Lake Alta Schist (Queenstown, Otago) EarthCache
Hidden : 3/9/2014
Difficulty:
4 out of 5
Terrain:
3.5 out of 5

Size: Size: other (other)

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Geocache Description:

Visit Lake Alta in the Remarkables during summertime and enjoy the wonderful alpine scenery! This cache can only be completed when the sign and rocks at GZ are visible, i.e. not during winter. NOTE: You MUST visit TWO waypoints and answer all four questions to log this cache. It is Difficulty 4, so expect to spend some time thinking about the answers. Your log date should be the day you visited GZ.

There is no physical cache at the location. Instead, you will work out the Textural Zone of the schist rocks at Lake Alta (in a simplified way suitable for us laypeople!) to gain this earthcache smiley. You are also asked to walk 200 m further along the lakeside and describe an interesting stone structure beside the track.

Drive up the winding 13 km Remarkables road (a 30 minute drive up) and park at the road end carparking. Then it is a 1 km walk up to Lake Alta - an easy 1 hour 30 minute return trip including caching time. See the Department of Conservation track information. From the skifield base building, follow the signposted access road to the top of the skifield. From there, the Lake Alta track is marked with yellow pegs and stepping stones across lovely alpine wetlands. You’ll disturb grasshoppers at every step – how do they survive winter up here when it is covered in snow?

BE PREPARED:The terrain rating given is for a fine summer day. In bad weather terrain rating can be extreme.
This is an alpine environment and the weather can change rapidly and severely at any time of year. We were snowed on close to Christmas- that is normal here! As well as wearing your sunhat, sunscreen and sunglasses ALWAYS carry warm and waterproof clothing.

The earthcache coordinates are at a viewpoint between Lake Alta and the skifield, where you will see an interpretive sign and many large schist boulders. Answer Questions 1 and 2 here. Then follow the track another two hundred metres down to the lake and along the lakeshore to the waypoint. Beside the track you’ll see a stone structure to answer Question 4.

If you are properly equipped and conditions are suitable, continuing up the route towards Wye Saddle cache will give you a nice view down on Lake Alta.


Earthcache Questions

To complete the earthcache, answer the following questions. Email your answers to us at the same time as you log your find. If your answers are insufficient, we will contact you for more information. Photos are very welcome, but please ensure there are no spoilers in your log.

1. From the sign at GZ, Lake Alta is a perfect example of … ?
Use the technical description as given on the sign.

2. Look at the schist boulders around GZ. In your email, describe the appearance of the rocks.
Include all the following details:
- what is the colour of the rock?
- does the rock have a lustre, i.e. are there shiny mica minerals in the schist?
- does the rock show a tendency to split into sheets (cleavage) along the layering of the rock?
- are there visible layers of different minerals in the schist? If so, describe the colour, thickness and shape of the layering.

3. Look at the photos showing examples of textural zone I, II and III schists, and read the information on the page. (Larger photos of the textural zones are attached below.)
Based on your description of the rocks at GZ, what is the textural zone of the schist rocks at Lake Alta?

            Textural Zone I schist               Textural Zone II schist                Textural Zone III schist


TZ1 schist: Caples greywacke, without any cleavage
TZII schist: Caples semischist with strong cleavage (splitting along layering)
TZIII schist: Caples schist showing cleavage and intense veining
Photos: P J Forsyth & I M Turnbull

4. Follow the track a hundred metres down towards the lake and then a hundred metres along the shore. You will see a stone construction on the left-hand side of the track.
What is this structure, and how has schist has been used to build it?

Lake Alta

Like much of the Wakatipu landscape, Lake Alta was formed by glaciation. During the Ice Ages, the head of a glacier here plucked out rocks from the surrounding cliffs, forming the classic steep cliffs and semi-circular cirque basin. When the ice finally melted 14,000 years ago, water filled the bowl-shaped depression left behind. In winter, divers hold an annual ice dive in the lake - brr!

Lake Alta is (yet another) Lord of the Rings location! The Rastus Burn where it leaves the lake became the River Silverlode. After escaping from the Mines of Moria without Gandalf, Aragorn led the Fellowship across the Silverlode and down the steep slopes of Dimrill Dale towards Lothlorien.

Haast schist

Haast schist is the basement rock of Otago.

It began as sand and silt deposited off the Australian coast of the ancient continent Gondwana from about 300 million years ago. As the Paleo-Pacific plate moved westward and dived under the Gondwana plate, these sediments were scraped off and gathered together in an “accretionary prism”.

About 200 million years ago this wedge of sediments became buried deep in the oceanic trench. Over 80 million years sediments progressively changed under heat and pressure (metamorphosed) into schist rock.

As temperatures and pressures increased, various minerals in the rock became unstable and formed new minerals. The deeper the rocks got buried, the more altered they became. Dull grainy greywacke sandstone changed into shiny, splitting, semischist and eventually into quartz-veined, high-grade schist.

A mountain-building period followed when the rocks were uplifted and eroded. By about 110 million years ago the once deeply buried schist was exposed at the earth’s surface. Erosion continued while New Zealand parted company from Gondwana and drifted to its present position.

With the initiation of the Alpine Fault some 25 million years ago, mountain building began again. To the present day there’s been about 20 km of uplift and almost as much erosion. As a result, across Otago we can see the whole range of grades of schist rock, from greywacke and moderately metamorphosed semischist through to high-grade schist that was deeply buried and thoroughly recrystallised.

The map showing the location of Haast schist in the South Island also shows how horizontal movement along the Alpine Fault has displaced part of the schist 450 km northwards to Marlborough.

Textural Zones

In the field, geologists use “Textural Zones” using the Roman numerals I to IV as a quick way of classifying schist rocks by appearance. With progressive metamorphism from greywacke to semi-schist to high-grade schist, there is an increasing tendency of the rock to split and an increase in quartz veining. (Note that Textural Zones are independent of “metamorphic facies” i.e. they don't necessarily indicate which minerals are found in the different rocks.)

As we all know, the deeper inside the earth you go the hotter it is. As temperatures and pressures increase, various minerals within the rock progressively become unstable and reform into new minerals.


Initially, the buried sediments are compressed into rock - a hard, resistant sandstone called greywacke (from German meaning 'grey rock'). This is the most common rock in New Zealand, forming much of the Southern Alps. [TZ I schist]


At about 8 km underground, shiny mica minerals start forming – first green chlorite, silvery muscovite and, at higher temperatures, biotite. These micas have flat, plate-shaped crystals which make the rock appear shiny. Flat mica crystals ‘line up’ in layers and cause the schist to split into characteristic flat sheets (cleavage). This means schist is easily split for building stone, but also makes it prone to landsliding. [TZ II schist]


At even greater depths, and higher heat and pressure, quartz is ‘sweated’ out of the rock to form white layers in the schist. [TZ III schist]




You’ll notice that in this earthcache only photos of TZ I, TZ II and TZ III are included. This is because the Caples terrane rocks at Lake Alta come from volcanic sediments, which are lower in quartz, and therefore they never develop the very thick quartz veins characteristic of TZ IV schist.

The rest of the writeup is optional, for interest only -

"Hat, boots, pack and hammer...I didn’t have anything else on..."

Schist textural zones were first recognised in the early 1930s. Frank Turner was on a geology trip along the Haast River in West Otago. (Yes, it's called the Haast schist because it was first described at Haast!). It began to rain and the group had to walk out quickly before the river flooded.

Frank recalled “I suppose we looked a bit curious because it was pouring with rain and we had so many creeks to cross, I did the trip in my hat and boots, I didn’t have anything else on – hat, boots, pack and hammer. But we made it... [I was] collecting rocks, grabbing them where I could, sticking them in my pack and getting on before this blasted river could catch us down the end of the trail... When I came back I looked at these [rock specimens]... and could see that the grade of metamorphism was rising as you go down the Haast”.

The use of textural zones in mapping schists was later refined, most recently in 2001. A very simplified version for us laypeople has been used in this cache. For the proper scientific details download the full article from New Zealand Journal of Geology & Geophysics, 2001, Vol. 44: 171-183. Similar rocks around the world are also classified using our New Zealand system.

Metamorphic facies

In the lab, geologists classify different metamorphic grades of schist from the minerals identified under a microscope.

Characteristic groups of minerals formed by metamorphosis under similar pressures and temperatures are called metamorphic facies. The facies are often named after the main minerals found in the rocks. The actual minerals produced during metamorphosis depend on the chemistry of the original rocks.

Otago schist metamorphism is an example of Barrovian metamorphism, which is the usual metamorphic sequence associated with mountain-building.





Otago schist grades (shown in map at left) progress from slightly-metamorphosed zeolite facies, then prehnite-pumpellyite facies formed at around 250°C, through pumpellyite-actinolite facies (described by Turner; not in diagram above), to chlorite (greenschist) facies, and finally to garnet-biotite-albite (amphibolite) facies, where the garnet and biotite minerals were formed at depths of perhaps 30 km and temperatures of at least 380°C.

Key to metamorphic zones:
pp: prehnite-pumpellyite
pa: pumpellyite-actinolite
chl: chlorite
gt-bi-ab: garnet-biotite-albite



Information and illustrations from:
- A Continent on the Move: NZ Geoscience into the 21st Century (2008) editor Ian J Graham
- The Reed Field Guide to NZ Geology (2003) Jocelyn Thornton
- I. M. Turnbull , N. Mortimer & D. Craw (2001) Textural zones in the Haast Schist—a reappraisal, New Zealand Journal of Geology and Geophysics, 44:1, 171-183
- GNS: Science Topics NZ Geology & GNS: The Geology of NZ
- James Madison University, Virginia, USA Barrovian metamorphism
- Wikipedia Metamorphic facies
- Geology of the Dunedin Area- GNS Science (1996) compilers D G Bishop & I M Turnbull
- Geology of the Wakatipu Area- GNS Science (2000) compiler I M Turnbull download: www.gns.cri.nz/content/download/4190/23154/file/wakatipu_‎
- The Lord of the Rings Location Guidebook (2003) Ian Brodie

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