Extreme Earth Events: New Zealand Volcanoes

The Pacific Ring of Fire

Volcanic eruptions since 1300

What is a Volcano? Subduction: E.g. volcanoes of the Taupo volcanic zone, Mt Taranaki and the Kermadec volcanoes. When the edge of one plate dives under the edge of another, i.e. is subducted, at a certain depth rock melts forming magma Rising magma forces its way up through vents in the crust forming subduction volcanoes.

A “typical” volcano

Volcanoes are made by two processes: Subduction: E.g. volcanoes of the Taupo volcanic zone, Mt Taranaki and the Kermadec volcanoes. When the edge of one plate dives under the edge of another, i.e. is subducted, at a certain depth rock melts forming magma Rising magma forces its way up through vents in the crust forming subduction volcanoes.

Hot Spots: Some volcanoes are found away from a plate edge above a hot spot in the mantle. Upper mantle rock is melted and a plume of magma forces its way from about 100 km deep to the surface through a vent in the crust. The Auckland volcanic field is a hot spot. Volcanoes never erupt in the same place, a new volcano is formed each time a new eruption event happens. Magma is dry and has very little gas

Plume of magma

A volcanic field like Auckland

Auckland volcanic field

Shield – basalt volcanoes Built of silica poor basalt magma Magma comes from the mantle, about 100 km deep – the “hot spot” Magma has very little water or dissolved gas so eruptions are very gentle, any gas can escape easily. Also eruptions are gentle because the magma is runny and very hot so it travels a long way before cooling. Therefore the magma builds up volcanoes with gentle slopes.

Rangitoto

Mt Eden

Runny lava

Fire fountains

Cone volcanoes (subduction) Made up of andesitic lava which has an intermediate amount of silica. The lava is semi-sticky and cooler which means that it is not so runny and it cools more quickly forming cone shapes. Eruptions can be gentle, erupting lava, or explosive, erupting hot ash. The cone is formed when layers of lava and ash build up over time. Examples are Ruapehu, White Island and Ngauruhoe.

Santorini

In Zealandia this is happening along the Kermadec trench. Most volcanoes are under water except for Raoul island. Note the sea level.

The Kermadec volcanoes are a line of jagged, cone volcanoes as high as Mt Ruapehu (2,797 metres), rising up from the ocean floor. Many have large craters, formed by powerful eruptions.

Mt Taranaki – an andesite volcano

Zone A Areas likely to be affected most severely and most frequently by future landslides, lahars and associated floods. Zone B Areas likely to be affected less severely and less frequently by future landslides, lahars and associated floods. Zone C Areas likely to be affected infrequently by very large landslides or lahars

On the Taranaki peninsula, a line of three cone volcanoes is surrounded by a huge ring plain of avalanche, lahar, and tephra deposits

Ruapehu erupting Ngarahoe erupting

Ruapehu plume Kermadec volcanoes

Aerial view of Ruapehu crater on 17 July 1945 showing lava dome that grew over a period of 4 months displacing all the water from Crater Lake.

A very full crater lake 2 months before disaster

The Tangiwai disaster

Lahars 18 th March 2007

Ngauruhoe erupting

Mt Ngauruhoe

Caldera and dome volcanoes - subduction formed from rhyolitic magma which is very viscous because it has a high amount of silica. Gas, especially water vapour and carbon dioxide from melted carbonates, gets trapped in the sticky lava which results in very explosive eruptions forming calderas. Sometimes the gas escapes so that the magma is still very sticky without trapped gas. This forms dome volcanoes.

Formation of magma in subduction volcanoes As plate subducts it gets deeper and hotter (Earth gets hotter) Subducted watery sediments also get heated up At a certain depth the water is super-heated and lowers the melting point of the surrounding rock A slurry of melted and unmelted rock is formed, melted rock (magma) is less dense and rises through weaknesses in the crust

How gas makes volcanoes explosive When magma is formed gases are dissolved in it As the magma rises the gases come out of solution and tiny bubbles form These bubbles expand as the magma rises, lowering the density so the magma rises more Sticky magma traps these bubbles The gaseous magma puts pressure on the ground above cracking it Magma rushes up these cracks (vents) and the bubbles expand so much that an explosive eruption occurs

The centre of the North Island is dominated by volcanic features The two largest lakes, Taupō and Rotorua, occupy calderas, formed by collapse after huge eruptions. Ignimbrite underlies much of the flat areas in the region. These were formed by huge eruptions from calderas, most of which are now buried. In the south a number of large cone volcanoes (Ruapehu, Ngāuruhoe, Tongariro and Pīhanga) are surrounded by gently sloping ring plains formed mainly from lahars

Mt Tarawera showing a volcanic dome The summit of Mt Tarawera consists of a group of rhyolite lava domes extruded in the final stages of an eruption, about 1314 AD. This shows the steep sides and flat top typical of a rhyolite lava dome.

Mt Tarawera showing rupture from eruption Mt Tarawera erupted again on 10 June An explosive eruption of basalt blasted a line of craters through the domes, draping them with layers of basaltic scoria and ash

 Pyroclastic fall ash and pumice deposits erupted 13,500 years ago. This was probably erupted over a few days.

Road cutting showing tephras from Mt Tarawera eruption

Lake Taupo Lake Taupō occupies a caldera (collapsed crater), formed by the Ōruanui eruption, and modified by later eruptions.

Area covered by the Taupo ignimbrite (shown in gray). The ignimbrite extends in all direction from Lake Taupo for about 80 km and covers and area of 20,000 square km.

Taupo eruption about 120 AD

Ash from the Ōruanui eruption from the Taupō volcano, about 26,500 years ago, covered a huge area.

A large submarine volcano on the Kermadec ridge, north of New Zealand, was discovered by undersea surveys, and has been named Healy volcano. It includes a large caldera, and the black cross-hatched area in the image is a reconstruction of what the volcano may have looked like before it erupted and collapsed.

View of the caldera of Macauley submarine volcano. The caldera size varies from 6 to 10 kilometers across. Depths range from 1000 meters (~3300 feet) at its deepest point.

Brothers Volcano as looking into the caldera from the south. The smooth cone (left foreground) is the site of recent volcanic eruptions.

Solander Island lies north-west of Stewart Island. It is an extinct volcano with andesite rocks 1–2 million years old. It is the only volcanic feature above water associated with the subduction of the AP beneath the PP

Geothermal Systems It is essential that geothermal areas have: A deep heat source e.g. magma Water from rain or rivers that percolates down through porous rocks or cracks in the crust Water percolates down and is heated when it gets near the magma. Hot water is formed, is less dense and so rises to surface, forming geysers, boiling mud etc.

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