Accessing the Earthcache
This earthcache can be accessed by parking at the Bowentown lookout car park on Seaforth Road. From the Bowentown Lookout, you will need to walk back along the road to the trailhead and then follow the path down to the beach. NOTE: The earthcache is accessible at high tide though should not be attempted during stormy weather. Be aware of the dangers of falling rocks near the cliffs.
Please do not try access the earthcache through the Bowentown Beach Holiday Park as this is for guests only.
For the practical tasks, you will need:
1. A magnet (as a geocacher, I'm sure you have a magnetic probe on your person at all times).
2. Some form of magnifying glass (a geological lens is perfect but a mobile phone will do at a pinch).
The Geology of Bowentown
This site is geologically interesting in several ways. Bowentown Heads itself consists of two rhyolite domes, a smaller one to the south-west and this larger one to the north-east. These erupted between 2.8 and 2.5 million years ago during the Pleistocene geological epoch, originally foming volcanic islands. They were originally classed as the southern-most, and most recent, part of the Coromandel Volcanic Zone, which was active between 18 and 4 million years ago but chemical similarities in the lava now suggest they are part of the Minden Rhyolite Subgroup, which also includes Mauao / Mount Maunganui. Therefore, they are now, tentatively, seen as the earliest, and most northerly aspect of the Tauranga Volcanic Zone, which was active from about 2.95 to 1.53 million years ago, the most recent eruption being Minden Peak. This slow, south-wards movement of volcanic activity is due to the movement of the boundary between the Indo-Australian and Pacific tectonic plates and means that current activity is centred on the Taupo Volcanic Zone.
The ocean has eroded most of the northern half of the larger dome, allowing the interior structure of the rhyolite dome to be examined. This earthcache will examine the effects of that erosion. Rhyolite is an acidic volcanic rock, with a high content of light minerals such as silica and orthoclase. Due to the high silica content, rhyolite lava is very viscous. It flows slowly, like tooth paste squeezed out of a tube, and tends to pile up and form lava domes. If rhyolite magma is gas rich it can erupt explosively, forming a frothy solidified magma called pumice (a very lightweight, light-coloured, vesicular form of rhyolite) along with ash deposits, and / or ignimbrite. In certain situations extremely porous rhyolite lava flows may develop. The extreme porosity of such flows allows degassing and subsequent collapse of the flow, forming obsidian (dark coloured volcanic glass). In Māori, this is known as Tūhua. As lava flows erode (or even shatter if they contact cold water when hot), they produce black volcanic sand. These contain minerals such as magnetite and so are often magnetic.
When sea-levels stabilised at their current levels during the Holocene, around 6,500 years ago, sand was deposited between the domes and behind along the coastline to form a tombolo, or sand spit, that cut off the ocean, forming the northern entrance to Tauranga Harbour. A similar tombolo and tied island, Mauao, form the southern entrance. On a clear day, Mauao can easily be seen from the lookout, emphasing the 40km length of Tauranga Harbour. A further sand spit, built on a Pleistocene basalt ridge, forms Matakana Island, seen across the harbour channel from the Bowentown lookout. Inside the harbour, limited tidal flow has allowed many shallow sand banks to form, as can be easily seen at low tide. Interestingly, since around 2020, Tauranga Harbour seems to have become a nursery for Great White Sharks with at least 6 adults and numerous juveniles and sub-adults observed over the last few years.
Tūhua / Mayor Island
From GZ, you will get an excellent view of Tūhua / Mayor Island. Inhabitants of Tauranga are very familar with its silhouette on the horizon - I, for example, see it every day on my drive to work - but few realise that it is a very unique volcano. Lying 35 km north of Tauranga, Mayor Island is the smallest caldera volcano in New Zealand. The 4km wide island represents the top of a dormant 750m high and 15km wide volcano, is dominated by a 3km wide caldera collapse crater and contains numerous vents.
There have been at least 52 eruptions of different types over the last 130,000 years, including phreatic (produced by exploding steam), plinian (explosive eruptions producing ash columns extending many kilometres into the stratosphere) and pyroclastic flow (dense, fast-moving flow of solidified lava pieces, volcanic ash, and hot gases). The caldera was formed as a result of 2 or 3 separate eruptions, ending with a Plinian eruption around 6,300 years ago (5060BCE) which produced a 70cm thick ash layer (tephra) on the mainland. The most recent eruption after this was probably 500-1,000 years ago.
So, why is Tūhua unique? Firstly, it is a shield volcano (a low, wide volcano like Rangitoto, rather than the classic steep stratovolcanic cone typified by Taranaki or Fujiyama). Shield volcanoes are almost always formed from low-viscosity ('"runny") basaltic lava rather than the much more viscous ("sticky") rhyolitic lava. Rhyolitic lava usual results in the formation of domes, such as Bowentown Heads, or calderas, such as Rotorua and Taupo. The recent caldera formation on Tūhua is much more typical of rhyolitic lava.
Secondly, Tūhua is chemically unlike any other volcano in New Zealand. It's rhyolitic lava is peralkaline, meaning that minerals such as sodium oxide (Na2O) and potassium oxide (K2O) dominate rather than aluminium oxide (Al2O3). This is in stark contrast to the rocks at Bowentown Heads, which are acidic rather than alkaline. The nearest match, chemically, are the volcanoes of the Afar region in the Ethiopian Rift Valley in Africa, where the tectonic plates are pulling apart, forming a new continent. The answer may be that Tūhua lies on the very edge of the continental shelf. The ocean between Tūhua and Bowentown is about 75m deep but, beyond the island, it plunges quickly to 400-500m and continues to over 3,000m deep. This means that Tūhua is at the western end of a deep underwater structure called the Ngatoro Basin. This is a graben, a depression caused by the faulting and pulling apart of vast blocks of crust, causing the area in the middle to sink. (The Waikato is an onshore example of a graben, as can be seen as you drive over the Kaimais on SH29).
On a final note, it has been estimated that the worse-case scenario of an eruption of Tūhua could produce a 35m high tsunami hitting Tauranga and Mount Maunganui with only 30 minutes warning, causing total destruction of these low-lying coastal resorts. Bowentown, of course, would have almost no warning. Enjoy your earthcaching!
Earthcache Tasks
Theory
Firstly, just to make sure you have been paying attention (and have not just skipped down to this bit), some questions, based on the information above.
T1. The Bowentown volcano lies on the boundary between which TWO volcanic zones?
T2. Of what type of rock is the Bowentown volcano made and why does this form domes?
T3. Explain how a tombolo forms.
T4. What must happen to rhyolitic lava before it can form obsidian (tūhua)?
T5. What TWO features make Tūhua unique amongst New Zealand volcanoes?
Practical
At the Bowentown Lookout waypoint:
P1. Observe the general surroundings. Identify Mauao, Matakana Island, Tauranga Harbour and the andestic Kaimai Ranges. (No answer is required for this task).
At the trailhead waypoint:
P2. Observe the Waihi Beach tombolo and Athenree lagoon.(No answer is required for this task).
At the Earthcache:
E1. Find a sample of white sand. Pick it up and run it through your fingers. How does it feel? Test the sand with a magnet. What happens? Examine the sample under a lens. How big are the crystals?
E2. Find a sample of black sand. Pick it up and run it through your fingers. How does it feel? What size are the grains? Test it with a magnet. What happens? Examine the sample under a lens. How big are the crystals?
E3. On the rock face, find these two features. Give your thoughts on how they formed.
 |
 |
| Feature 1 - Southern face of cliff |
Feature 2 - Western face of cliff |
E4. On a fallen block, identify the different layers and estimate the thickness of the thickest and thinnest layers. By examining their fine structure, can you suggest how they may have formed?
E5. Some layers on the fallen blocks are reddish in colour. Examine them closely. What do you notice? How do you think they might have formed?
E6. Please take a photograph of yourself (or your GPS) at GZ and post it with your log.
TO CLAIM THIS EARTHCACHE, PLEASE E-MAIL YOUR ANSWERS TO BOTH THE THEORETICAL AND PRACTICAL TASKS TO THE CACHE OWNER. YOU MAY THEN LOG THE FIND STRAIGHT AWAY (although logs without answers after a few days may be deleted). PLEASE DO NOT POST THE ANSWERS IN YOUR LOG. HOWEVER, YOUR LOG SHOULD INCLUDE YOUR PHOTO AT GZ.
Bibliography
New Zealand geology: An Illustrated Guide - Geoscience Society of New Zealand
Geology of the Tauranga Area - Briggs, Hall et al
University of Auckland Geology Department website
Tectonic Setting of Mayor Island - Cole
Pyroclastic deposits and volcanic history of Mayor Island - Buck, Briggs et al.
Integrating LiDAR to unravel the volcanic architecture and eruptive history of the peralkaline Tūhua (Mayor Island) volcano - Kosik et al.
This Youtube video covers the formation of Tūhua / Mayor Island in more detail.