Marlborough Sounds Drowned Coastline

The Marlborough Sounds’ intricate network of waterways began as a steep, mountainous landscape with peaks up to 2,000 metres in height. Over time, many rocks throughout the landscape were incised and eroded by rivers, forming characteristic V-shaped valleys separated by sharp ridgelines. One million years ago, continued erosion had reduced the maximum height of most of the peaks to around 1400 metres.

The transformation from a mountainous landscape to the drowned valley system observed today has two significant components; tectonic subsidence and rising global sea levels.

The tectonic subsidence responsible for the submergence of the Marlborough Sounds occurs due to flexure – the bending of tectonic plates. Beneath the sounds, the Pacific and Australian Plates are locked together. As the Pacific plate subducts, it drags the Marlborough Sounds region down with it. While many people associated earthquakes with uplift, because of the relationship between the two plates in this region, earthquakes here tend to result in subsidence. The Sounds have been subsiding for approximately 1.5 million years at a rate of around 0.3mm/year.

The effect of tectonic subsidence in the sounds was exacerbated by significant rises in sea level beginning around 14,000 years ago. This sea level rise was associated with the end of the last glacial maximum (LGM), the most recent global ice age, which spanned from roughly 14 - 25,000 years ago. The termination of the LGM resulted in significant melting of global ice bodies and, with it, rises in sea level of about 130 metres globally.

The dendritic coastline of the Marlborough Sounds is described by geologists as a ‘ria coastline’ – a coastline formed by the flooding of river valleys. This process is responsible for the elongate islands present throughout the sounds, which are analogous to the ridges of the paleo-landscape; these areas of once-heightened elevation are now the only parts which protrude above sea level.

Similarly, the extensive bays, inlets and coves of the sounds have a modern-day analogue – the tributary valleys of the river valley systems common throughout New Zealand. The main sounds represent major past rivers, while the inlets which extend off the sounds were formed by smaller tributary rivers feeding into the main systems.

Take a minute to consider an area you’re familiar with which has a landscape shaped by the erosive power of rivers. Can you see the similarities between your region and the sunken Marlborough Sounds?