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Types of volcanic rock

Rocks are not all the same. Some are heavy, some are light. Others are dark, while some can be almost pure white. Even igneous rocks that are all formed from magma in the Earth’s mantle can look very different.

Igneous rocks

Rocks are broadly classified into three groups – igneous, sedimentary and metamorphic. Igneous rocks are formed from magma in the Earth’s mantle. They generally don’t contain fossils, don’t react with acids, don’t usually contain obvious layers, can be made of different minerals, sometimes have holes or bubbles and may be glassy in appearance. Volcanologists look for these igneous rocks so that they can learn more about where these rocks have come from and whether they were formed during a volcanic eruption.

Geologists use the visual appearance of the rock as an initial clue to its composition but will then verify their ideas using specialised techniques. For example, scientists at The University of Auckland use an electron microprobe to measure the exact quantities of silica, iron, magnesium and many other chemicals that are in rock samples they collect. This information helps them to classify the rock and may give them direct clues about the volcano and the eruption that formed the rock.

Lava solidifies to rock

New Zealand has 3 main types of volcanoes, and each has been formed from a different type of magma. Once the lava has erupted, it cools and solidifies into rock:

  • Basalt magma often forms shield volcanoes.
  • Andesite magma often forms cone volcanoes.
  • Rhyolite magma often forms calderas. Depending on how much gas the magma contains, it can also form cone volcanoes.

Basalt

The Earth’s crust is mainly basalt rock. It is a heavy, dark, grainy rock. Basalt is associated with great rock columns that are found in many places around the Earth, for example, the Organ Pipes in Dunedin or the Giant’s Causeway in Ireland.

Basalt magma is formed at high temperatures (around 1,200ºC). When it comes out of the volcano, it is hot and liquid. It contains very little silica (less than 50%) and a lot of magnesium and iron, which makes the rock look dark.

The Auckland volcanic field has erupted this type of hot, runny iron-rich lava, and the landscape is dotted with mountains made from basalt and scoria (a red-coloured rock that contains large amounts of iron-rich minerals). Both rock types are excavated for building materials and landscaping.

Andesite

Andesites are lighter coloured than basalt because they contain less iron and more silica (50–60%). Some scoria rocks fall within the andesite classification because of their chemical composition.

Magma that contains andesite is generally around 800–1,000ºC and forms steep-sided cone volcanoes (stratovolcanoes). Mount Ngāuruhoe is an example of an andesite volcano.

Rhyolite

Rhyolite is light-coloured or white – this is a clue that the rock contains a lot of silica (more than 70%) and not much iron or magnesium.

Rhyolitic magmas are associated with low temperatures (750–850ºC) and are often thick, which means gases can’t escape. Some rhyolitic rocks are quite light, for example, pumice, which may still have evidence of the bubbles of gas trapped as the rock solidified.

Nature of Science

Classification helps scientists organise things into groups. In rock classification, such grouping can help geologists see patterns and perhaps explain the reasons for rocks looking similar.

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