The Coromandel Peninsula EarthCache

The Coromandel Peninsula has played an important role in the history of New Zealand. A small fraction of the rock in the Coromandel has been responsible for most of the modern human settlement in the area - gold was found in the 1800's.

The foundations of the peninsula were laid down 150 million years ago. Sandy sediment, which had been accumulating undisturbed on the floor of the sea, was uplifted and deformed by colossal forces. New Zealand’s North Island is a convergence zone, where the plate of the floor underlying the Pacific Ocean collides with the crustal mass that includes Australia and New Zealand. The Pacific plate, some 7-10 km thick, descends under the Australian plate in this part of New Zealand, scraping off some of the sedimentary rocks that have formed on it as it does so. This sediment has been converted over millions of years into the sandstone rock, greywacke.

The Pacific plate and its remaining sediments warmed up as they descended into the lithosphere. The heat tens of kilometers below the surface melted the lighter minerals in the Pacific plate, causing them to rise towards the surface. The magma erupted through the greywacke about 18 million years ago. An arc of volcanoes arose from Great Barrier Island southward to Te Puke. The eruptions created a type of lava called andesite. This rock is typical of subduction zones and is named for the Andes Mountains. The andesite eruptions covered the greywacke with lava, ash and debris.

Approximately 6 million years ago geothermal activity was prominent in the Coromandel, in a manner similar to the Rotorua area today. The current theory is that superheated solutions from below filled cracks in the greywacke and lava formations, depositing quartz, gold and silver.

The Hauraki gulf also defines the Western edge of the Coromandel. It is a rift valley or graben. It was created about 2-3 million years ago when a huge block of continental crust dropped downward along faults running Northwest to Southeast. The depression was in the range of 500 to 2000 meters. It created a long deep valley which received the outflow of the Waihou and Piako rivers (and the ancient Waikato River for several periods). The bottom filled up with sediments from these sources, creating the flat Hauraki plain which extends inland as far as Te Aroha and Morrinsville.

The Firth of Thames is the part of the plain that has been flooded by the ocean. The flooding has also isolated several highlands in the northern peninsula, creating offshore islands.

The rhyolite can be seen in lava flows on the East coast of the Coromandel near Cathedral Cove.

The greywacke can be seen in only a few areas of the Coromandel now. The peninsula rose at a slight angle, being lower in the Southeast. Erosion of the surface rock reveals the greywacke more in the North west, where it is closer to the surface. Outcrops can be seen along the coast north of Tapu and Cape Colville. This rock is typically shows folding as a result of the distortion caused by the uplifting process.

Submit your answers to the following questions to the cache owner and include a photo of yourself in front of the mine entrance in your log. Finds that are not supported by correct answers and a photo will be deleted.
1. Describe the character of the rock outcrop at the main coordinates. What are its colour, texture and hardness? Why does it have these features? (GZ is a few meters away from the entrance to the attraction and can be seen from the sidewalk).
2. Under which natural feature was the Shotover Reef discovered?
3. How much bullion did the Waiotahi and Caledonian mines respectively produce?