The steep-sided and seemingly flat-topped Mana Island is a prominent and distinctive physical feature of Wellington’s west coast. The name is an abbreviation of “Te Mana o Kupe ki Aotearoa”, which acknowledges the achievements of Kupe, the legendary 12th century Polynesian navigator, who discovered this land — Aotearoa.
There is an unusual flatness of the top of Mana Island, especially when compared with the contours of its neighbour, Kapiti Island. Maori legend has it that Te Awarua o Porirua, the taniwha who lives in Porirua Harbour, caused the island’s distinctive shape when he crash-landed on it in an attempt to fly.
In fact, the island’s flat-top appearance is due to its uplifted peneplain surface and its cliffs, which are about 100 metres above sea level. Mana Island is one of the oldest and best examples of preserved marine terraces in the Wellington area.
Geology
Mana Island is of moderate relief and has a plateau-like surface sloping gently to the south-east. The eastern half of the island is drained by a deeply incised stream system, and the northern and western coasts are backed by steep cliffs up to 115 m high.
Mana Island is bounded by faults to both the east and west and, while it has been uplifted, it has not been appreciably tilted, unlike much of the uplifted Wellington landscape. Its flat surface was once thought to be a remnant of the Kaukau erosion surface (peneplain) formed in the Oligocene (33.9 – 23.0 Ma ago).
The basement rocks of Mana Island are formed from sandstone and siltstone. At one stage, when sea levels were lower, an isthmus connected the island to the mainland. Today this “Mana Bridge” lies 7-8 m under the sea. Shingle Point, a prominent gravel spit on Mana Island, marks the western end of this submerged shoal area, suggesting that the shoal is a secondary, “built” feature, rather than a remnant of basement rock.
This earthcache focuses on the marine terraces of Mana Island, and in particular on its highest and oldest one – the Mana Surface.
Marine terraces
It takes a unique combination of geologic processes to form a marine terrace.
First, high wave energy is needed to carve a wave cut platform into bedrock. A wave cut platform is the broad, gently seaward dipping platform located at the base of the modern sea cliff. Wave cut platforms are the initial step in the formation of marine terraces, but not every wave cut platform becomes a marine terrace.
Second, the wave cut platform must be removed from interaction with the high-energy wave regime. This is accomplished by two processes: 1) changing global sea level due to glacial-interglacial cycles, and 2) tectonically rising landmasses. Global sea level fluctuates with the amount of water stored in the polar ice caps. When water is stored in polar ice caps, global sea level falls. Conversely, sea level rises when there is less water stored in the ice caps. Fulfilling our second criterion for terrace formation, the lower North Island region is an actively rising landmass, which aids in removing the wave cut platforms from the high energy wave regime. To the east of the North Island is a collision zone between the Pacific Plate and the Australian Plate. The Pacific Plate is being subducted under the Australian Plate, compressing the Wellington Region. The landmass is being squeezed which causes the island to pop-up or actively rise due to the tectonic environment.
Finally, once the wave cut platform is raised above sea level it is removed from subsequent reoccupation from the high energy wave regime and is preserved, usually as a flight of marine terraces along the coastline.
Mana Island’s Mana Surface and other younger marine terraces
The Mana Surface is the highest and most extensive of a series of well-preserved surfaces on Mana Island. It occupies the island’s “table-top” area 78 m above sea level and higher. It is considered to be the oldest marine abrasion platform on the island.
A peneplain origin was proposed for the Mana Surface. It had previously been correlated with the K-surface, remnants of which, on the mainland, dip westward and seawards. The flat-topped hills on the mainland west of the Wellington Fault, in Belmont Regional Park for example, are known to geologists as a peneplain or ‘K Surface’ – after Mt Kaukau, a city landmark. However, continuity of the Mana Surface with similar surfaces on the mainland is now considered by recent studies to be unlikely, because of the Pukerua Fault to the east of Mana Island and the possible presence of other faults between the island and the mainland. But correlation with the Quartz Hill Surface, of probable early Pliocene age (5.3 – 2.6 Ma ago), is thought possible. Quartz Hill in Makara is where the Meridian Energy’s West Wind farm is presently located.
The Mana Surface is gently sloping and dips south-east towards the mainland. It slopes from about 120 m above sea level in the north-west to about 80 m in the south-east. It is possible that such a dip is an initial dip on the erosion surface, but as the island is unlikely to have escaped the deformation known to have affected the mainland, the dip of the Mana Surface is considered more likely to be of tectonic origin, and to represent the net result of tilting since its formation.
Below the Mana Surface are seven marine benches and gravel terraces. The term “bench” refers to the abrasion platform cut across the basement rocks. The term “terrace” is used for the surface of the sediment layers lying on top of the bench.
These benches and terraces are preserved at various levels from 74 m down to 5 m above sea level. The marine benches and the overlying terrace deposits are considered to have formed in the later stages of sea-level rises, during successive interglacial periods, although each does not necessarily represent a separate interglacial period. Seven such terraces or benches have been identified (A to G in Figures 1 and 2), but others may have been formed that are not now preserved.
The Mana Surface has no overlying deposits except late Quaternary (younger than 12,000 years ago) loesses, which are thought to have been formed during glacial periods. Thus, the numerous surfaces younger than the Mana Surface probably represent successive interglacial periods and glacial periods. Marine deposits preserved beneath these lower surfaces suggest that the Mana Surface itself is the oldest of the suite of marine benches preserved on Mana Island, any residual deposits on the Mana Surface having since been eroded away or buried beneath the loess.
Please remember
As a Scientific Reserve administered by the Department of Conservation, everything on Mana Island is protected and nothing may be removed. Overnight camping, fires and smoking are not permitted. Dogs and other pets may not be taken onto the island.
Bring reasonable walking shoes, warm clothing, raincoat, water bottle and some snack food.
How to get there
Mana Island is open to visitors 08:00 – 17:00. Access may be made only in the bay between the old woolshed and the wharf. Boats must remain offshore after visitors have landed. Two moorings, marked by red buoys, are available in the bay for daytime use. They are not available for overnight mooring. Dinghies may be pulled up the beach after transporting passengers from their boat.
Earthcache tasks
You have two main tasks for this earthcache. The first is to measure the gradient and direction of the dip of the Mana Surface (Q1 to Q4). The second is to identify one of the lower marine terraces on Mana Island (Q5). There is also the photo task (Q6), which is optional but encouraged.
- (Q1) Go to the published coordinates (WP1) at S 41° 04.919 E 174° 47.082. At this level, you are already well on the Mana Surface. From your GPS reading or topographic map, what is the elevation at WP1?
- (Q2) Go to WP2 at S 41° 04.739 E 174° 46.870. Here you are on the highpoint of Mana Island. From your GPS reading or topographic map, what is the elevation at WP2?
- (Q3) From your GPS compass or topographic map, what is the distance and bearing back to WP1?
- (Q4) Both WP1 and WP2 are on the Mana Surface. Verify that the surface is gently sloping down in the direction from WP2 to WP1 and beyond. From the measurements you made for Q1 to Q3, estimate (a) the gradient and (b) direction of dip. Give your answers in degrees.
- (Q5) On your walk up to or down from the Mana Surface you will have traversed one or more of the lower marine terraces on Mana Island shown in Figures 1 and 2 above. Select one such spot on the ground and (a) take a GPS mark of its coordinates, (b) identify which of A to G this terrace surface is, and (c) describe any evidence of the terrace on the ground at this spot.
- (Q6) (Optional) At WP2, take a photo showing the landform of the island, with your GPS clearly visible in the photo. Attach it to your online log.
Note: Your elevation readings from your GPS may well differ from the height marks and contour lines shown in topographical maps. Measurement of height with a GPS can be problematic. As well, your GPS is designed to give ellipsoidal height measurements based on the WGS84 Ellipsoid earth model, whereas topographical maps give orthometric height based on local survey measurements. The two may differ by about 10-20 metres on Mana Island. However, as long as you measure Q1 and Q2 consistently with your GPS, it should still be able to provide reasonable estimates for Q4. Do not mix GPS elevations with topographical map heights between Q1 and Q2.
In your answer, indicate whether you have based your height measurements on GPS readings or topographical maps. For bonus points, if you use both GPS and topographical maps for the height measurements at each of WP1 and WP2, then provide both sets of answers to Q1 to Q4, and comment on the differences between the two sets of results.
Earthcache logging requirements
To log this cache you must visit Mana Island and undertake the above earthcache activities. Once you have answered the above Questions 1 to 5, email them to the CO and go ahead and log your find. Do not include your answers in your log. The CO will check to see if your answers are reasonable. Dubious finds will be deleted.
You are encouraged to post your photo with your log.