|Exploration of the Oceans|
For centuries, exploration of the oceans was primarily limited to exploration on the surface of the oceans. Explorers sailed or rowed ships across the seas in search of new lands or natural resources.
Biological limits prevented humans from exploring beneath the surface. Three main issues prevented humans from exploring great depths of the ocean. First, humans must breathe air to survive, and humans can hold their breath for several minutes or less.
This does not provide much time to dive, explore, and return to the water’s surface. Second, the weight of water increases greatly as a diver descends into deep water. Finally, water temperature decreases with increasing depths. The temperature near the ocean floor is near freezing.
In the last half of the twentieth century, humans made great advancements in ocean exploration. Technological advancements greatly increased knowledge of marine biology (ocean life) and marine geology (ocean floor composition and structure). Humans and machines can now dive to great depths to explore the hidden world that lies below the surface of the ocean. Most of the vast ocean however, still remains unexplored.
Until the last several hundred years, humans had to rely solely on free diving to explore beneath the ocean’s surface. When free diving, the diver simply holds their breath underwater. Ancient peoples used free diving to gather pearls, mother-of-pearl, and sponges, and some pearls are still gathered today by free diving.
The depth of a free dive is limited by the diver’s ability to hold his breath and the risk of hypoxia. Hypoxia is a condition in which body tissues do not receive enough oxygen to function efficiently. Hypoxia can lead to anoxia, or the absence of oxygen in tissues, and death.
The invention of the diving bell in the sixteenth century allowed divers to remain underwater for a longer period. A diving bell is a large metal bell that is placed underwater, trapping air from the surface inside the bell.
This principle can be observed by turning a glass upside down and plunging it into a full sink or bathtub. A diver could explore underwater, but was required to return to the diving bell for fresh air. The diver returned to the surface before the oxygen supply in the trapped air inside the diving bell was exhausted.
By the eighteenth century scientists improved the diving bell and also created diving suits. Like diving bells, diving suits relied on air supplied from the surface to fill the helmet of the sealed suit. Often a long air hose and a series of hand pumps supplied the air to divers.
These improvements allowed divers to explore underwater to depths of 60 feet (18 meters) for over one hour. By the nineteenth century scientists began to develop diving systems that did not rely on fresh air from the surface. Divers instead carried a supply of air or oxygen with them.
Any diving system in which a diver does not rely on surface air is called scuba diving. Scuba stands for Self Contained Underwater Breathing Apparatus. As pure oxygen can be harmful to the central nervous system at depths below 25 feet (7.6 meters), modern scuba equipment contains a mixture of helium, nitrogen, and oxygen.
French ocean explorer Jacques-Yves Cousteau (1910-1997) along with Canadian engineer Emile Gagnan invented modern scuba gear in 1943. Cousteau and Gagnan’s scuba equipment contained a tank of air with a tube for the diver to breath through.
Cousteau and Gagnan perfected a regulator for scuba gear that allowed divers to obtain compressed air from a tank simply by breathing normally through a tube. Until this invention, divers had to turn a valve on and off to control the flow of air from a diving tank.
The scuba equipment of Cousteau and Gagnan made scuba diving a popular sport for millions of people who enjoy the underwater world of coral reefs (a tropical marine ecosystem made up of tiny coral animals and the structures they produce) and aquatic animals and plants that remained hidden for most of human history.
humans and equipment from the cold. Bathyscaphes also carry large supplies of air that allow humans to breathe underwater for hours.
Bathyscaphes also protect humans and equipment from the pressure exerted by deep water. At sea level, air produces a pressure of 14.7 pounds per square inch. Scientists label this standard one atmosphere of pressure. The human body functions best at one atmosphere of pressure. At 33.8 feet (10.3 meters) below the water, the pressure doubles to 29.4 pounds per square inch, or two atmospheres.
The pressure increases by an additional atmosphere, 14.7 pounds per square inch, for every additional 33.8 feet below water. The deepest point of the ocean is Mariana Trench, at 35,802 feet below sea level, almost 7 miles (11 kilometers) below the ocean’s surface. At this depth, the water pressure is nearly 16,000 pounds per square inch.
In addition to manned vessels, scientists have invented numerous types of unmanned submersibles called autonomous underwater vehicles (AUVs) or remote-operated vehicles (ROVs). These unmanned vessels prevent lives from being placed in danger while exploring the oceans; they often enter shipwrecks and other places usually dangerous for manned submersibles. Unmanned underwater vehicles are normally more economical to operate than manned submersibles.
Mapping the oceans
Below the surface of the ocean, there is an underwater world of topographic (surface) features similar to that on land. Mountain ranges, hills, volcanoes, and trenches lie on the sea floor. Most of these features remained undiscovered until the twentieth century.
Scientists now map the features of the oceans to provide information for the military, geologists (scientists who study earth and rock formations), seismologists (scientists who study earthquakes), and other marine scientists.
In the nineteenth century, scientists and shipping companies attempted to map the ocean near the coast. They were mainly interested in discovering reefs and underwater rocks that could pose a problem to ships. A process called sounding was used to produce these early ocean maps.
Sounding involved dragging a weighted rope along the sea floor. The rope would slacken when dragged up an underwater hill. The amount of rope taken up by the slack indicated the height of the hill. A crude map could then be made indicating the position of hills and valleys.
In the early twentieth century sonar allowed scientists to produce better maps of the ocean. Sonar stands for SOund Navigation and Ranging. Sonar equipment sends out a pulse of sound energy (all energy travels in waves) that travels about 4,500 feet (1,372 meters) per second.
When the sound wave hits an object, such as the sea floor, it bounces back to the source. By determining the length of time that the sound wave takes to return, scientists can calculate the distance of an object. When mapping the floor of the ocean, a sonar signal would take less time to return after striking a hill or mountain than when striking the bottom of a trench.
Using these calculations scientists are able to produce maps of the ocean floor. The drawback to conventional sonar is that a sonar beam covers a very narrow area, making mapping the entire ocean with sonar impractical.
A newer form of sonar, called sidescan sonar, allows scientists to map larger areas of the ocean at once. Sidescan sonar equipment is placed in the water and towed by a boat.
The equipment is usually towed several hundred yards (meters) above the ocean floor. Unlike active sonar, sidescan sonar emits signals over a wide path instead of straight down. This allows the sidescan sonar to create maps of an area tens or hundreds of miles (kilometers) across.
Until the last century humans were not able to explore far below the surface of the ocean. Scientists could only study species of plants, animals, and other organisms that lived near the surface. The invention of deep-sea submersibles has exposed a world of living organisms that lay hidden for millions of years. Scientists had long assumed that all organisms depended on sunlight for life.
Plants require sunlight to conduct photosynthesis, or the conversion of sunlight, water, and carbon dioxide into their food. Animals then rely on plants as the bottom of their food chain (the relationship between plants and animals where one species is eaten by another).
In the 1970s discoveries at the bottom of the ocean changed the assumption that organisms require sunlight for survival. Scientists found small communities of organisms on the ocean floor that were living without sunlight. These organisms depend on hydrothermal vents for survival.
Powered by volcanic activity, hydrothermal vents are geysers (hot springs) that spew out a fluid rich in chemicals and minerals. The temperature of some of the fluids from hydrothermal vents is nearly 750°F (399°C). The animals that live near these vents rely on chemosynthesis for survival. Chemosynthesis is the use of chemicals, rather than sunlight, for the production of energy
In addition to the discovery of new species of plants, animals, and microorganisms, recent ocean exploration has also led to new findings about animals that scientists assumed were extinct. In 1938 fishermen near South Africa caught an unusual looking fish.
Scientists later determined that the fish was a coelacanth. Before this discovery scientists had believed that the coelacanth had become extinct between 65 and 80 million years ago. Unchanged for hundreds of millions of years, many scientists call the coelacanth a living fossil.
The coelacanth is a fish that has a pair of lobed-fins in the front and an extra lobe on its tail. The coelacanth can use its front lobed-fins to "walk" on the ocean floor. In 1991 scientists used a submersible to record the first images of living coelacanths in their natural environment.
Ocean exploration has also revealed a deep sea landscape that is similar to land. Marine geology is the study of the formation and structure of underwater land and rock formation. Mountain ranges, hills, valleys, volcanoes, and trenches cover the floor of the ocean. Most of these features remained undiscovered until the twentieth century.
Advancements in ocean mapping and submersibles revealed the geology of the ocean floor. The Mid-Atlantic Ridge, a mountain range that stretches the length of the Atlantic Ocean, was not discovered until 1952. Mariana Trench, the deepest point in the ocean, was not discovered until 1951.
Ocean exploration has also increased the understanding of plate tectonics. The entire surface of the earth and the ocean floor is composed of large masses of land called tectonic plates. These tectonic plates constantly move over, under, or collide with each other. The movement of these tectonic plates creates mountains.
The movement of tectonic plates also causes volcanic eruptions and earthquakes. Most volcanic and seismic activity (earthquakes) occurs at the edges of tectonic plates. The area surrounding the Pacific Plate is one of the most volcanically and seismically active areas of the world. The Pacific Plate is a large tectonic plate that lies beneath the Pacific Ocean.
Volcanic eruptions and earthquakes occur as the Pacific Plate moves under several other tectonic plates. About three quarters of the world’s active volcanoes lie around the Pacific Ocean. For this reason, the area surrounding the Pacific Ocean is called the "Ring of Fire."