Theory goes that the earth is a coalescense of materials in our area of the orbit, not once, but twice (including the moon forming collision). Helium being a noble gas would have to settle in using its own form. But that form would be incredibly light and likely rise to the top of our atmosphere, if not get blown off by solar wind, right? So how do we have helium here at all?
In: Earth Science
So you are right about the formation of the earth. What you are missing is that earth has a lot of radio active metals. Some of these decay into Helium atoms. That is how all helium came to be on earth.
Helium is a decay product of several nuclear reactions involving heavier elements (uranium, thorium, etc.).
Since this mostly happens in rocks, the helium typically can’t go anywhere and is stuck in place. It was generated here, and will continue to do so as long as we’ve got radioactive elements decaying within the earth.
The helium in our crust is actually the result of radioactive decay. Other unstable, heavier elements break down into helium.
Basically, the rocks are turning into helium.
We are weirdly lucky in this sense because nature for some reason decided that one of the main ways radioactive decay happens is by releasing a helium nucleus.
Radioactive elements often undergo alpha decay which releases a helium nucleus, and the electrons for this are just stolen from nearby atoms. This has built up over billions of years inside the earth by uranium and other elements decaying.
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