Rapaki Dyke

Rapaki Dykes is a 30m thick, trachytic intrusion sourced from the magmatic system of the Lyttelton Volcanic Complex. The dyke cross cuts lava flows of the Lyttelton Volcanics. As the dyke cooled joints formed. Some of these are now seen as vertical cracks running through the cliff. Others are at right angles to the cliff face and have a rough shape of columns.

Dyke: A dyke is a type of later vertical rock between older layers of rock. But while technically, it is any geologic body which cuts across: flat wall rock structures, such as bedding; massive rock formations, and is usually igneous in origin

Cooling Joints: are columnar joints that result from the cooling of either lava from the exposed surface of a lava lake or flood basalt flow or the sides of a tabular igneous, typically basaltic, intrusion.

Rockfall: Significant rockfall occurred in the volcanic deposits of Banks Peninsula during the Canterbury Earthquake Sequence (2010-2014). The slope on the southern side is littered with rock fall blocks, which have toppled from Rapaki Dyke. The size and shape of rock fall blocks are controlled by joints. It has been found that recent rock fall have travelled further than those in the past, due to the loss of native vegetation, which slowed and stopped boulders. Rockfall areas and talus (loose rock slopes) provide delicate ecosystems for native flora and fauna; if you are still you may be able to spot sunbathing skinks.

From this location you can see into the eroded interior of the Lyttelton Volcanic Complex (12-9.7 million years old). You need to use your imagination to visualize the volcano reaching heights of 1500m. Looking across the harbour the highest peaks on the peninsula are Mt Herbert (left) and Mt Bradly (right). These are part of the Mt Herbert Volcanic Group, which erupted on the outer flanks of the Lyttelton Volcanic Complex (9.7-8.0 Ma). The lava flows (grey cliffs) at the top of Mt Bradley are almost flat lying reflecting the control of topography during these eruptions.
The Diamond Harbour Volcanic Group is best seen from here as the Diamond Harbour Dip Slope. Diamond Harbour is the small community on the eastern side of Lyttelton Harbour. The smooth slope above the community is the upper surface of a series of lava flows that erupted to the north-east of Mt Herbert. These lavas flowed downslope and into an eroded Lyttelton Harbour, almost reaching the opposite side (forming Shag Reef), and flowing as far down the harbour as Ripapa Island, Shell and Pile Bays.
Quail Island in the centre of Lyttelton Harbour is comprised of Lyttelton Volcanic Group lavas, earlier volcanics, and lavas of the Diamond Harbour Volcanic Group.

Rapaki dyke is composed of trachyte, a type of lava which commonly has crystals of alakali feldspars. Another way that trachyte is distinguished by its subtle green colour. Find a rock fall boulder and examine the dimensions, can you see where it potentially fell from? Why would rock fall boulders make a good habitat for skinks? Rapaki Dyke is a popular rock climbing area, what rock properties are used by the rock climbers?