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If the core of the Earth is super hot and the mantle is liquid magma, why is the crust so much cooler?
Answer 1:

First, let’s talk about heat: a hot object has more energy with a cold object, and will share its energy if the two are touching. They can share energy in three ways, from fastest to slowest:

1 – direct contact
2 – indirect contact, like heating the air around the objects
3 – radiation of light (This doesn’t need air to work.)

When the Earth was formed, it was very hot, with molten rock at the surface of the planet. There wasn’t a quick way for the Earth to cool, though, since it wasn’t in direct contact with something cooler and there’s no air in space to absorb the energy. Instead, it could only lose heat by radiation, which is a slow process. So, the Earth cooled slowly and a solid crust formed that was cooler than the inside. This layer also insulates the molten rock below, by preventing heat from escaping by radiation. Instead, the molten rock has to heat the crust and atmosphere, which then radiates the heat into space. So, the Earth is cooling from the outside in but this process is very slow.

You can observe this process the next time you have soup. When you pour hot soup into a cold bowl, the bowl heats up quickly since the soup is in direct contact with the bowl. The top layer of soup, in contact with air, will cool next. If it’s a creamy thick soup, the top layer may become a solid film that you can poke with a spoon. The film is cool but the soup at the bottom of the bowl is hotter.


Answer 2:

The mantle is actually solid, because of the pressure it's under.

The crust is cooler because heat from the crust escapes into space.



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