tongariro Volcanic Centre (Ruapehu and Tongariro), through Taupo, Rotorua, and out to sea to form the Kermadec Ridge. Activity was initiated around 2 Ma, and continues to this day. The Tongariro Volcanic Centre is composed of andesitic volcanoes, while the areas around Taupo and Rotorua are largely rhyolitic.

The Tongariro Volcanic Centre developed over the last 275,000 years and contains the active andesitic volcanic cones of Ruapehu, Tongariro, and Ngauruhoe (really a side cone of Tongariro). Ruapehu erupts about once a decade, and while the eruptions cause havoc for skiers, plane flights and hydroelectric dams, the eruptions are relatively minor. However, the sudden collapse of the crater wall caused major problems when it generated a lahar in 1953, that destroyed a rail bridge, and caused 151 deaths at Tangiwai. The last significant eruption was 1995-96. Ngauruhoe last erupted 1973-75.

Lake Taupo, the largest lake in the North Island, is a volcanic caldera, responsible for rhyolitic eruptions about once every 1,000 years. The largest eruption in the last 65,000 years was the cataclysmic Oruanui Eruption 26,500 years ago, producing 530 cubic kilometres of magma. The most recent eruption, around 233 AD was also a major event, the biggest eruption worldwide in the last 5,000 years. The eruption caused a pyroclastic flow that devastated the land from Waiouru to Rotorua in 10 minutes.

The Okataina volcanic centre, to the East of Rotorua, is also responsible for major cataclysmic rhyolitic eruptions. The last eruption, of Tarawera and Lake Rotomahana in 1886, was a relatively minor eruption, which destroyed the famous Pink and White Terraces, and covered much of the surrounding countryside in ash, killing over 100 people.

Many lakes around Rotorua are calderas from rhyolitic eruptions. For example, Lake Rotorua erupted around 13,500 years ago.

Earlier eruptions between Taupo and Rotorua around 1.25 Ma, and 1 Ma, were large enough to produce an ignimbrite sheet that reached Auckland, Napier, and Gisborne.

Vast pumice deposits generated from eruptions in the Taupo Volcanic Zone occur throughout the central North Island, Bay of Plenty, Waikato, King Country and Wanganui regions.

Every so often, there are swarms of earthquakes within an area of the Taupo Volcanic Zone, which last for years. These earthquake swarms indicate that some movement of magma is occurring below the surface. While they have not resulted in an eruption in recent times, there is always the potential for a new volcano to be created, or a dormant volcano to come to life.

White Island, in the Bay of Plenty, is a very active andesitic volcano, erupting with great frequency. It has the potential to cause a tsunami in the Bay of Plenty, as does the dormant Mayor Island volcano.

A line of undersea volcanoes extends out along the Kermadec Ridge.

Intra-plate basaltic volcanic eruptions occurred near the Bay of Islands in Northland, in the Late Miocene (10 Mya), and again more recently (0.5 Mya). The South Auckland volcanic field was active in Pleistocene times (1.5-0.5 Ma). The Auckland Volcanic Field started around 250,000 years ago, and is still active. It includes around 50 distinct eruptions, with most of the prominent cones formed in the last 30,000 years, and the most recent eruption, which formed Rangitoto Island, around 600 years ago.