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| Sunday, 8 January 2006 |
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Junior Observer |  |
| News Business Features Editorial
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The depths of oceans Have you ever wondered how the ocean would look without all that water? How it would look if it were just dry land? Well, if you're standing on the shore, watching the sandy beach disappear beneath the waves, you may think that it is probably like a big sandy desert. It's not! The ocean floor is as varied and irregular as the land we can see. It has mountains and plains, and valleys and ridges, and volcanoes and just about any other land feature you could name. The deepest ocean trenches could easily swallow up the tallest mountains on land. Around most continents are shallow seas that cover gently sloping areas called continental shelves. These reach depths of about 650 feet (200 m). This shelf is relatively shallow, tens of metres deep, compared to the thousands of metres of depth in the open ocean, and extends outward to the continental slope where the deep ocean truly begins. Sediment from the erosion of land surfaces, washed into the sea by rivers and waves, nourishes microscopic plants and animals. Larger animals then feed upon them. These larger animals include the great schools of fish, such as tuna, menhaden, cod and mackerel, which we catch for food. The continental shelf regions also contain the highest amount of benthic life (plants and animals that live on the ocean floor). The continental shelves end at the steeper continental slopes, which lead down to the deepest parts of the ocean. Beyond the continental slope is the abyss. The abyss contains plains, long mountain ranges called ocean ridges, isolated mountains called seamounts, and ocean trenches which are the deepest parts of the oceans. In the centres of some ocean ridges are long rift valleys, where earthquakes and volcanic eruptions are common. Some volcanoes that rise from the ridges appear above the surface as islands. Other mountain ranges are made up of extinct volcanoes. Some seamounts, called guyots, are extinct volcanoes with flat tops. Most seamounts began life as volcanoes formed over hot spots in the ocean floor. After the crust moves off the hot spot, the volcanic activity stops. Seamounts are usually 25 miles (40 kilometres) in diameter and can be 10,000 to 15,000 feet (3000 to 4500 metres) tall. In fact, some are so tall that their peaks pierce the ocean surface, forming a volcanic island or, if there are more than one seamount, a volcanic island chain (e.g. the Hawaiian Islands). Oceanographers (people who study the ocean) know these features exist because much effort has been spent on mapping the ocean bottom. In order to make maps of the ocean floor, the depth of the ocean must be known in many places. In the early days of ocean exploration, sailors made depth determinations called soundings, by means of a lead line. This was simply a long piece of rope, marked off in fathoms (six-foot intervals) and having a lead weight at one end. The depth was measured by dropping the weight into the water, and noting how much line went out when the lead weight reached the bottom. Taking soundings this way is time-consuming, especially in deep water. Today, most depth measurements are made using an echo sounder. Instead of dropping a weight, a pulse of sound energy is transmitted electronically towards the bottom. The time it takes the pulse to travel to the bottom and be reflected back up to the surface is measured. From this time interval, the depth of the water can be calculated. When pulses are sent out and received in quick succession, an almost continuous recording of the ocean depth called a bottom profile may be obtained. The sedimentary rocks that exist on the ocean bottom are much younger than any similar rocks found on the continents. The cores of mud and rock, brought back by deep-sea drilling ships, vary greatly in age, but no deposits from the ocean floor seem to be more than about 200 million years old. This makes oceanic crust very young compared with the continents, which contain rocks up to about four billion years old. *** Facts about the ocean Area: about 140 million square miles (362 million sq km), or nearly 71 per cent of the Earth's surface. Average depth: 12,200 feet (3,720 m). Deepest point: 36,198 feet (11,033 m) in the Mariana Trench in the western Pacific. Mountains: The ocean ridges form a great mountain range, almost 40,000 miles (64,000 km) long, that weaves its way through all the major oceans. It is the largest single feature on Earth. Highest mountain: Mauna Kea, Hawaii, rises 33,474 feet (10,203 m) from its base on the ocean floor; only 13,680 feet (4,170 m) are above sea level. |
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