Making waves in Marlborough Sounds
On 02 December, 2017, the Operational Land Imager (OLI) on Landsat 8 acquired this natural-colour image of the rocky landscape of Marlborough Sounds, New Zealand. The shallowest (light blue) parts of the inlets are estuaries, where sediment from rivers has discoloured the water and accumulated on the bottom. Deeper water is darker blue. The inlets that make up the Sounds are mostly shallow; none have depths that exceed 50 metres (160 feet).
Māori mythology tells us of the origin of the intricate network of inlets, coves, bays, islands, and forested hills around the northern reaches of South Island. According to legend, a boy named Aoraki-along with three of his brothers and a crew-were boating in the open sea when a storm god crashed their vessel on a reef and flipped it. As the brothers scrambled out of the water, a bitter wind froze them in place and turned everything into stone. The overturned hull became New Zealand's South Island. The petrified body of Aoraki, the tallest of the brothers, turned into Mount Cook; the others became the Southern Alps. The partially submerged prow of the canoe formed the maze-like Marlborough Sounds and other landforms along the northern coast of South Island.
Geologists have a different way of explaining the distinctive landscape. They see evidence that the northern end of coastal mountains in this area began to tilt and sink about 1.5 million years ago. Meanwhile, as global temperatures increased, the rising seas swamped coastal valleys and turned them into features known as rias—drowned, funnel-shaped river valleys that connect to the seas. While sea level has risen and fallen many times with the comings and goings of Ice Ages, geologists think the current coastline began to take shape about 7,000 years ago when the most recent Ice Age ended.
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Credit: NASA / Joshua Stevens, using Landsat data from the U.S. Geological Survey.