Auckland Islands Formation

Volcanoes and Glaciers

Level 1 Achievement Standard 90952 Science 1.13Demonstrate understanding of the formation of surface features in New Zealand

Requirements of the standard

Thoroughly explain the links between external and/or internal processes and the formation of surface features in New Zealand.

Key external and internal processes relevant to the Auckland Islands include:• Hot spot volcanism• Glacial Erosion• Sea level change

Key surface features include:• Shield volcanoes• Glacial Inlets/Fiords

Auckland Island - Location

• The Auckland Islands lie approximately 450km South of mainland New Zealand.

• They sit on the continental shelf of the Pacific Plate.

Auckland Islands

New Zealand Volcanism

New Zealand’s volcanoes can be classified into two types:

Subduction volcanoes Magma comes from melted crust. Forms tall steep sided volcanoes or large calderas. e.g. volcanoes of the central North Island.

Hot Spot volcanoesMagma comes from the mantle. Forms low, wide, shield volcanoes. e.g. volcanoes of Auckland city as well as the Auckland Islands.

Subduction

Ocean crust (lithosphere) is more dense than continental (land) crust.

When these two types of crust collide the ocean crust is pushed beneath the continental.

As it moves downwards it heats up and melts forming liquid magma.

The magma expands and rises through the overlying continent and is released during a volcanic eruption.

Features of subduction volcanoes

• Subduction volcanoes form when melted crust from a subduction zone rises to the surface of the Earth and is released during an eruption.

• The magma is viscous (sticky) and explosive.

• Because of this the lava does not flow easily and the volcanoes are typically steep and pointed (Mt Taranaki) or large calderas (explosion craters – Lake Taupo).

New Zealand’s Subduction Volcanoes

Mt Taranaki

Lake Taupo

Hot spots

• Heat from the Earth’s core causes magma in the mantle to heat up and rise.

• The magma can push through the crust and is released during a volcanic eruption.

• Magma originating from the mantle is basaltic. Basaltic magma is very hot and has low viscosity (runny).

• When released, the lava is able to flow very easily and this results in low, wide, shield volcanoes.

Hot Spots

Crust

Mantle

Hot magma rising through mantle

New Zealand’s Hot Spot Volcanoes

Rangitoto IslandA typical low, wide, shield volcano

Auckland IslandFormed from two ancient basalt volcanoes

Types of magmaSummary Table

Magma Type

Solidified Rock Chemical Composition

Temperature(oC) Viscosity Gas Content

Basaltic Basalt45-55 SiO2 %, high in Fe,

Mg, Ca, low in K, Na1000 - 1200 Low Low

Andesitic Andesite55-65 SiO2 %, intermediate

in Fe, Mg, Ca, Na, K800 - 1000 Intermediate Intermediate

Rhyolitic Rhyolite65-75 SiO2 %, low in Fe, Mg, Ca, high in K, Na.

650 - 800 High High

External Processes – Glaciers and Erosion• A glacier is a slow moving build-up of ice. They are often referred to

as frozen rivers.

• As a glacier moves it scrapes the rocks on which it sits.

• Some of the ice can melt and then refreeze in cracks and around pieces of rock. As the water freezes it expands and this can be strong enough to break rock.

• Over time these erosive processes carve out valleys in the rocks.

• Glacier valleys are typically steep sided, U-shaped valleys.

Franz Joseph Glacier, NZ

Earth’s temperature and sea level change• Earth’s temperature has naturally fluctuated over time.

• Linked to temperature is the Earth’s sea level.

Earth’s temperature and sea level change• 70% of the Earth is covered in water – mainly in oceans.

• Massive amounts of water are also frozen at each of the Earth’s poles.

• During cold periods of Earth’s history more water is frozen at each pole and snow and ice is also found on land closer to the equator.

• Because all this water is not in the oceans, it results in a lowering of the Earth’s sea level.

• During warm periods, snow and ice on the land close to the equator melts and flows into the ocean causing the sea level to rise.

The Auckland Islands

• The Auckland Islands at 50o south would have been covered in snow and ice during cold periods (ice ages).

• Glaciers have eroded the basalt rocks forming steep, U-shaped valleys.

• As the Earth warmed the glaciers melted and sea level rose.

• The U-shaped valleys were flooded by the rising ocean and have become the many deep, steep-sided inlets and fiords of the eastern coast of the islands.

Mclennan Inlet, Auckland Island.A classic deep, steep sided, U-shaped inlet.

Aerial view of a U-shaped glacial valley, Auckland Island.