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The Sea Floor • 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.
But • 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.
Geography of the Sea Floor • Continental Shelf • Submarine Canyons • Abyssal Plain • Sea Mounts • Volcanic Islands • Coral Reefs • Deep Ocean Trenches
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 • 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 • It is 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 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.
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. • Abyssal plains are among the smoothest surfaces on the planet, with less than five feet of vertical variation for every mile
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.
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.
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, 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. 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 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.
Coral Reef • 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. • They begin life as tiny, microscopic creatures 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. • Over many thousands of years, the accumulation of these tiny bits of reef can build a very substantial structure
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. • An example of a deep ocean trench is Marianas Trench in the Southeast Pacific.
