The Hydrosphere

Over 70% of the Earth is covered by seas full of salty water. This salty water makes up about 97½% of all Earth’s water. The fresh water persons can drink is mostly ice. Only a very small amount is at hand in rivers and under the earth, for humans to drink and use. The air above the Earth stops the water from going away into outer space.
An ocean is a large area of salt water between continents. Oceans are very big and they join smaller seas together. Together, the oceans are like one “ocean”, because all the “oceans” are joined. Oceans (or marine biomes) cover 72% of our planet. The largest ocean is the Pacific Ocean. It covers 1/3 (one-third) of the Earth’s surface.
The smallest ocean is the Arctic Ocean. Different water movements separate the Southern Ocean from the Atlantic, Pacific, and Indian Oceans. The Southern ocean is also called the Antarctic Ocean because it covers the area around Antarctica. Older maps may not use the names the Arctic Ocean and the Southern Ocean.
The deepest ocean is the Pacific ocean. The deepest point is the Mariana Trench, being about 11,000 meters (36,200 feet) deep. The deep ocean is characterized by cold temperatures, high pressure, and complete darkness. Some very unusual organisms live in this part of the ocean. They do not require energy from the sun to survive because they use chemicals from deep inside the Earth.
The ocean floor, also known as sea-bed, also has mountains valleys and plains like that on land. A continental shelf is the part of the continent that is under water. The shelf was part of the land during the ice ages in the glacial periods. Beyond the continental shelf, the ocean floor goes down to much greater depths. The continental shelf is a shallow ocean. It varies in depth, up to 140 meters deep. It varies greatly in its width. At the leading edge of a moving continental plate, there will be little or no shelf. The shelf on a passive edge of a plate will be wide and shallow. The widest shelf is the Siberian shelf in the Arctic Ocean: it is 1500 km (930 miles) in width.
The shelf usually ends at a point of decreasing slope (called the shelf break). The sea floor below the break is the continental slope. The character of the shelf changes dramatically at the shelf break, where the continental slope begins. With a few exceptions, the shelf break is located at a remarkably uniform depth of roughly 140 m (460 ft); this is likely a hallmark of past ice ages when sea level was lower than it is now.
Most of the oceans have a common structure, created by common physical phenomena, mainly from tectonic movement, and sediment from various sources. The structure of the oceans, starting with the continents, begins usually with a continental shelf, continues to the continental slope – which is a steep descent into the ocean, until reaching the abyssal plain – a topographic plain area, the beginning of the seabed, and its main area. The border between the continental slope and the abyssal plain usually has a more gradual descent and is called the continental rise, which is caused by sediment cascading down the continental slope.
The mid-ocean ridge, as its name implies, is a mountainous rise through the middle of all the oceans, between the continents. Typically a rift runs along the edge of this ridge. Along tectonic plate edges, there are typically oceanic trenches – deep valleys, created by the mantle circulation movement from the mid-ocean mountain ridge to the oceanic trench.
An ocean current is a continuous movement of ocean water from one place to another. Ocean currents are created by the wind, water temperature, salt content, and the gravity of the moon. The current’s direction and speed depend on the shoreline and the ocean floor. They can flow for thousands of miles and are found in all the major oceans of the world. Ocean currents can be found on the water surface and deeper down.
Currents on the surface often depend on the wind. They travel clockwise in the northern hemisphere. They travel counterclockwise in the southern hemisphere. They are found up to 400 meters (1,300 ft) below the surface of the ocean. Deeper currents depend on water pressure, temperature, and salt content.

Ocean currents are classified into cold currents and warm currents. Generally, currents flowing from warm tropics to poles are warm currents and currents flowing from cold tropics to tropics are cold. They also regulate the temperature of the area through which it flows and causes variation in climate.
Hydrological cycle or water cycle is the cycle that water goes through on Earth. It includes the movement of water from water bodies to the atmosphere by evaporation and return of water from the atmosphere back by condensation and precipitation.
First, water on the surface of the Earth evaporates and gets collected as water vapor in the sky. This water vapor condenses when gets cooled to makes clouds. The small droplets in clouds coalesce to form larger clouds and the water falls from the sky as rain, snow, sleet, or hail which is called precipitation. This water sinks into the surface and also collects into lakes, oceans, or aquifers. It evaporates again and continues the cycle.

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