Oceans occupy about 70 per cent of the Earth’s surface.

Oceans occupy about 70 per cent of the Earth’s surface. There are five oceans in the world. They are the Atlantic, Pacific, Indian, Arctic and Antarctic oceans. The surface under the oceans is called the ocean floor. Like land, the ocean floor also has natural features like plains, valleys and mountains. Quick Q’s: 1. Which is the largest of all oceans? The Pacific Ocean is the world’s largest and deepest ocean. It has an average depth of over 4,000 metres (13,100 feet). It has the world’s deepest trench – the Mariana Trench near Japan. The Challenger Deep in the Mariana Trench is the deepest point on Earth – about 11,033 metres (36,200 feet) deep. 2. What causes an ocean current? An ocean current is a mass of water that keeps moving in one direction. Surface currents are caused by wind and the Earth’s rotation. Underwater currents are the result of differences in temperature and salt content of the water. 3. What is a black smoker? When water seeps into the crust of the ocean floor through cracks, it may be heated by the magma below. As pressure builds up within the crust, the hot water shoots up through these cracks. These jets of warm water are often black due to their mineral content, so they are called black smokers. Q Which ocean is also known as the Southern Ocean? A The Antarctic Ocean is also called the Southern Ocean. Until recently, the Antarctic Ocean was considered to be a part of the other main oceans, as it was actually formed from parts of the Pacific, Atlantic and Indian oceans. In the year 2000, however, it was officially named the ‘Southern Ocean’. Q What is the ocean floor like? A The ocean floor is far from flat. The edges of islands and continents gently slope into the surrounding water to form an area called a continental shelf that is higher than rest of the ocean floor. A continental shelf usually extends about 75 kilometres (47 miles) out to sea but some, like the Siberian shelf in the Arctic Ocean, can extend up to 1,500 kilometres (932 miles). The continental shelf contains large deposits of petroleum, natural gas and minerals. It also receives the most sunlight, so marine life thrives here. The point where the continental shelf starts to plunge steeply towards the deep ocean floor is called the continental slope. It is here that the deep canyons of the ocean are found. 34 Q What causes the formation of mountains and valleys on the ocean floor? A Like the rest of the Earth, the ocean floor is divided into tectonic plates. The movement of these plates is responsible for features like ridges, trenches, and valleys. Ridges are formed when two plates drift apart. Boiling rock from inside the Earth, called magma, oozes out through the cracks between the plates and cools to form a ridge. Trenches are formed when a heavier plate sinks down under a lighter one. Not a flat floor The ocean floor has mountains and valleys. Some of the mountains are tall enough to rise above the surface and form islands. Ocean currents The water in the oceans and seas is always in motion, due to the rotation of the Earth, the gravitational pull of the Sun and Moon, and the difference in temperature and salt content of the water. These movements form strong currents in the oceans, both at the surface and deep down. Oceans of the world The five oceans of the world cover over 70 per cent of the surface of the Earth. Pacific Ocean Atlantic Ocean Indian Ocean Antarctic Ocean Arctic Ocean Island Arc Active Volcano Oceanic Trench N. Equatorial N. Equatorial N. Equatorial S. Equatorial S. Equatorial S. Equatorial West Wind Drift GrBigBk_Q_A 026-041.qxd 21/5/07 12:15 pm Page 34 Oceans 35 Q What is a mid-ocean ridge? A The ridges on the ocean floor are connected to form a single chain called a mid-ocean ridge. The mid-ocean ridge is over 80,000 kilometres (50,000 miles) long and is the longest mountain chain on Earth. On average, these mountains lie about 2,500 metres (8,200 feet) below the ocean surface, with their peaks sometimes breaking above. Mountains down there The mid-ocean ridge exists on the floor of all the oceans in the world. The diagram shows a part of the mid-Atlantic ridge, which extends below Iceland. Many volcanoes lie along the ridge. The breaker Breakers are higher when the seabed slopes down quickly from the shore. The beaches of Hawaii and Australia are famous for their high breakers. Q How are waves different from tides? A Waves are caused by wind, while tides are the regular rise and fall of the ocean’s surface caused by the gravitational pull of the Sun and the Moon on the water that is in the ocean. Waves are formed when winds blow over the surface of the ocean. Stronger winds create larger waves. The water in a wave normally moves in circles. As a wave approaches the land it is slowed down by the rising slope of the seabed. But it is the bottom portion of the wave that is slowed down. The top part of the wave keeps moving and crashes on to the shore as a breaker. Feeling the heat The temperature of the water from an underwater hot spring can be as high as 400 °C (752 °F). However, this water is rich in minerals, helping some unusual creatures like giant tubeworms and eyeless shrimps survive in an environment where nothing else can live. North American Plate Iceland Atlantic Ocean Mid-Atlantic Ridge Eurasian Plate Deep water Beach Surf Q How are volcanic islands formed? A Volcanoes under the sea are responsible for the formation of volcanic islands. As magma keeps oozing out of a volcano, it can collect, causing the volcano to grow and rise above the ocean surface as an island. Q What is special about the Hawaiian group of islands? A Volcanic activity does not always take place near plate boundaries. There are some places deep inside the Earth that are much hotter than others. As a result, there is constant volcanic activity above these spots, known as hot spots. This activity leads to the formation of underwater volcanoes. The constant movement of tectonic plates eventually shifts the volcano away from the hot spot. Soon, another volcano is created in the area near the hot spot. This often leads to the formation of a chain of islands, such as the Hawaiian Islands.