To date, the natural history noticings of this blog have focused primarily on the living world. There’s a lot more to natural history than the daily biotic frenzy, though. The backdrop of it all— the rocks, the minerals, the geology— might not be alive, but is certainly just as active. This is especially true in New Zealand, home to earthquake hotspots and volcanoes and the world’s fastest-growing mountain range.
All of this has been particularly on my mind since I spent last weekend scrambling around the rocky cliffs of Mt. Owen, in Kahurangi National Park. In this sort of landscape, it’s impossible not to wonder what the rocks are doing and how they got to be that way– so on returning to Christchurch, I spent a bit of time learning about Mt. Owen, and New Zealand geology in general. Where did this land come from?
To tell this story right would take an impossibly long time. The living world mostly takes place on an immediate time scale. The time periods that count are days, seasons, lifespans, even generations. That’s nothing compared to the time periods covered by geology, where processes stretch out over centuries and millennia and millions of years.
But I’ll try to tell a brief history of New Zealand geology. Mostly, it’s the story of the continent Zealandia. Have you heard of it? I hadn’t. Between the name, and the fact that it’s so often overlooked— I’m totally charmed by this continent. I just feel like it’s got a lotta personality.
So what’s the deal with Zealandia? A long time ago (the inconceivable length of 120 million years ago, to be precise), most of the Southern Hemisphere’s landmass was joined into one supercontinent called Gondwana. Antarctica, Australia, even current-day India— everyone was in there. Over millions of years, large chunks of land began to break off from Gondwana and move out into the Pacific Ocean. Around 85 million years ago, Zealandia made its own congé (side note: just learned that word and I love it). Independent rebel that it is, Zealandia was not content just breaking off from the mother continent— nope, to complete its isolation from the rest of the world’s landmass, it sank beneath the ocean, and stayed there for many millions of years. The continent wouldn’t stay suboceanic forever, though. Zealandia is situated across two tectonic plates, the Pacific and the Australian. That’s not a static place to be. As these plates ground against each other, parts of Zealandia crunched together and were forced back above sea-level. The land we call New Zealand is the portion of Zealandia that emerges above sea-level— it’s like the tip of a continental iceberg.
The meeting of the Pacific and Australian plates is still an active border today. You can see the fault line with a glance at a topographical map. It’s marked by a range of mountains— the Southern Alps— that rise jagged and steep in a diagonal line across the South Island. The tectonic plate grinding continues even now, and as a result this mountain range is actually still growing, and growing faster than any other in the world.
Mt. Owen is one peak of many in the Southern Alps. It lies to the west of the faultline, and it’s made up mostly of mudstone and limestone. There’s granite in there, too, no surprise– that forms the mineral base for most of today’s continents. Mudstone and limestone are rocks that are formed of oceanic sediment, the build up of calcium from many billions (trillions? quadrillions?) of dead sea organisms. It’s a reminder of Zealandia’s once fully-aquatic existence. Now, these sedimentary rocks find themselves above the sea (1875 meters above the sea, to be precise). The shift from marine to terrestrial existence exposes them to a whole new set of rock-shifting forces.
The most notable of these terrestrial forces is rain. Limestone is a water-soluble rock, particularly if the water is slightly acidic (and rainwater often is). When rain hits the mountain and flows downhill, it dissolves some of the rock with it. Small cracks formed by drips and drops of rain eventually become larger gullies and sinkholes, until the limestone has been channeled into a vast drainage system. This drainage system cuts above and below the top of the rock, sometimes visible from the surface as a network of crevasses, sometimes hidden as subterranean caves. Bulmer Cavern is the longest cave network in New Zealand, and it spans at least 66 kilometers beneath the Mt. Owen peak. Mt. Owen, then, is not the solid rock it appears, but a mountain peak made out of this karst (to use the geologic term), this cave-and-gully-riddled limestone.
The karst on Mt. Owen is interspersed with big gravel patches. These rubble deposits are also the result of water on the mountain. When temperatures drop, the water in the karst drainage system freezes, and as it freezes it expands. The result? Cracks get forced open, and rock bits of all sizes break off from the mountain. These rocks collect in valleys and gullies, forming scree fields.
As a tramper on Mt. Owen, this made for an exciting climb. My summiting experience involved (nervously!) sidestepping sinkholes, leaping gullies, and traversing steep scree fields. And as I scratched my palms against the limestone ridges or scrabbled along the scree, I was touching rock that formed millions of years ago deep under the ocean. Physical processes have been acting for the last 85 million years, at least, to form these rocks, to force them to the surface, to carve and crack them. And so here’s the mind-boggling part of mountain tramping, the part where the immense timescale that matters to a mountain forms the backdrop for a living being’s weekend. When I looked out at my fellow trampers, they stood tiny against the peaks. Our two-day scramble across the peaks was just a blip of a moment compared to the time these rocks have existed.
And that’s all the abridged version! For more information, head to Te Ara, the New Zealand Encyclopedia and fall into the rabbit hole of geological stories they share.
And otherwise, just sit back and marvel at the way geology puts things into perspective.
Until next time,
Update: On February 9th, the Geological Society of America published a paper that formally classifies Zealandia as the eighth continent. While this name and theory have existed since the mid-nineties, this paper is the first to accumulate decades of data into a unified argument. After its publication, it made the rounds of popular news reporting, including mentions in the Huffington Post and NBC– and Zealandia was briefly internationally famous. But why should we care? As the authors of the paper write, the story of Zealandia is an illustration “that the large and obvious in natural science can be overlooked”. Check out the original paper here.