Abyssal Plain. Continental Shelf

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1 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 plain. It averages over two miles deep and accounts for as much 79% of the sea floor over the entire planet. The top layer of sediment found on the surface of the abyssal plain 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 mid-water ranges of the ocean. 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 muck supports untold numbers of species whose diet thrives on the crumbs and the leftovers that have settled into the mud of the seabed. 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. The continental shelves are not considered part of the ocean floor proper, but as the flooded margins of a continent. Under the United Nations Convention on the Law of the Sea, the name continental shelf was given a legal definition as the stretch of the seabed adjacent to the shores of a particular country to which it belongs. With a few exceptions, the shelf break of the continental margins is located at a remarkably uniform depth of about 140 m (460 ft). Scientists believe this depth marks the ancient shorelines of the continents during past ice ages, when sea level was lower than it is now.

2 Mid-Ocean Ridge A team of scientists on board the H.M.S. Challenger first discovered the existence of a large rise in the sea floor in 1872 when they were looking for suitable undersea locations to set down telegraph cables. Later in 1925, soundings of the sea floor were taken to get a picture of the topography of the sea bed in these locations. They were astonished to discover that there were not only mountains on the sea floor, but there were mountain ranges longer than any mountains on land. After years of mapping the sea floor, it was revealed that the longest, unbroken mountain chain in the world was running through the middle of the Atlantic ocean. They called it the 'Mid-Atlantic Ridge' and determined that it was over 40,000 kilometers long! What surprised them even more was the discovery of active volcanoes along the entire length of the Mid-Atlantic Ridge. Scientists were at a loss to explain such extensive undersea vulcanism. At that point in time, no one believed it was possible for volcanic eruptions to occur under the immense pressure and depth of the sea bed. Yet there was lava oozing out from the cracks in the sea floor without sending any hints of its occurrence to the surface of the ocean. All this had been happening silently, out of sight, under the sea for who knows how long. Geologists collected samples of lava rock from the sea bed surrounding these areas of volcanic activity, carefully recording the location and began piecing together the puzzle of these undersea lava flows. Scientists used the magnetic anomalies recorded in the lava rocks to date them - to tell how old they were and when they formed. Using this technique they discovered that the newest, youngest rocks were at the very center of the ridges and the oldest rocks were furthest away. That told them that the undersea volcanoes were the site of formation of new ocean floor and that the oceanic crust was being pulled apart along these rifts, or tears, in the ocean's crust. These were determined to be spreading boundaries of tectonic plates. A spreading boundary is where the tectonic plates are separating, or coming apart. Some spreading boundaries are places where the crust is sinking downward as it is stretched thin - like in the East Rift Valley of Africa, where the Dead Sea is located. Many of the spreading boundaries are located deep in the ocean on the sea floor. These are places where volcanic activity is at a premium because the crust is being torn apart. New crust is forming when magma from the mantle deep down is forced upward out of the cracks where the plates are coming apart. Long chains of undersea mounts (like the mid-atlantic Ridge) and volcanic islands (like Iceland) typically characterize these type of plate margins.

3 Hot Spots About 30 years ago a Geophysicist named J. Tuzo Wilson came up with an idea to explain why there was volcanic activity out in the middle of the Pacific Ocean, in the middle of the huge Pacific Plate. At the time, scientists thought that volcanoes only happened at plate boundaries, but nobody could explain why they were happening out in the middle of a tectonic plate. Dr. Wilson said that there are hot spots, under the earth s crust in some places. These are called hot spots because they are places where a lot of heat is concentrated in a small area. The heat causes the overlying rock to melt. Since the magma is liquid and is lighter than the surrounding rock it "floats" to the surface and forces its way out of fissures in the crust. once magma erupts through the crust it is known as lava. Over time, the continual outpouring of lava can form a sea mount or island volcano if the hot spot is under the ocean floor, as in the case of the Hawaiian Islands. There is just one hot spot that never moves. But the Pacific Plate continually (and slowly) moves north over the hot spot, forming a new volcano on the overlying plate each time. Deep Ocean Trench In a few places on the ocean floor there exist unusually deep zones. Nearly three times deeper than the average depth of the sea floor, these zones are known as deep sea trenches. The trench zones are characterized by complete darkness and unimaginable pressures of up to 16,000 pounds per square inch. Yet, despite the seemingly inhospitable environment of the deepest depths, life is inexplicably found thriving. It is in the Marianas Trench, of the Marianas Islands of the Southeast Pacific Ocean, where the deepest known place in the ocean was found. 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.

4 Volcanic Island 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. Submarine Canyon 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 in the middle of the Bering Sea called the Zhemchug Canyon. It is deeper than the Grand Canyon (1.83 km deep) at 2.6km deep. Zhemchug Canyon is the largest submarine canyon in the world, based on drainage area (11,350 km2) and volume (5800 km3). Deep, cold, oxygen-rich waters well up from the deeps into the canyon, providing sustenance to an enormous array and variety of life forms.

5 Seamount Seamounts are 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!

6 Guyots Seamounts and Guyots are volcanoes that have built up from the ocean floor, sometimes to sea level or above. Guyots are seamounts that have built above sea level. Erosion by waves destroyed the top of the seamount resulting in a flattened shape. Due to the movement of the ocean floor away from oceanic ridges, the sea floor gradually sinks and the flattened guyots are submerged to become undersea flattopped peaks. We know that the tops of guyots were once at the surface because they contain evidence of fossils such as coral reefs that only live in shallow water. Seamounts conversely represent volcanoes that did not reach sea level so their tops remain intact and are shaped like volcanoes on land. A volcano erupts above sea level somewhere in the ocean After a long time, waves have eroded the portion above sea level. Gradually the sea floor subsides as it moves away from the oceanic ridge, and the guyot becomes submerged.