Inside the earth the molten matter solidifies very slowly so large crystals are formed in these rocks. Such rocks are called Extrusive 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 silicairon, 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.