|The location of New Zealand and the Kermadecs in the South-West Pacific, and the major features of the sealer. (on this page)|
|The tectonic plates responsible for the uplift of the Kermadec Ridge and the downdraft of the Kermadec Trench. (on this page.)|
|The geology of (submarine) volcanoes and a geologic description of each of the islands, Raoul, Macauley, Curtis & Cheeseman and Esperance. (on this page) For charts of the islands, visit the chapter on dive sites.|
|Topographical map, as can be obtained from the Department of Land Information NZ (LINZ). Homepage: www.linz.govt.nz Detailed (1:25,000) topographical map of Raoul (free, 2MB): http://ortho.linz.govt.nz/nz_offshore_islands/raoul_high_res.jpg. Also some photographs of the island.|
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-- home -- issues index -- marine reserves index -- site map -- Rev:20020719,
This map shows New Zealand in relation to Australia. It also shows NZ's outlying islands and its Exclusive Economic Zone (EEZ). The Kermadec islands lie NE of New Zealand, roughly in the direction of NZ's main axis. Their distances apart and their distance to the North Island happen to be of the right size to add a considerable area of EEZ. As can be seen, Norfolk Island which is administered by Australia, lies roughly at the same latitude of about 30ºS.
The detailed map of the sea bottom, shown below (Map courtesy NIWA) reveals that the sea bottom has major features such as the 8km deep Kermadec Trench on right, and next to it the Kermadec Ridge, which provides the volcanic activity to bring the Kermadec Islands to the surface. further back the Colville Ridge, which is of older date. In the middle the South Fiji Basin, a large area of deep ocean floor of 3-4km deep. To the left the Three Kings Ridge, also formed much earlier than the others, and finally the Norfolk Ridge with the submarine volcano of Norfolk Island, which has a very large plateau on it. At the very bottom of the chart, the northern tip of the North Island can be seen.
Please take note of the location of the Kermadecs, in order to locate it at world maps used further down this chapter (and the chapter on ecology), on which they are not marked.
Islands Marine Reserve
The marine reserve created around the Kermadec Islands consists essentially of a number of circles of 12 nautical miles (22km) radius, centred around every outcrop. It amounts to three disjoined areas adding up to 7480km2, but separated by gaps of 15-25 km. As can be seen from the map, most of the marine reserve consists of blue ocean to a depth of 1000m and some of it even to 2000m. Most of the sea bottom is found deeper than 500m, out of reach of daylight.
The main habitat it protects is thus blue ocean with very little life,
and a muddy sea bottom, as dark as the night, with here and there a rocky
outcrop with filterfeeders attached. How much of it is there and what it
looks like, is not known, because nobody has sent cameras down that deep.
On the world map of tectonic plates, one can see that an important subduction zone between the Indian-Australian plate and the Pacific plate, runs through the Kermadecs and through the main axis of NZ. These two plates mainly slide past each other, which makes the subduction component weak. As the Pacific plate slides below the other, it pushes the other margin up, resulting in a ridge. The melting plate which is drawn down, causes volcanic activity in the area of the ridge, rising from about 100km depth, resulting in volcanoes rising up above the ridge. Such volcanoes formed the islands of the Kermadec Group. Subduction zones are usually found where ocean plates meet continents, as the map shows, and the situation of one ocean plate subsiding underneath another ocean plate is uncommon. The two plates move relatively to one another at the speed of 7cm per year, which is relatively fast for such a process.
diagram shown here (courtesy Institute of Geological and Nuclear Sciences),
illustrates to scale the extent to which the Pacific Plate is subducted.
From the locations of deep earthquakes, it is concluded that the plate
is drawn down to a depth of over 600km. But shallower, at around 100km,
very hot magma is formed, which is pushed upward through cracks in the
Australian Plate, forming volcanism. Rising heat also buckles up the crust
to form the Havre Trough.
This volcanic area is closely monitored because eruptions could be hazardous to passing ships and overflying aircraft. Jet engines are extremely sensitive to volcanic ash. Serious submarine slumps could also threaten New Zealand with a tsunami - dangerous because of its short warning time (1-2hours).
All the islands in the Kermadec Group have been formed by volcanic eruptions. Usually the first phase of a volcano starts with violence and the release of trapped gases under very high pressure. As the liquid rock is blasted into the air, the dissolved gases expand, pulverising the rock to ash, pumice and scoria, most of it falling down on the volcanic cone (tephra).
During the last ice age, when the ocean level was about 120m lower than today, the islands must have been much larger. Ironically, both Curtis and Macauley are volcanoes with very flat tops at 120m depth, and they may have been as large or larger in area than Raoul at the time. Now these gently sloping flatlands lie just within reach of daylight, hosting fish life much richer than that found deeper down.
The outlying Meyer and Herald Islands were formed from underwater eruptions,
0.5-1.5 million years ago.
|Stratovolcano: (L: stratum=layer) These
are steep volcanic cones built by both pyroclastic (thrown by fire)
and lava-flow eruptions. They are composed of volcanic rock types that
vary from basalt to rhyolite, but their composition is generally
Stratovolcanoes may erupt many thousands of times over life spans of millions
of years. A typical eruption would begin with ash explosions and end with
extrusion of thick viscous lava flows. The cone-shaped form begins gradually
and becomes steeper (up to 35°) toward the summit, which generally
contains a crater.
Tephra: (Gk: tephra= ash) loose material, e.g. ash and pumice, deposited by volcanic eruptions.
Dacite: An extrusive igneous rock in composition between rhyolite and andesite, consisting mainly of the minerals feldspar and quartz.
Tholeiite basalt: A basalt resembling that found in Tholey, Germany.
See also the geology glossary on this web site. Right-click on this link and open it in a new window for ease of reference.
Raoul is a stratovolcano, composed of material ejected from several eruption centres, mostly andesitic tuffs. The Herald Islets were the first to be active, and the Denham Bay crater is estimated at 2100 years of age. Raoul erupted 14 times above water. What happened before, is not known.
Macauley is quite a substantial volcano, standing on a base over 900m deep, where it measures 380km2. Built from tholeiite basalt and interbedded dacite tephra, the island measures about 3.1km2. Continued eruption of thin basalt flows built a large shield volcano with a smooth but hard top at 150m high, where a small crater can be found.
|Curtis and Cheeseman (chart)
The total land area of Curtis and Cheeseman is about 60 Ha (147 acres). Curtis remains active with daily earthquakes and small eruptions destroying the vegetation in its caldera. The islands are the eroded peak of a large volcano, measuring 25x15km at its base on the Kermadec Ridge. The rocks of these islands stem from pyroclastic poorly-bedded ash deposits. Curtis has been rising by 18m in the past 200 years.
Previously known as French Rock, l'Esperance is but a pinnacle rising up steeply from the surrounding sea floor: 100m directly off the pinnacle, while its submerged mountain rises from an 800m deep ridge. 250m in diameter, it measures optimistically 4.8 Ha (12 acres). l'Esperance has no volcanic activity and has been like that for many thousands of years.
|Topographical map and photos
The topographical map of Raoul shown here is part of the map which is freely available from the LINZ web site. The inset shows the location of the buildings, and it covers the most inhabited part of the island. The map shows graphically that Raoul is completely covered in forest, where the steep cliffs are and the sandy beaches. Best of all, its shading shows the shape of the crater's deep impression, the caldera. It can now also be seen that Denham Bay forms another crater, to the west of it.
A large part of the rain falling on Raoul, ends up in the crater lakes, which are 3-6m above sea level. Excess water drains through the porous rocks out to sea.