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Geology
Thousands of commuters use Mortimer Pass every day to get from the Southern Motorway's Gillies Ave offramp down into Newmarket, and yet few are aware of the lava rift cave they pass by, or it's geological significance.
While there are a number of locations around New Zealand where one can find caves within lava, Auckland is the only region that has lava tubes - all of the others around the country have been formed by erosion or collapse. Furthermore, of all of the lava caves in the Auckland region, the Mortimer Pass Cave is the only example of a rift cave.
The Mortimer Pass lava cave is located in a pahoehoe lava flow originating from the Mt Eden volcano, consisting of alkalic olivine basalt. It is a rift structure, or pull-apart cave, in a direction transverse to the lava flow direction and roughly parallel to the steep lava front just metres away. The cave presents an S shaped cross section and is easily accessed from Mortimer Pass.
The rift cave has formed during the cooling of the thick, viscous, pahoehoe lava flow. As the lava cooled and solidified, still flowing slowly towards what is now Newmarket, tension built within the flow. Eventually the tension in the flow became too great and the cavity tore open and solidified forming the thin S shaped cave that can be seen at the published coordinates.
The cave extends around 40m from Mortimer Pass under the historic Highwic grounds and residence. Small encrustations from within the cave have been identified as opal and calcite.
This Earthcache
In order to log this earthcache you will need to visit the published coordinates and discover for yourself this rather special cave. You will need to perform the following tasks:
1. Measure the height of the opening, from the grass level beside the road, and the width of the opening at its widest point.
2. Describe, in your own words, the appearance of the rock in which the cave is situated.
3. Using your GPS unit, please take a reading of the elevation from sea level of the cave opening.
4. Take a photo of the cliff face including cave entrance, or into the cave itself. You must include your GPS unit in this photo.
The measurements and descriptions required must be emailed to the cache owner. The photo should be uploaded with your online log.
So long as you meet the requirements above you can visit this earthcache any time of day or night.
You can log this cache straight away after you have emailed your answers to the cache owner, no need to wait for confirmation. Please include the name of this earthcache in the email - you'd be surprised how many people forget. Also, when contacting us with answers, if you want a reply, please include your email address. Any problems with your answers we'll be in touch.
Happy Earthcaching!
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