Geography of the Sea

In the brief history of ocean exploration some of the most fascinating discoveries about the geology of the sea floor have changed the way humans had viewed the sea for hundreds of years. No longer is the sea considered to be a bottomless, black abyss, the sea floor a vast expanse of cold, flat, lifeless watery desert. In the last 30 years, modern technologies have been used to map the topography of the sea floor, revealing a profile of the sea floor that is anything but flat and lifeless. The sea bed has been found to be riddled with massive geologic formations that mimic those we have on land, but on a much, much grander scale. Some of the most notable discoveries have been deep ocean canyons, which rival the Grand Canyon, seemingly endless underwater mountain chains that run thousands of miles; deep trenches, sea mounts, and coral reefs and atolls formed not by the geologic processes of earth, but by the diligence of tiny organisms over thousands of years.

Continental Shelf – Along the shores of most of the major continents around the globe, the sea floor slopes gently downward so gradually that a wide shelf forms just offshore. The width of these relatively shallow shelves varies from just a few miles to several hundred miles from shore. At the outermost edges of these shelves the sea floor slopes abruptly and steeply to the abyssal plain. Scientists believe that the continental shelves mark the true outline of the land forms that are our continents and were exposed as shorelines in the geologic past when sea levels were much lower than today.

Submarine Canyons - In many continental shelves submarine canyons have been found carving deep fissures that stretch from near shore out to the deep sea edge of the shelf. There are a number of theories as to what carved these giant cracks into the shelves, but the most prominent one states that sediment transport carved these canyons. Sediment transport in the sea occurs primarily as underwater landslides of enormous masses of rock and sediment, usually triggered by turbulent waters during a storm, or ground movement from an earthquake. The depth at which the submarine canyons have been cut depends on the make up of the underlying rock – how susceptible it is to being carved – and how much, how often and how heavy the materials are that are transported downslope during an underwater landslide. It is also believed that some canyons were carved above ground, at a time when sea level was a mile or more lower than it is today. Those canyons that are now submerged may have once held rivers and waterfalls that carved the canyon walls, carrying the sediment and debris down into shallower sea.

The largest and deepest submarine canyon ever discovered is off the west coast of the Australia, carving the continental shelf 25 miles out from the coastline to a depth of over 6,000 feet. Its reach spans over twenty miles wide and its habitat supports one of the richest underwater ecosystems in the world. Deep, cold, oxygen-rich waters well up from the deeps into the canyon, providing sustenance to an enormous array and variety of life forms.

A video animation showing the draining of the oceans

Abyssal Plain – The vast majority of the real estate at the bottom of the sea is a relatively flat expanse of level floor. Called the abyssal plains, these average over two miles deep and may account for as much 79% of the sea floor worldwide. The top layer of sediment found on the surface of the abyssal plains is rich in organic matter that has drifted down from above and contains the remains of virtually everything that lives in the upper reaches to the midwater ranges of the ocean. For everything that lives, must also die. All creatures that spend their lives at sea are also buried at sea, their remains being eaten by other organisms, or left to sink downward to the abyss. This organically rich and nutritious muck supports untold numbers of species whose diet thrives on the crumbs and the leftovers that have settled into the mud of the seabed.

Sea Mounts – Seamounts are isolated submarine elevations, or underwater mountains. They are at least 700 meters in height and were created by volcanic activity. They are mountains, just like we see on land, but the tops of them don’t break the surface of the ocean. Many of the sea mounts throughout the world have their peaks far below the ocean surface, often over a mile down. For many years, people thought that the bottom of the sea was a flat plain. Imagine the surprise when surveyors discovered there were entire mountains under the sea! Some of the sea mounts were found to rise up over a mile from the ocean floor. There is a newly formed, recently discovered sea mount south of the big island of Hawaii where a new volcano is building up from the ocean floor. Named Loihi, the sea mount will one day become a new island, part of the Hawaiian Island chain, when it breaks the surface of the sea. Right now, the summit of Loihi is 3,178 feet below sea level. Scientists expect it will take approximately 10,000 to 40,000 years for Loihi to lose the title of “sea mount” and become an island.

Under sea mountains don’t always occur singly, in isolation. Entire mountain ranges have been discovered on the sea floor. Equipped with the latest in sonar equipment for taking soundings of the sea floor, oceanographers in the 1950s conducted the most thorough investigation of the ocean floor to date. The sonar maps they generated, which profiled the sea bed, showed incredibly high mountains, in long chains, some of them extending unbroken for thousands of miles. In the years since the first discovery of undersea mounts, the Mid-Atlantic Ridge has been measured and mapped to reveal an underwater mountain range that is over 46,000 miles long, winding from the Arctic Ocean, around Africa, Asia and Australia, to North America. Compare that to the longest mountain range on land – the Andes Mountains in South America are only 4,700 miles long. The Mid-Atlantic Ridge is almost 10 times longer than the Andes!

Volcanic Islands – Another type of mountain in the sea is an island. It rises from the sea floor, but reaches above the ocean’s surface, sometimes just barely. An island is a solitary mountain formed by volcanic activity. Lava erupting from the sea floor builds up on the sea bed over thousands, or even millions of years. The lava cools rapidly as it reaches the ocean water and forms solid rock. Each eruption of the sea floor volcano builds up the volcano a little bit higher. Until one day, the eruptions have built the top of the undersea mountain so high, that it rises above the ocean’s surface.

Sometimes, volcanoes occur in close proximity to each other on the sea floor, creating a very large island. For example, the big island of Hawaii is actually five, side-by-side volcanoes that have grown together. The island chain of countries that make up Southeast Asia; Indonesia and Papua New Guinea, the Philippine Islands were all created by volcanic activity on the sea floor. New Zealand, the Island country off the Southeast coast of Australia, was also formed by ancient volcanoes.

A volcanic island in the South Pacific Ocean is also the biggest mountain in the world. Mauna Loa, one of the volcanic islands that make up the big island of Hawaii, is the most massive single mountain in the world because of its sheer bulk. Only about 13,448 feet of Mauna Loa are above sea level, so it may not seem like a very tall mountain. But, when you start measuring Mauna Loa from its true base on the bottom of the ocean, in the Hawaiian Trough, the total height exceeds that of Everest by over 3/4 of a mile. Mauna Loa is Hawaiian for "Long Mountain", probably because of its long, gently sloping shape. Mauna Kea, a neighbor of Mauna Loa on the same island of Hawaii, is actually the tallest mountain in the world. Mauna Kea is about 350 feet taller than Mauna Loa, but its mass doesn't compare to that of Mauna Loa. Mauna Loa takes up a lot of space because it's mass is 9,700 cubic miles of mountain!

Coral Reefs – Another fascinating feature of the ocean that is part of the topography are coral reefs. Coral reefs are found in the sea around at the equator all around the world, but they are not formed by geologic processes. Reefs are hard-as-stone structures that have been built up over many thousands of years by the activities of tiny organisms called corals. Corals are related to jellyfish, in that they are gelatinous creatures, which come from the same family tree. They begin life as tiny, microscopic polyps that swim about and float freely in the seas, but eventually take root on the structure of a reef and spend the rest of their lives living under the protection of the reef. Their tiny bodies extract minerals from the sea water and use them to form a hard shell around their bodies. The coral now has a safe haven from all the creatures in the sea that might like to eat it. After the coral dies, its protective shell remains behind, adding just a tiny bit of new “masonry” to the existing reef. This process is called accretion and, as you can imagine, is a very slow process. Over many thousands of years, the accumulation of these tiny bits of reef can build a very substantial structure.

Coral reefs are usually found in warmer, tropical seas surrounding the equatorial belt. Typically, corals will invade the seas around a sheltered volcanic island, taking root on the fresh rock below the surface. The Great Barrier Reef, off the south coast of Australia, is the largest coral reef in the world. It stretches for 1,260 miles and covers over 80,000 square miles of ocean floor. It is estimated that over 350 different species of coral live on the Barrier Reef and that it took over 600 million years to build the reef to its current size.

 

Deep Ocean Trenches – These are rare occurrences on the sea floor, accounting for less than 2% of the topography of the sea bed. Trenches occur when heavier tectonic plates of oceanic crust are subducting under lighter tectonic plates of continental crust, dragging the leading edge of the continental crust downward. The pocket of exceptionally deep space where the two plates are grinding past each other forms a trench that runs along the margin where the subduction is happening.

Examples of some deep ocean trenches that are subduction zones are off the West coast of South America, in the Peruvian Trench. The Aleutian Trench, in the North Pacific Ocean, and the deepest, most famous one in the world, the Marianas Trench, off the coast of the Marianas Islands in the Southeast Pacific. These are all places where oceanic plates are subducting beneath continental plates.

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