The volcanic shape of the Port Hills is very apparent even at a distance. Close to, this impression is confirmed by the obvious lava flows that are exposed in cliffs and road cuttings around the peninsula. These features are so clear and so distinct that it is surprising to discover that geologists date the last volcanic activity in this area at nearly six million years ago.
In fact the main volcano that contributed most to the local landscape, and which was centred roughly on Charteris Bay, was active some twelve to nine million years ago. At that time the volcano formed an island, because the connection to the mainland came about only in the last 20,000 years. Two younger volcanos (Lyttelton and Akaroa) ceased erupting 6 million years ago and the three make up what is now Banks Peninsula.
Built up layer by layer over millions of years, the massive volcanic cone of the older volcano was probably more than 1500m high at its maximum development, whereas the highest feature in today's landscape (Mt Herbert) is a mere 920m high. From the time that the volcano became extinct, wind and rain, streams and rivers, together with the relentless washing of the sea, have reduced its size and changed its shape to lay bare most of the features that we see today.
Sea Caves
A sea cave, also known as a littoral cave, is a type of cave formed primarily by the wave action of the sea. The primary process involved is erosion. Sea caves are found throughout the world, actively forming along present coastlines and as relict sea caves on former coastlines
Formation
Littoral caves may be found in a wide variety of host rocks, ranging from sedimentary to metamorphic to igneous, but caves in the latter tend to be larger due to the greater strength of the host rock.
Sedimentary
Sedimentary rocks are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water.
Metamorphic
Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form".
Igneous
Igneous rock is formed through the cooling and solidification of magma or lava.
Logging Tasks
- At posted coordinates have a look at the caves and count the number of caves you can see.
- Tell me what you think the host rock for the formation of these caves from the three choices in the above description.
- Describe the texture and colour of the host rock at the additonal waypoint of the other sea cave.
- Optional: Take a photo of the caves from your watercraft.
This Earthcache can only be accessed by boat and please becareful in and around these caves as waves, swell and tides can be unpredictable.