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Why are fjords often very deep below sea level?

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Snow-capped, rugged Atlas Mountains encircle a green valley with scattered trees, terraced fields, and a few small buildings in Morocco. — valley The rugged Atlas Mountains surround a valley in Morocco.

Fjords are often exceptionally deep below sea level because they were carved by massive, thick glaciers that possessed the erosive power to scour valley floors far beneath the ocean surface before the ice began to float.12

Unlike rivers, which cannot erode deep basins because water cannot flow uphill, glaciers are capable of flowing over obstacles and exerting immense pressure on the underlying bedrock. The extreme depth of these U-shaped valleys is directly related to the thickness of the ice that once occupied them; for instance, to reach the current depths of some Norwegian fjords, the glaciers would have needed to be nearly 2,000 meters thick.21

Once these glaciers melted, the sea inundated the deep, glacially carved troughs. While some researchers suggest that tectonic activity may have contributed to the initial relief of these valleys, the consensus remains that the profound depths are primarily a result of intense glacial erosion.12