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The Ocean Floor

• The ocean itself is 20 times older than the oldest seafloor. The oldest rocks in the present ocean floor are only 160 million years old, but fossils of marine bacteria have been found in rocks 3.7 billion years old.
  
  
• Maps of the magnetic field in rocks on the seafloor show a pattern of stripes parallel to the mid-ocean ridges. When molten lava cools into rock it remains magnetized in the direction of the Earth's magnetic field. The Earth's magnetic field has reversed many times in the past, with the North and South Magnetic Poles changing places, and these reversals are recorded in the sea floor.
  
  
• The youngest rocks in the seafloor are generally found near the mid-ocean ridges. The age of seafloor rocks increases with distance from the mid-ocean ridges, with the oldest rocks found near the edges of the oceans.
  
  
• In 1992, Fred Grassle and Nancy Maciolek extracted over 100 different species from deep-sea mud samples the size of a dinner plate (900cm2). More importantly, each new sample added more and more new species in such a way that it is now estimated that there are between one and 10 million species living in the deep-sea sediments.
  
  
• On March 7, 1873, the British research vessel Challenger dredged up small lumps of almost pure manganese peroxide from the tropical Atlantic Ocean floor. We have since learned that these 'manganese nodules' are widespread on the ocean floor, particularly the North Pacific, and are a potentially valuable mineral resource as about 16 million tons of new material are annually.
  
  
• Approximately 1.2 billion tons of sand and gravel are mined from the ocean floor annually. The United Kingdom and Japan each take 20 per cent of their total sand and gravel requirements from the seafloor.
  
  
• Most of the sediments on the floor of the open ocean are turned over, eaten and pass through bottom animals. Deep-sea photographs show an endless vista of burrows, mounds, tracks and trails.
  
  
• Modern surveys of the ocean floor use multi-beam echo sounders with many sound sources and receivers mounted on a single vessel to provide complete coverage of the bottom over a swath about twice as wide as water is deep. In water 10m deep a person using a lead weight and line can make about 20 depth measurements per hour, while a multi-beam sounder can make about 293,000 measurements in the same period.
  
  
• The composition of rocks formed from marine sediments suggests that the composition of seawater has not changed significantly for several hundred million years. Rocks older than 2.5 billion years retain evidence that sodium chloride was the principal salt, as it is now.
  
  
• Proportions of elements such as copper, zinc and uranium in black marine shales from ancient deposits are similar to those in comparable sediments now being deposited in the Black Sea.
  
  
• The deep sea floor can have 'storms'. The effects of large surface weather events can be transmitted several thousand meters down to generate high bottom currents that stir up large volumes of sediment.
  
  
• The Earth's crust under the oceans ranges from five to 15km in thickness. Under continents thickness ranges from 30 to 80 km. Oceanic crust is about 59 per cent of the total area of crust, but continental crust makes up about 70 per cent of the total volume of crust.
  
  
• Continents are largely granite, which is much lighter than the basalt that makes up most of the seafloor rock. When a plate of oceanic crust is pushed against a continent, the continent simply floats up over the heavier ocean plate, forcing it down into the interior of the Earth where it melts.
  
  
• There is no ocean crust older than 160 million years because it is continually being recycled. The depth of earthquakes under the edges of continents is generally greater the farther inland the earthquake is. Scientists believe that the earthquakes are caused by the oceanic crust sliding under the continent to be melted back into the Earth's interior.
  
  
• The ocean crust moves away from the mid-ocean ridge spreading centers at rates of between one and 20cm/year. New ocean crust grows about as fast as human fingernails!
  
  
• Parts of the deep ocean floor are very flat, with changes in height of less than one meter per kilometer. These 'abyssal plains' are formed when thick sediment blankets the rough oceanic crust.
  
  
• Burrowing animals in the ocean floor are important in the carbon balance of our planet. Dying plants and animals which fall to the ocean floor are reused and recycled by the burrowing animals, but many of these animals are then eaten by fish, who carry the carbon back up into the water column.
  
  
• A special ship has been busy since 1968 getting scientific samples of sediments and rocks from beneath the deep sea floor. Started as the US Deep-Sea Drilling Project, using the Glomar Challenger, the project grew into the international Ocean Drilling Program, using the JOIDES Resolution. Similar in appearance to a freighter, but with a tall well-drilling derrick amidships, the drill ship is equipped with a highly accurate satellite-controlled navigation system and elaborate equipment to position and steady the ship while on station. The scientific operations carried out on board consist of continuous seismic and magnetic surveys while underway, in-hole measurements, and laboratory analysis of the cores recovered.
  
  
• On Dec. 9 1998, the 143m long scientific drill ship JOIDES Resolution is scheduled to sail from Fremantle Australia to begin a 40-day drilling program near Kerguelen Island in the southern Indian Ocean. More than 30 nations subscribe to the international drilling program. Scientists in each country have the opportunity to use the vessel to address their scientific priorities.