The volcanoes of Lyttelton and Akaroa are primarily made of an igneous rock called basalt. Igneous rocks form from the crystallisation of minerals from hot molten rock. Volcanic rocks have minerals cooled quickly from molten magma or lava at or near the surface of a volcano. As a result, they are dominated by small crystals, often unable to be seen with the naked eye.
Plutonic rocks represent slow cooling of molten magma deep within the ground, often representing the magma chambers that feed the volcanoes at the surface. They have large crystals that are all very visible to the naked eye.
Basalt is a dark coloured volcanic rock, rich in heavy metals like iron. Because it contains so much iron, it often rusts red or orange as the minerals are exposed to air. It is the most widespread of all igneous rocks, and comprises more than 90% of all volcanic rocks on Earth (primarily as oceanic crust under our oceans is all made of this material). Because of its relatively low silica (SiO2) content, basalt lava has a comparatively low viscosity (is "runny") and forms thin flows that can travel long distances. It is also the hottest type of lava/magma. This often produces an overall volcano shape with a large, low profile, called a shield, rather than the more commonly recognised steep-sided cone shape that is most often thought of as a stereotypical volcano.
The basalt rocks used in this part of the garden have originally come from various working quarries around Banks Peninsula but been here on the UC campus for some time! They are one of the most common rock types you will see around campus for many landscape and garden designs, including retaining walls. In fact, many historical buildings and famous landmarks all over tautahi Christchurch have been built using the same material!
What famous buildings that you have observed in the tautahi Christchurch city centre do you think have been built using this dark, durable local stone?
