Trachyte dikes crossing over. Photo/ K Pedley
The northern end of the peninsula at the narrow neck exposes a basalt cinder cone and related volcanic ash deposits. These rocks are known as the French Hill Formation (9.0 - 8.3 million years old). A series of criss-crossing dikes (vertical intrusions) cutting through these rocks are commonly composed of trachyte (see definition below) and can be up to 6m thick.
On the highest point of Ōnawe Peninsula, at Ōnawe Pā, the plutonic Ōnawe Syenite intrusion is exposed as a group of boulders next to the Ōnawe Track. These rocks are of unknown age but younger than about 9 million years.
It is thought that the Akaroa Volcano evolved in a two stage process with a lower level magma chamber replenishing and mixing vigorously with an upper level magma body, resulting in the variety of rocks types we observe.
What are these rock types?
Volcanic rocks have cooled quickly from molten magma or lava at or near the surface of a volcano. As a result they have 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. It is the most widespread of all igneous rocks, and comprises more than 90% of all volcanic rocks. Because of its relatively low silica content, basalt lava (which forms basalt rock when it cools) 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.
Trachyte is a lightish grey coloured rock. The major mineral component of trachyte is alkali feldspar (e.g. orthoclase), and it generally contains no quartz. Because of the higher silica content, trachyte lava runs more sluggishly and at a cooler temperature than basalt. It is therefore also more explosive in eruptions than basalt. Trachyte is the volcanic equivalent of the plutonic rock syenite.