LECTURE 3: HYDROSPHERE
The abundance of water on Earth's surface is a unique feature that distinguishes the "Blue Planet" from others in the solar system. The Earth's hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters down to a depth of 2,000 m. The deepest underwater location is Challenger Deep of the Mariana Trench in the Pacific Ocean with a depth of −10,911 m.[61][62] The average depth of the oceans is 3,794 m, more than five times the average height of the continents.[60]
The mass of the oceans is approximately 1.35×1018 metric tons, or about 1/4400 of the total mass of the Earth, and occupies a volume of 1.386×109 km³. If all of the land on Earth were spread evenly, water would rise to an altitude of more than 2.7 km.[63] About 97.5% of the water is saline, while the remaining 2.5% is fresh water. The majority of the fresh water, about 68.7%, is currently in the form of ice.[64]
About 3.5% of the total mass of the oceans consists of salt. Most of this salt was released from volcanic activity or extracted from cool, igneous rocks.[65] The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.[66] Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir.[67] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño-Southern Oscillation.[68]
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