What causes the Earth to cool down?

It is actually quite simple. According to thermophysics, heat always moves from and area of high heat to an area of low heat. Space has no heat at all. It is extremely cold. So, the heat from the burning hot earth dissipated into space. It's just like a hot coal cooling down of you keep it in the freezer. Also, It is impossible for water vapour to condense into magma. Magma is made of rock. It is just melted rock. Water vapour can never ever become magma, because they are two completely different substances. Also, I think you are under a misconception. the tectonic plates were formed before the water vapour condesed. The earth is made of several layers. the outmost layer is the one with tectonic plates. However, the plates are very closely fitted together. when the earth was forming, there were deep hollows. These giant depressions, or pits, were filled up by the condensed water vapour and became the oceans

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Why doesn't the magma in the Earth's core cool down and solidify? What is the source of energy for maintaining its molten state?

The Earth's core does, in fact, cool down over time, and eventually it will solidify completely. Since the Earth's magnetic field (which protects the atmosphere and biosphere from harmful radiation) is generated by molten iron in the core, the solidification of the core might seem quite foreboding. Fans of the 2003 sci-fi film The Core will know what I'm talking about. Fortunately, the reader need not worry: let's see why.

First things first, let's establish the facts. The diagram below shows the interior structure of the Earth as a whole. Right now, the Earth's core has both solid and liquid components, which respectively make up the inner and outer core (shown in light and dark grey).

When the Earth formed, it would have been entirely molten due to the release of gravitational energy; at this time, the Earth became chemically differentiated, meaning that heavy elements (notably iron) mostly sank to the center to form the core while relatively light elements remained in the mantle and crust. The energy released by the formation and differentiation of the Earth is often called primordial heat.

It turns out that, if primordial heat had been all the Earth had to work with, the core would have completely solidified long ago, which is inconsistent with observation. As the question suggests, something else must provide additional heat to slow the solidification of the core; this alternative heat source so happens to be radioactivity.

As we noted before, heavy elements mostly ended up in the Earth's core. One might think, therefore, that the core contains most of the Earth's budget of radioactive substances. It turns out, however, the most important radioactive species on Earth -- uranium-235 and -238, thorium-232, and potassium-40 -- are lithophilic or 'rock-loving.' Lithophilic elements readily form chemical bonds with oxygen, which helps them to remain in the crust and mantle while others sink to the core.

As they decay, radioactive atoms release energy as radiogenic heat in the mantle. Much as an electric blanket keeps you warm on a cold winter's night, radiogenic heat has allowed Earth's core to remain hot and molten far longer than primordial heat. Specifically, the timescale for the core to cool and solidify is related to the half-lives of the species that supply radiogenic heat, which range between 700 million and 14 billion years. The Earth is currently about 4.57 billion years old, so there is plenty of "fuel" left to maintain a partly molten core.

(By the way, don't worry about the radioactivity in Earth's interior -- it is in no way dangerous to you, your loved ones, or your cat.)

In summary, the Earth's core is cooling very, very slowly; some of it has solidified, but it will take many billions of years for the rest to follow suit.

Correction: An earlier version of this article stated that most of the Earth's radioactive elements were in the core rather than the mantle. The Curious Team apologizes for this error.

In all the discussions about climate change, one thing is often overlooked – the upper atmosphere of Earth is actually cooling while the lower atmosphere grows warmer, and this strange paradox is a clear fingerprint of greenhouse gases at work.

Carbon dioxide in the lower atmosphere helps trap heat from the sun’s solar energy reflected off the Earth’s surface and so, like a greenhouse, the lower atmosphere grows warmer. But at high altitudes it is a different story, because the upper atmosphere is so thin that the carbon dioxide releases its energy into space and so the upper atmosphere cools. And that cooling is also making the upper atmosphere contract.

Past studies have shown that the cooling trend is driven by greenhouse gases, as well as shifts in the Earth’s magnetic field and the roughly 11-year cycle in the sun’s solar activity. A recent study disentangled these different factors by simulating the upper atmosphere at 100-500km altitude from 1950 to 2015. The results confirmed that rising carbon dioxide levels were the main driving force cooling the upper atmosphere. Shifts in the Earth’s magnetic field and variations in the solar cycle played much smaller roles.