The hydrosphere. The World Ocean

The hydrosphere is the Earth’s water shell; water occurs in it as liquid, solid (ice) and vapour. Of the hydrosphere’s water, 96.5 % is in the World Ocean and 3.5 % in waters on land; nearly half of the land water is in glaciers, nearly as much underground, and rivers and lakes hold the least but are the most convenient for people. Sunlight evaporates ocean water, the vapour falls as precipitation and returns to the ocean through rivers – this is the water cycle. The World Ocean consists of four oceans: the Pacific (largest and deepest; larger than all the land together), the Atlantic (second), the Indian (third) and the Arctic Ocean (smallest and coldest). The deepest place is the Mariana Trench in the Pacific, about 11 km (modern measurements ≈ 10 935 m; the textbook gives 11 022 m). Seas are smaller parts of an ocean, a gulf is a shallow part reaching into the land, and a strait is a narrow passage joining two water bodies (Gibraltar). The ocean floor has a continental shelf (to 200 m), a continental slope, abyssal plains, undersea mountains and deep trenches. Depth is measured by an echo sounder: sound travels in water at ≈ 1500 m per second, so depth = 1500 × (time : 2).