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Sponge and sea corn coral
Sponge (Sidenops atlantica) and sea corn coral (Primnoa resedaeformis) on a reef at Iverryggen.
Photo: MAREANO/IMR
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Iverryggen - Glacial Formations and Coral Reefs

Cruise report: 65 kilometers due west from the outer tip of Vikna in Nord-Trøndelag, just north of Haltenbanken, a ridge named Iverryggen is encountered. On the otherwise fairly flat topography Iverryggen rises more than 130 meters above the seafloor. On the slopes of Iverryggen many coral reefs are found, and this area is protected.

Iverryggen is nearly 15 km long — from north to south — and approximately 6 km wide. Projecting southwest is a ridge 6 km long that is speckled with coral reefs. The western side of Iverryggen has a rather steep slope, while on the eastern side the slope is more gentle down to seafloor depression reaching down to 350 meters.

How has this ridge been formed? Are these simply undersea mountains jutting up? Or, have glaciers had a hand in this? The latter case is most likely. Iverryggen is a good example of what is known in geological slang as “glacial tectonics”. This type of seafloor has been shaped by glacial movement, most likely during the last ice age 20,000 – 30,000 years ago. During periods with little movement, glacial ice became affixed to the seafloor. When the glacier again began to move, it pulled along huge sections of the sea floor. The sea-floor depression east of Iverryggen is an example of an area where glaciers dislodged large sections of the seafloor, and Iverryggen is where the substrate was deposited.

The shape of Iverryggen resembles a bit of a bird's head.

The shape of Iverryggen resembles a bit of a bird's head. The bird’s "beak" is speckled with coral reefs and extends nearly 6 km southwest of Iverryggen proper. This is material that the glacier has pushed out from the main ridge.

Photo: MAREANO

The depth profile clearly shows the steep western slope, and the more gradual eastern slope. The depression in the seafloor east of Iverryggen is irregular. It is also approximately 100 meters lower than the seafloor west of the ridge. The sediments that was here during earlier geological times has been dislodged and re-deposited at least 10 km west onto Iverryggen.

On top of Iverryggen and the surrounding ridges, elongated depressions (iceberg plough marks) are observed up to 100meter wide and 5 meter deep. These are tracks from icebergs that drifted around in the Norwegian Sea after the ice had retreated from the continental shelf.

Coral Reefs

Fishermen have known that coral reefs occur on Iverryggen since bottom-trawl fishing started here during the late 1990s. In 1999, IMR investigated the seafloor in five areas of Iverryggen; extensive trawl damage to coral reefs was observed within all areas. Available information indicated that there were plenty of coral reefs on Iverryggen, and to prevent their destruction, the area was closed to bottom trawling in 2000. Observations made during this survey confirm damage to coral reefs in the area. Most reefs are quite small, with limited amounts of live coral. This indicates that these reefs could have been further damaged if bottom trawling had not been stopped.

Redfish and corals

Redfish among living corals on one of the many reefs at Iverryggen.

Photo: MAREANO/IMR

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Pål Buhl-Mortensen
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Beate Hoddevik
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