Harrow and Hillingdon Geological Society

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Mountains in the sea

One of the mysteries of the sea are the large number of seamounts that rise up on the seabed and, in a few cases, break surface to form oceanic islands. Volcanic in origin, seamounts are widely scattered throughout the world’s ocean basins, especially in the Pacific. Recent estimates suggest that there may be as many as 200,000 seamounts with heights above the surrounding seafloor that are in the range 0.1 to 6.7 km.

Seamounts are generally circular in shape, have pointed, star-shaped, curved or flat tops and are often capped by a coral reef. They are of geological interest because they record the horizontal and vertical motions of the Earth’s tectonic plates, the strength of its hard outermost rock layer and the magmatic ‘pulse’ of its deep interior. They are also significant as ocean ‘stirring rods’, biodiversity ‘hotspots’ and as hazards for earthquakes, landslides and submarine navigation.

Unfortunately there are only a few (<300) sample ages from seamounts and so we do not know how many are like Loihi in the Hawaiian Island chain, which is building up on the seafloor and will eventually form an island or Horizon Guyot in the Mid – Pacific Mountain Chain, which was once an island and is now sinking. One problem is sparse ship track coverage that has made seamounts difficult to find. Statistical studies suggest that there may be as many as 24,000 seamounts higher than 1 km still to be discovered. The charting of these seamounts and the determination of their morphology, structure and evolution is one of the many challenges facing marine geoscientists in the future.

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