Layers of crust from millions of years ago are in layers below the outermost crust, which would make sense if the earth was constantly growing in mass and size. Like in millions of years we will fossilize and structures will be buried but unless things just sink into the ground or there is mass added how do older layers of crust get underground?
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The solid rock that forms the crust and the continents FLOATS on the more dense liquid rock that makes up the [inside of the Earth](https://www.sciencealert.com/images/2021-03/processed/EarthsInnermostInnerLauer_1024.jpg).
So two things happen:
* Volcanoes spew out molten rock which then solidifies and becomes islands or mountains, basically adding to the crust. Over millions of years the wind and the sea erode these mountains, and the resulting sand and dust are transported to the bottom of seas and lakes, and to valleys and plains.
* At the [edges of the continents](https://qph.fs.quoracdn.net/main-qimg-1f2a192aebc28f0419ae40fbc41b354a), there are [subduction zones](https://epl.carnegiescience.edu/sites/dtm/files/subduction%20volcanoes_whiteback.png) where the “extra” crust is pushed down into the molten magma layer and “re-melted” basically.
So basically volcanic ridges under the oceans push out the molten magma, which solidifies and pushes sideways on the crust plates, and this pressure causes the other side of the plates to sink under and re-melt. The crust doesn’t grow upward, it just moves sideways and gets re-melted and recycled, over millions and millions of years.
[Here’s how the continents moved](https://www.youtube.com/watch?v=QhldiOaFqpE) in the past. The ocean-covered “spaces” between the continents is crust that was formed from molten magma and “pushed under” when it hit the continental plates.
There’s lots of factors that contribute, for example wind and rain will spread dust and mud over the surface, give it enough time and layers will be formed. What’s even faster is nature. If you have a paved road that goes through a pine forest and just leave it alone for a few years, the pine needles will cover it and turn into soil, hiding it completely. Leave it a few decades and you’ll have to dig deep to reach it.
The crust is constantly moving and being redistributed. Things like wind, mudslides, floods, earthquakes, sink holes, plant growth, and animal activities are all moving the crust that we live on. Volcanic eruptions and the areas around the tectonic plate edges are moving some of the molten layer to the surface as well. So over great lengths of time, things get buried by the natural movement of the ground beneath us.
The North Pole is ice floating on top of water with no land. It’s not exactly the same ice but there’s always ice there. Ice falls into water, water falls on ice and becomes ice in a basically endless and closed cycle. The crust is doing that too (continental drift is a thing), just much much slower at much much higher temperatures.
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