Mountains under the Sea

            Passengers on a cruise ship probably don’t realize they are often sailing over mountain peaks at many points along their route.  The mountain under their keel may be part of the Mid-Ocean Ridge or a free standing seamount.

Mid-Ocean Ridge.  The seafloor is home to the longest continuous mountain range in the world.  The Mid-Ocean Ridge, with mountain peaks averaging 1,500 meters (5,000 ft) high, stretches unbroken for 65,000 km (40,000 mi) along our planet’s ocean floor.  This long stretch of connected mountainous terrain, which threads around the globe like the raised seams of a baseball, marks the boundaries of the 12 tectonic plates that make up the earth’s crust.

            The Mid-Ocean Ridge is split by a rift, or deep valley, that runs down the middle of the chain, with half the mountain range sitting on one tectonic plate and the other half on the adjoining plate.  Magma from the mantle, the molten layer under the crust, seeps up through the rift and constantly builds up the mountains, and at the same time continues to wedge the tectonic plates apart by cooling and hardening on the valley floor.  These seafloor rift valleys are called Oceanic Spreading Centers, and may be up to 20 km (12 mi) wide.  The spreading rate is the rate at which the flow of hardening magma continues to widen the rift and push the two plates farther apart.  The Mid Atlantic Ridge, which separates the North American and Eurasian plates, widens relatively slowly, spreading at a rate of only 25 mm (1 in) a year.  The East Pacific Rise running between Antarctica and the Gulf of California spreads much faster, up to 150 mm (6 in) a year.  The East Pacific Rise separates the Pacific, Antarctic, Nazca, and South American plates.

            The average depth to the ridge crest is 2,500 m (7,800 ft).  Depth to the ocean floor averages 4,000 m (12,500 ft).

            Seamounts.  Most seamounts are self-standing undersea mountain peaks not associated with the Mid-Ocean Ridge.  These are usually extinct volcanoes that can rise 5,000 m (15,600 ft) from the ocean floor.  The seamount base is normally in deep water, leaving a wide clearance between the peak’s summit and the surface of the ocean.  There are exceptions, one being the Bowie Seamount off the coast of British Columbia that lies in 3,000 m (9,200 ft) of water with a summit to surface clearance of only 24 m (75 ft).

            There are approximately 30,000 seamounts rising from the world’s seafloor, many of them still uncharted.  In 2005, the submarine USS San Francisco ran into an uncharted seamount at a speed of 35 knots, killing one seaman and causing extensive damage to the submarine.   The Muirfield Seamount in the Cocos Islands is a 5,000 m (15,600 ft) mountain with only 16 to 18 m (49 to 56 ft) of clearance from the summit to the surface.  It was named after the merchant vessel MV Muirfield which struck the uncharted mountain while sailing between Indonesia and Australia, damaging the ship’s keel.

            Most seamounts are located in archipelagos of volcanic mountains created when a tectonic plate passed over a hotspot’s deep mantle plume.  One example is the Emperor Chain of seamounts which is the northwest extension of the Hawaiian Islands.   There are 80 identified volcanoes, most dormant, in this underwater chain that runs 5,800 km (3,600 mi) from Lo’ihi near the Big Island of Hawaii to the Aleutian Trench.   Over many millions of years, the Pacific Plate moved slowly over the Hawaiian Hotspot in a northwesterly direction, creating volcanic islands that rose high above the sea, becoming dormant when the island cleared the hotspot, and gradually eroding till sinking below sea level.  The present Hawaiian Islands are part of that process, and they, too, someday millions of years hence, will be mountains under the sea.  The highest undersea volcano in the Emperor Chain is Deikakuji, a 4,000 m (12,600 ft) mountain that rises from a 5,000 m (15,600 ft) depth.  The undersea volcano Lo’ihi off the coast of the Big Island is now sitting atop the Hawaiian Hotspot, and will someday rise from the ocean to be the newest island in the Hawaiian chain.