Submarine earthquakes arising from tectonic plate movements under the oceans can lead to destructive tsunamis, as can volcanoes, huge landslides, or the impact of large meteorites.Ī wide variety of organisms, including bacteria, protists, algae, plants, fungi, and animals, live in the sea, which offers a wide range of marine habitats and ecosystems, ranging vertically from the sunlit surface and shoreline to the great depths and pressures of the cold, dark abyssal zone, and in latitude from the cold waters under polar ice caps to the colourful diversity of coral reefs in tropical regions. Tides may have a very high range in bays or estuaries. Tides, the generally twice-daily rise and fall of sea levels, are caused by Earth's rotation and the gravitational effects of the orbiting Moon and, to a lesser extent, of the Sun. Deep-sea currents, known as the global conveyor belt, carry cold water from near the poles to every ocean. The directions of the circulation are governed by factors, including the shapes of the continents and Earth's rotation (the Coriolis effect). Winds also create surface currents through friction, setting up slow but stable circulations of water throughout the oceans.

Winds blowing over the surface of the sea produce waves, which break when they enter the shallow water. Salinity varies widely, being lower near the surface and the mouths of large rivers and higher in the depths of the ocean however, the relative proportions of dissolved salts vary little across the oceans. The water also contains salts of magnesium, calcium, potassium, and mercury, amongst many other elements, some in minute concentrations.

The most abundant solid dissolved in seawater is sodium chloride. Humans harnessing and studying the sea have been recorded since ancient times, and evidenced well into prehistory, while its modern scientific study is called oceanography. The sea moderates Earth's climate and has important roles in the water cycle, carbon cycle, and nitrogen cycle. The word sea is also used to denote second-order sections of the sea, such as the Mediterranean Sea, as well as certain large, entirely landlocked, saltwater lakes, such as the Caspian Sea. The sea, connected as the world ocean or simply the ocean, is the body of salty water that covers approximately 71 percent of the Earth's surface. Coastal sea waves at Paracas National Reserve, Ica, Peru

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