North Island Volcanics
Volcanoes 
once dominated the rugged Coromandel skyline but today only their wind and rain-eroded remains can be seen thrusting up out of the dense forest. These volcanoes were part of a migrating volcanic arc that developed about 20 million years ago forming first the now extinct formations of the Coromandel peninsula and moving on to its present zone of activity in the Taupo volcanic region.
This volcanic arc is the result of differential movement along the tectonic plate boundary that New Zealand sits upon. The Pacific plate to the east is slowly being forced under the Indo-Ausatralian plate in a process known as subduction. Once the Pacific plate reaches a critical 
depth of about 85 kilometres, the crustal rock melts and rises under immense pressure to the surface as magma, exploiting fissures and weaknesses in the Indo-Australian plate, forming volcanoes, lava flows and calderas (giant explosion depressions).
Continual activity associated with this volcanic carc slowly migrated 80 kms from north to south along the Coromandel Peninsula from 18 million to 4 million years ago. The volcanic arc has continued to move further south and now lies beneath, and is the cause for, the volcanic activity in the Taupo Volcanic Zone (from White Island through to Ruapehu).
Coromandel Geology
The Coromandel volcanic formations are dominated by two types of eruptions and volcanic rock. The first eruptions were mostly of andesites and dacites - these molten rocks thrust up through cracks in the existing basement rock to build volcanic cones (like Ruapehu) made of alternating viscous lava flows and ash or debris.
Later, shallower, less dense rhyolites often erupted explosively forming Rotorua-like domes, caldera and ash-flow ignimbrite fields as well as spines. Table Mountain, the unmistakeable flat topped peak at the head of the Kauarenga Valley, is the remains of andesite lava flows that filled a large valley cut in the ignimbrites, and since exposed by erosion.
Both types of eruptions occured simultaneously in some places, evidenced by the discovery of overlapping andesitic and rhyolitic material.
Super heated water rose up through fissures in geothermal areas as a result of the volcanic activity to create the rich mineral deposits of gold and silver that formed the Hauraki Goldfields.
Kapowai Caldera
The geology of the area in the vicinity of the headwaters of the Kauarenga Valley is dominated by the Kapowai Caldera - a large silicic volcanic collapse centre. Multiple massive eruptions deposited thick sheets of pumiceous ignimbrite around 8 million years ago.
This activity was followed by renewed andesitic volcanism on the flanks of the caldera (for example, forming Table Mountain), the area east of SH26, and between Tairua and Hot Water Beach. Extensive rhyolite lava flows erupted towards Tairua from fissure vents on the Ruahine-Tanehua ridge, and rhyolite domes erupted through the ignimbrites within the caldera.
Pumiceous and carbonaceous sediments within the caldera centre indicate that a lake formed briefly after the initial ignimbrite eruptions and collapse of the caldera.
During the next 2 million years, an extensive drainage system eroded and carved out valleys to the north towards Whitianga. At around 5 million years ago, eruptions of rhyolite gave rise to domes and flows on the northern rim of the Kapowai Caldera. The Whenuakite dome, of the same age, has a distinctive crater, lava flows through a breach in the crater rim, and a tholoid (lava plug) within the crater.
The Kapowai mining area is unique in the Coromandel Peninsula for being the only proven gold-bearing mineralisation associated with an ancient caldera. The Big Beetle, Kapowai and Welcome Jack mines exploited quartz pipes and veins within both a rhyolite dome and the surrounding ignimbrite.
This Earthcache
To claim a find on this earthcache, please complete the tasks outlined below. Answers to the questions should be emailed to the cache owner and not mentioned in your online log.
1. According to the information panel at WP1, which overlooks Tauranikau, the volcanic activity on the Coromandel Peninsula deposited ash and pumice layers up to how many metres thick?
2. Also from the information panel at WP1, around how many years ago did volcanic activity cease in this area?
3. What are Tauranikau and The Pinnacles (published coordinates) examples of, and about how long ago did they form?
4. At the published coordinates you will be standing on the lookout platform at the summit of The Pinnacles. Having seen Tauranikau on the way up, please compare the shapes of these formations and if they are different, suggest how the original vents/fissures would have differed when they were originally formed.
5. Take an altitude reading from your GPS unit at the published coordinates.
6. (Optional) Please take a photo of Tauranikau, The Pinnacles, Table Mountain or the view from the Pinnacles, and include your GPS and/or yourself in the photo if possible - upload this with your online log. While this is optional, we'd really appreciate seeing your photos. Who would visit The Pinnacles without their camera anyway, right?!
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!!!
NZFlossy & NZBiscuit