Tekapo Glacier (McKenzie Country, Canterbury) EarthCache
Tekapo Glacier (McKenzie Country, Canterbury)
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This is an Earthcache. It is a "virtual cache" highlighting the geology of the area; there is no container at GZ. Instead, email the cache owners with your answers to the questions below.
This earthcache is at a viewpoint on the 30 minute Mt John Summit loop track. At the given coordinates there is an interpretive sign which names landmarks and describes how glaciers have shaped the landscape.
You will need to read the sign at GZ to be able to answer these questions:
1) Look at Lake Alexandrina, to your left. On the far side you can see [what?]. These were left behind [how?].
What was left behind, and how did they form?
2) Beyond Lake Alexandrina, you can see a line of [something?] halfway up the hills. This is [what?] that formed [how?].
What can you see, what is it, and how did it form?
Please email or message us your answers to the earthcache questions and log your find at the same time. If you cannot send the answers immediately then it would be better to log a note of your visit to GZ, and then change your note to a find log when you do send the answer email. Your log date should be the day you visited GZ. If your answers are incorrect or incomplete you will be asked to provide more details.
Walking or Driving to Mt John
To reach the summit of Mt John you can:
• Walk up from Tekapo township. This takes 2 – 3 hours for the round trip.
Map and information on walking tracks : Dept of Conservation Lake Tekapo Town Tracks
• Drive to the summit(9.45am - 3.30pm Thursday - Sunday). This costs $8 per car.
We recommend walking the entire Mt John summit loop track – it only takes 30 minutes, the views are stunning all the way around and there are other caches to find. However, you can also park (carefully) in a small pullover, on the left-hand side of the road going downhill. From here it is a 2 minute walk to the viewpoint.
Geological Background
Mt John, where you are standing, is made of greywacke scoured and shaped by successive ice ages.
Greywacke, a hard grey sandstone/mudstone, is New Zealand’s most common rock. It formed between 199.6 million and 271 million years ago from sediments accumulated in ocean basins off the Australian coast of Gondwana. During the Rangitata Orogeny (mountainbuilding period) 130-115 million years ago this new rock was uplifted and formed new land on the margins of Gondwana.
About 83 million years ago the opening of the Tasman Sea began to separate New Zealand from Gondwana. As the new country drifted to its present position, it cooled and sank almost completely underwater.
By 23 million years ago, a new plate boundary had formed between the Australian and Pacific plates. Here in the South Island, the boundary is the Alpine Fault. As the two tectonic plates collided, our sunken land was forced above sea level. During the Kaikoura Orogeny of the last 5 million years uplift became more rapid, creating the spectacular Southern Alps.
Overall, there has been 20 km of vertical uplift at the Alpine Fault; the mountains you see from Mt John are still rising at 5 – 10 mm each year.
Tectonic stresses also created other major fault lines. Here in the Mackenzie Plains the land between two fault lines has sunk, while the ranges on either side have been uplifted. This has formed the Mackenzie Basin. As the Southern Alps were pushed up at close to 1 cm a year, the mountains eroded very rapidly and river sediments washed down from the mountains filled in the basin.
Glaciation
Then came the Ice Ages, between 2.5 million years and 14 thousand years ago, when glaciers sculpted the landscape to what we see today.
During the Ice Ages in New Zealand there have been as many as 20 glacial advances, with nine South Island glaciations in the last 700,000 years. The rising Southern Alps developed their present pattern of ranges and valleys, shaped by advancing and retreating glaciers.
The most recent glacial period, the Last Glacial Maximum, was the Ōtira Glaciation between 75,000 and 14,000 years ago. The map here shows the glaciated area during this Last Glacial Maximum as a white overlay.
During the Last Glacial Maximum the Tekapo Glacier flowed over and beyond Mt John, shaping it into a roche moutonnee. The moving glacial ice also scraped out a depression in the bedrock of the valley 130 m deep.
About 18 - 20 thousand years ago the Tekapo Glacier reached its maximum extent where Tekapo township is now. As ice melted at the snout of the Tekapo Glacier, rock debris carried by the glacier was continually dumped at the forward edge. When the glacier eventually retreated, these dropped rocks formed the terminal moraine showing the furthest point the glacier had reached. Lateral moraines form the same way at the sides of the glacier, showing how high the glacier reached up the sides of the valley. After the last Ice age ended about 14,000 years ago, meltwater filled the glacial trough and formed a lake behind the terminal moraine damming the valley. Today, Tekapo township is built on this terminal moraine.
How a terminal moraine forms
After the Tekapo Glacier melted
When Lake Tekapo first formed, it was much larger than it is now. The ‘superlake Tekapo’ was about 30 m higher than it is today and Lake Alexandrina was at that time an arm of the enlarged lake. Then around 10,000 years ago there was a rapid retreat of the Tekapo Glacier and a rapid down-cutting of the lake outlet through the terminal moraine, so the level of Lake Tekapo dropped to where it is today. (Although, Lake Tekapo has been raised for hydroelectric storage; when full, Lake Tekapo is 10 m higher than its natural level.)
The GNS New Zealand Geology Webmap shows that the Tekapo Glacier has gone, but it can't be forgotten.
The map shows Mt John as a blue greywacke outcrop surrounded by yellow glacial deposits:
Blue = greywacke, 200 - 270 million year old
(Mt John and the mountain ranges)
Yellow = glacial deposits, 12,000 - 24,000 years old
White = river outwash, present - 24,000 years old
Sources & Further Information:
• Lake Tekapo walkways map and information: Download Tekapo walking map
• Department of Conservation track information: Lake Tekapo Town Tracks
• "Geology of the Aoraki Area" GNS Science 2007
• GNS New Zealand Geology Webmap: http://data.gns.cri.nz/geology/
• Geological Society of New Zealand Annual Conference 2003 Field Trip: Southern Alps Tectonics and Quaternary Geology
• General distribution and characteristics of active faults and folds in the Mackenzie District, South Canterbury GSN report 2010 http://ecan.govt.nz/publications/Reports/fault-report-mackenzie.pdf
• "Landforms of New Zealand" (2nd edition) Eds J M Soons & M J Selby, Longman Paul 1992
• The Geological History of New Zealand https://ncealevel2sci.wikispaces.com/Geology
• Wikipaedia Moraines
• Aoraki Mackenzie International Dark Sky Reserve
Additional Hints
(Decrypt)
Fhttrfgvba: Gnxr n cubgb bs gur fvta. Gung jvyy uryc lbh jvgu gur nafjref.