South Island Landscape
New Zealand’s South Island Landscape
The South Island landscape owes its spectacular scenery to the very recent processes that formed it. Next time you’re in one of those big valleys – say at the Mt Cook Hermitage, or the Ohau Lodge, or even just in Queenstown – take a moment to look up to the landscape above. If the valley is U-shaped (and these ones are) then it was carved out by glaciers. The one pictured above, looking up the East branch of the Matukituki River valley in Mt Aspiring National Park is a classic example. As little as 10,000 or 12,000 years ago, you would be sitting under a kilometre of ice. Look a little closer and you will see where the smoother shape of the valley floor and lower slopes abruptly change to the more jagged rocks near the peak. That’s where the ice went up to. Everything below it was ground smooth by the passage of the glacier. No man made construction would have been able to stop them.
Then about 10,000 years ago, for reasons that we still don’t understand, the Earth entered a major warming period and the glaciers began to melt, forming those rivers that are such a part of today’s landscape. And the rivers quickly took over as the dominant landscape forming influence, creating very wide, flat river gravel plains that now have all those long, straight roads on them. In Canterbury, those plains reach all the way from the Southern Alps to the Pacific coast.
In Otago those flat alluvial plains of course are constrained within the hill and mountain ranges. But they were so important to the early settlers that their names live on in today’s maps – ‘Piano Flat’, ‘Hawea Flat, and so forth.
Taking a moment to look over some of the flat terraces as you drive down a picturesque river valley will give a clue as to the real nature of the forces that formed this South Island landscape – plate tectonics. Or more specifically, the earthquakes that are one of the consequences of plate movement.
New Zealand sits on one of the Earth’s most active tectonic plate boundaries, between the Pacific and the Australian Plates. The relative plate motion is presently around 30mm per year, which is very fast by geological standards; vertical movement – the type that builds mountains – is also fast, around 1.5 mm per year. The fault is currently “locked”, but every 200-300 years it unlocks and all the accumulated strain lets go at once, resulting in a rather large earthquake. This can locally move the ground up or down by several metres, rivers change course and terraces are formed. So each terrace represents a period of stability between earthquakes.