system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: The center of the earth

From what I understand thus far the center of the earth is a molten mass and we're living on the crust, which occasionally leaks by way of volcanoes and other vents. How is the earth still molten after all this time? Will it ever "burn out" like the sun is predicted to do? How important is the moltenness of the center for life to exist?
gpt
You're asking a LOT of questions here, too much for a single ELI5, I'll just answer the main one.

First, your description of the earth isn't quite accurate there fam. The centre of the earth is a ball of super-hot iron, then there's a massive layer of rock all around it that's so hot that it's plasticky. Then on top of that there's a crust.

The earth is still hot even after all this time for a few reasons.

* It's super super massive, and it would take an extraordinarily long time to cool down. There's so much heat down there that the insulation on the Earth's outside and the vacuum of space means it's not lost yet. It's kind of like its own thermos.
* The rock inside it is moving. It has a moon and a sun, and the gravity from these pulls the rocks just like it pulls the ocean tides. Heavier material sinks, lighter material floats. This movement of material creates force and friction which helps heat it just like rubbing your pant leg with your hand will warm your hand.
* The earth is radioactive inside, not much, but enough to generate quite a bit of heat.
* The sun continuously has provided a fair bit of warmth too, replenishing some during the "day" that bleeds into space at "night".

Some of these sources of heat are way more powerful than others, but they all add up to enough energy to keep the earth's "mantle" - the layer between the core and the crust - hot enough to occasionally form pools of underground liquid rock. These become volcanoes if they break through the crust to the surface.