Many of us have stared in awe at stunning mountain ranges all across the globe, but have you ever thought about mountains that exist below sea level? This almost seems impossible, since the term ‘mountain’ evokes images of huge, imposing peaks at high altitude. Even so, there’s a whole world below sea level – and underwater mountains are a part of it.
Entire mountains, even entire mountain ranges exist below the surface of the ocean. Underwater mountains house an abundance of marine life and are some of the most ecologically diverse areas of the ocean.
What Are Underwater Mountains?
Underwater mountains are known as seamounts and are extinct volcanoes rising hundreds to thousands of meters above the seafloor. While active, these volcanoes create large piles of lava that make up underwater mountains. Sometimes these mountains rise above the surface of the ocean, as is the case with Mauna Kea on Hawaii. When measured from base to summit, this dormant volcano is the tallest mountain on earth. Although it only stands 13,000 feet meters above sea level, when measured from its base below sea level it stands over 30,000 feet tall. This is even taller than Mount Everest!1
It has been estimated that seamounts encompass approximately 28.8 million square kilometers of the surface of the earth. This is larger than any above-ground biome! 2
Seamounts are technically defined as rising at least 1,000 meters above the seafloor. Smaller underwater mountains are known as sea knolls, while underwater mountains with a flat top are referred to as guyots. Guyots form when a seamount breaks the surface of the ocean. Over time, the peak begins to flatten and erode due to waves and weather. The guyot eventually sinks back below the surface.3
The mid-oceanic ridge system is the most extensive chain of mountains on earth, clocking in at 65,000 km long. The remarkable aspect of this chain is that over 90% of the mountain range is completely underwater.
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Mid-ocean ridges occur along divergent plate boundaries, which occur when two tectonic plates begin to move away from each other. As the plates separate, magma rises from the earth’s mantle to the seafloor. This produces major volcanic eruptions and eventually, a chain of mountains is formed. This happens very slowly. Underwater mountain ridges only spread between 1 and 20 centimeters per year. Ridges that spread slowly are characterized by steep and irregular topography, while those that spread faster have gentler slopes.4
Seamounts Versus Islands
Seamounts are often formed by “hotspots” which is an area in the earth’s crust where hot plumes of magma rise upward. This is due to tectonic plates moving over an unusually hot part of the earth’s mantle. This volcanic activity ends up creating seamounts below the surface of the ocean. When these seamounts continue to grow and finally break the surface of the ocean, they are then known as islands.
The plumes that form hotspots are known to be fairly stationary, while tectonic plates are constantly moving. As volcanoes are formed, the plates continue to move and the volcanoes end up moving away from the hotspot. When this happens, the volcanoes eventually cool and become extinct. This happens very slowly, as some seamounts may remain volcanically active for two to three million years before finally cooling off. Because the tectonic plates continue to move over the hotspot, more volcanoes are formed.
The Hawaiian islands are a great example of this. The Hawaiian hotspot is believed to have been active for at least 70 million years, explaining the chain of islands that extends 6,000 km across the Pacific Ocean. While the island of Hawaii has been an active volcano for many years, the volcanic activity is slowly moving to the nearby seamount known as Loihi.5
The Importance of Seamounts
Seamounts are incredibly ecologically diverse and offer a lot of opportunities for scientists to learn more about life under the ocean. Although seamounts are abundant in the ocean, it is estimated that less than one-tenth of a percent of them have been explored.
From what has been explored, scientists have found that they often provide a habitat to endemic species. Endemic species are those found only in one single location.6 The deep-sea corals that live on seamounts host over 1,300 different species of marine life. These extensive ecosystems are still largely unknown, and more in-depth scientific research was only begun recently.
Because of the abundance of marine life, seamounts are often targeted by deep-sea fisheries. Nets that are weighted with heavy chains are often dragged across seamounts to catch the large schools of fish that live around them. This practice can destroy unique corals and other species, often taking hundreds or thousands of years to become reestablished.7
Final Thoughts on Underwater Mountains
Many of the mountains we know have been submitted by mountaineers, and much of the flora and fauna around the globe has now been documented. Seamounts are a whole new world of biological diversity that has only just begun to be researched. It is possible that the unique life forms that reside on these underwater mountains could offer major breakthroughs in scientific research.
PhotoCredits: littlesam/shutterstock.com, EthanDaniels/shutterstock.com, VeronikaSurovtseva/shutterstock.com