Ōnawe Peninsula

Accessibility: EASY
Dike along the shore with chilled margin (pale colour at edges). Photo/ K Pedley
Ōnawe Peninsula exposes a variety of both volcanic and plutonic igneous rocks near the centre of the Akaroa Volcano. It provides us with a unique view into the initial stages of volcanism, an aspect not exposed at any other location on Banks Peninsula.
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.
Close-up of patterns in weathered trachyte. Photo/ K Pedley
The colour contrasts between the red basalt and the yellow, white and orange of the weathered trachyte (would normally be pale grey in colour) is quite striking and makes for great photographs!
Can you identify any pyroclastic bombs in the basalt French Hill Formation? These round shaped blobs of (now hardened) magma have been thrown out of the volcano during explosive eruptions.
Have a closer look at some of the dikes along the shore platform. Can you spot the glassy hard lighter coloured edges of the chilled margins? Can you also identify columnar cooling joints regularly cutting across the dikes? In dikes, these cooling joints run at right angles to the edges. While most of these dikes have no visible crystals, you may see the occasional pale coloured feldspar crystal in the very centre.
If you visit the syenite outcrop at the very top of the peninsula, compare the crystal sizes of the minerals in the rocks to what you saw in the trachyte dikes at the northern end. Similar minerals are present, just in much bigger size.

Access is off SH75 from Christchurch to Akaroa. If coming from either way look for the Ōnawe Flat Rd signposts. Carparking is at the end of the small peninsula at the start of the Ōnawe Track.

Avoid at high tide.
This is a sacred/tapu Māori site as Ōnawe Pā was the site of an invasion and massacre of the local Ngāi Tahu by Te Rauparaha in 1832. Please walk with respect, leave everything as you find it, and refrain from eating. Please just observe, do not use rock hammers.

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Accessibility: EASY

From the carpark follow the Ōnawe Track down to the beach. There are plenty of outcrops to look at along the beach and shore but you can also follow the Ōnawe Track (1 hr return - MODERATE) up to the old Pā site for great views of the volcanic surrounds and to visit the syenite outcrop.

Volcanic Plutonic Landform
Geological Age
Late Miocene, Akaroa Volcanic Group ( about 9 million years old)
Zealandia Evolution Sequence
Māui Supergroup (Emergence): 25 – 5 million years ago