Agates: A Blast from the Past

In February, 2022 the Ashburton Museum team was hard at work developing a new exhibition on agates with the help of Malcolm Luxton from The Agate Orphanage in Tinwald. Agates are categorised as a form of chalcedony, which is a type of quartz comprised of microscopic crystals. These precious formations take shape over the course of about fifty million years and can eventually fall into the hands of a very lucky (or determined) collector. But how exactly do agates form and what can they teach us?

A long time in the making

The Ashburton District is considered one of the most promising areas for finding agates in New Zealand. How did we gain this reputation?

Rakaia River, 2007. Photo reference 05.2014.1108.

The process of agate formation begins when a volcanic eruption occurs. It just so happens that our district is bounded to the west by the great and mighty Southern Alps. These snow-topped peaks we see looming over our little slice of New Zealand were not always such a fixed feature. If we were to step into a hypothetical time machine and travel hundreds of millions of years into the past, we would find ourselves among a totally different landscape.

Around 540 million years ago, the land which became Aotearoa New Zealand was part of an ancient supercontinent known as Gondwana. The modern countries and continents that made up this huge land mass included Australia, the Indian subcontinent, Antarctica, Africa, South America and little old New Zealand. Oh how small our wee future islands must have felt as they clung tightly to Australia and Antarctica, essentially nothing more than a rocky outcrop.

Gondwanan rivers deposited sediments into the sea, and offshore volcanoes deposited ash on the sea floor. What resulted was a hardened, sunken layer of rocks through which mountains sprang forth, eventuating in the infancy of our islands. Between 100 and 85 million years ago, Zealandia broke off from Gondwana and drifted into the Pacific like a teenager leaving home for the first time. After all, could you blame it? If your family were in the habit of causing massive volcanic eruptions and earthquakes, you would leave too.

Fast-forward another 60 million years, and this new large land mass known as Zealandia split apart and was pushed upwards by tectonic action caused by the Australian and Pacific plates. Aotearoa as we know it was almost there.

Right place, right time

The last 1.8 million years have seen the Southern Alps rise thousands of metres, and volcanic eruptions have contributed to our islands’ transformation. Somewhere between this point and the initial formation of New Zealand, agates came into the picture.

As volcanic lava cooled and solidified from the outside-in, bubbles of gas and liquids were trapped inside the igneous rock. These cavaties, and those created by other geological events, lead to the formation of agates when silica and other minerals find their way inside. Layers upon layers of these deposits build up inside these hollows, and millions of years later, you have an agate!

90 million years ago, when New Zealand was still part of Gondwana, a series of powerful eruptions occurred in the Mid Canterbury foothills known as the Mt Somers Volcanics (lava flows). Molten rock pushed up through the region’s agate-producing rocks, predominantly andesite, melting and absorbing their distinctive minerals. These eruptions can account for much of our region’s geology.

Early photo of the Ashburton Tramping Club taking a break at the Petrified Gully, Mt. Somers, 1930s. Photo reference 03.1985.0932.

Agates can be found across our district (if you know what to look for), thanks to the flow of rivers that brought agates down from the mountains and scattered them around our hills, riverbeds and seashores. In the Ashburton District alone, agates can be found in areas such as:

* The Ashburton, Rangitata, Hinds and Rakaia rivers

* Mount Somers, Alford and Barossa areas

* Wakanui and Ashton beach

Agates are not exclusive to these places, they can be found in all sorts of other nooks and niches too. It should be noted that most of the areas where agates are found are protected either by private ownership or the Department of Conservation, but every now and then luck can be found in publicly accessible areas by the rivers and beaches.

By Connor Lysaght

Unless otherwise stated, photographs and research materials on this page are owned by the Ashburton Museum & Historical Society Inc. This post was modified for this blog and was originally published in the Ashburton Guardian, 26 February 2022.


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