Banks Peninsula Geology (Wikipedia)
Banks Peninsula forms the most prominent volcanic feature of the South Island. Geologically, the peninsula comprises the eroded remnants of two large composite shield volcanoes (Lyttelton formed first, then Akaroa), and the smaller Mt Herbert Volcanic Group.These formed due to intraplate volcanism between approximately eleven and eight million years ago (Miocene) on a continental crust.
The peninsula formed as offshore islands, with the volcanoes reaching to about 1,500 m above sea level. Two dominant craters formed Lyttelton and Akaroa Harbours.
The Canterbury Plains formed from the erosion of the Southern Alps (an extensive and high mountain range caused by the meeting of the Indo-Australian and Pacific tectonic plates) and from the alluvial fans created by large braided rivers. These plains reach their widest point where they meet the hilly sub-region of Banks Peninsula. A layer of loess, a rather unstable fine silt deposited by the foehn winds which bluster across the plains, covers the northern and western flanks of the peninsula. The portion of crater rim lying between Lyttelton Harbour and Christchurch city forms the Port Hills.
Lakes Waihora and Wairewa
Lake Ellesmere / Te Waihora is located in the Canterbury Region of the South Island of New Zealand. It is actually a broad, shallow lagoon located directly to the west of Banks Peninsula, separated from the Pacific Ocean by a long narrow sandy spit called Kaitorete Spit, or more correctly Kaitorete Barrier. It lies partially in extreme southeastern Selwyn District and partially in the southwestern extension of the former Banks Peninsula District, which now (since 2006) is a ward in the city of Christchurch. The lake holds high historical and cultural significance to the indigenous Māori population and the traditional Māori name Te Waihora, means spreading waters. It has officially had a dual English/Māori name since at least 1938.
Lake Forsyth (known to Māori as Te Roto o Wairewa) is a lake in the Canterbury region of New Zealand. It is fed by the Takiritawai River, a short arm of the Okuti River, and exits via a gravel bank into the sea in the vicinity of the small community of Birdling's Flat. State Highway 75 leading to Akaroa and part of the Little River Rail Trail follow the north-western shore of the lake.
Wairewa was important for providing tuna and eels as food for the Ngai Tahu tribe (indigenous Maori people of South Island). It is the only Ngai Tahu customary lake. Wairewa Runanga one of the 18 Ngai Tahu Runanga are the guardians or kaitiaki of the lake. However deforestation of the surrounding hills has led to erosion and silting up of the lake.
The lake is hypertrophic, leading to eutrophication with corresponding poor water quality.This decline in water quality has been known since the early 1900s. So far, attempts of opening up the lake to the ocean to rectify the problem have had limited success. In 2016, after a spell of dry weather, the water quality deteriorated and recurring algal blooms made the water toxic. Animals, including pets and sheep, died after drinking the water.
Questions:
1. At the waypoint co-ordinates listed part way to the Summit Hill trig you will encounter a prominent rock outcrop. What do you think created this, and what sort of rock is it?
2. Looking south you will see two large waterbodies as described- Lakes Wairewa and Lake Waihora. The Kaitorete Spit blocks their drainage into the Pacific ocean. How do you think this spit has formed? Where have the materials come from to create the spit?
3. (Optional) At the GZ cords (Saddle Hill Trig) you will find a commanding view of the Akaroa Harbour, the eroded remnants of one of the three main volcanism sites in Banks Peninsula history. If you can see the Harbour below (subject to weather) take a photo of the remnant basin with your GPS/Mobile in shot