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erates huge amounts of heat, which makes the rock of the man- tle extremely hot. The mantle rock is a lot hotter than the lava that pours from volcanoes, but it stays solid because it is under pressure due to the huge weight of rock above it. This raises the melting point of the mantle rock and stops it from becoming liquid. Yet, despite this, the mantle rock can flow sluggishly, like soft clay. As heat diffuses from the Earth’s core, it generates convec- tion currents , just like the currents you see churning through a pan of boiling soup on the kitchen stove. And just as the soup boils up and around, so the mantle rock surges up toward the cool, hard crust of the Earth. As it moves away from the core, the mantle rock spreads out, cools, and sinks again. Basalt is formed from molten minerals that boil up from the mantle. The basalt of the ocean floor is lighter than mantle As the Earth was cooling down after its formation 4.6 billion years ago, huge clouds of volcanic gases poured from its interior. These included vast amounts of water vapor that eventually cooled and condensed to form the oceans. The volcanic gases also included chlorine—the gas that gives public swimming pools their peculiar smell. The chlorine was dissolved in the early oceans, and it is possible that they smelled a bit like swimming pools. Overtime, however, rivers pouring off the continents picked up other sub- stances from the rocks and carried them into the oceans. These included sodium, a metal that reacts chemically with chlorine to produce sodium chloride, or salt. It is sodium chloride that gives ocean water its distinctive salty taste. Where Did Earth’s Water Come From?

What Are Oceans?

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