Eli5 Where does the dirt come from?


When looking at a geological timescale, typically ‘the deeper you dig, the older stuff gets’, right?
So, where does this buildup of new sediment come from?
I understand we’re talking about very large timeframes here, but I still dont really get it.

In: 19

The new dirt on top of one location came from dirt eroding away from another. The history in the one place gets buried, and the history of the other place gets erased. Whether a location gets buried or eroded can change with time, so some location might not have any dirt from the period of 5000YA-7000YA for example, but when digging the dirt from 7001YA will still be below the dirt from 4999YA.

From erosion, which is why river valleys and deltas have traditionally some of the best farming. Also from bio matter breaking down from plants and animals.

Decaying plant material, volcanic dust, erosion from mountains and rocks all create new layers and deposits burying ancient cities. https://youtu.be/EofirRBIh28

On land, you can easily see dirt forming around you. It’s the natural remnants of a crumbling world: trees decay, drop leaves, animals poop, living things die, rocks erode and turn to dust …all this matter becomes dirt. Now imagine a new volcanic rocky outcrop emerging from the ocean. It’s barren. But over time, life happens: seabirds poop on it. Lichen spores blow in on the breeze, settle down, begin growing on the rock, and eating into it. They emit acid which helps break down the rock a litttle, but also hold onto dust in the air, and trap it, accumulating it. Rainfall and wind helps grind down the rock. In just a few years, a barren rocky island in the middle of the ocean begins to form dirt on top, and soon offers a home to seaborne seeds which happenstance washes ashore, to become the pioneers of a future forest.

The rock cycle, and the life cycle.

[Rock cycle](https://en.wikipedia.org/wiki/Rock_cycle): liquid rock in the form of magma or lava comes to the surface and cools, becoming solid rock. This happens because of [plate tectonics](https://en.wikipedia.org/wiki/Plate_tectonics) and convection currents in the mantle. This process results in [mountains made of this new rock](https://en.wikipedia.org/wiki/Orogeny). Then, these mountains weather and erode. Weathering is when big rock breaks into small rock. Could be because of chemical reactions, water and wind wearing it down, or a bunch of other processes. Erosion is when the small rocks are moved away from where the big rock used to be. Wind, water, and gravity are the big movers. Eventually, a mountain will disappear and somewhere else gets a whole bunch of sand, silt, and dust.

Example: The great Plains of North America used to be [underwater](https://en.wikipedia.org/wiki/Western_Interior_Seaway). A mountain range we call the Ancestral Rockies weathered and eroded away and filled in this ancient sea. Then, plates collided and lifted up the whole western side of the continent even higher, draining whatever was left, and then weathered and eroded again, leaving behind today’s Rocky Mountains and Great Plains.

Life Cycle: Things live, grow, die, and decay. Soil is made of both tiny bits of rock (sand, silt, and clay) AND tiny bits of dead things (plants, microorganisms, bugs, etc). Healthy soil will also still have a lot of living things in it, mostly microorganisms (bacteria, fungi, really tiny worms, etc).

Example: Plants. Plants grow by taking CO2 out of the air and turning it into very complex carbohydrates like starch, cellulose, or lignin through photosynthesis. They quite literally create themselves out of thin air. By making this gas into something solid, they can add to the amount of dirt when they die. A tree spends its life turning CO2 into wood, and then it dies and all that wood decays and becomes dirt.