Why are there distinct, meter-long layers in the sediment or stone, as seen in blasted hillsides, as opposed to a smooth blending of one layer to the next?


Basically, why aren’t the micro-layers of earth seamless? Or even the larger layers like, like from the mantle to the crust, for that matter.

In: Earth Science

Because stone is created over long periods of time. When you’re walking in the forest, the stuff you stand on is a layer of leaf litter.

Below that layer is a layer of decomposing leaf litter and other organic and inorganic material.

Below that layer is a layer of mulch, thick rich earth made of even older and more decomposed organic and inorganic materials.

The organic components of the soil are eventually used up by plants and other organisms and the inorganic remainder just become more and more compacted by pressure. As it compacts, it becomes less granular and porous until it eventually turns to rock.

How these layers get deposited can happen in many ways. Layers of leaves in a forest. Mud is deposited by rivers and tides. Sand deposited by the wind etc.

Since this is a gradual process that is fed by depositions on top, the cross-section looks like a bunch of stacked layers because that’s exactly what it is. Loose materials are added on top and the whole thing compacts in layers downward.

In terms of why the mantle and the crust are clearly differentiated, this is because they are each made of a different blend of minerals, forming completely different rock types. That is in fact the very definition of the change from crust to mantle.

Very broadly speaking, the crust is made from rock like basalt (oceanic crust) or stuff ranging from andesite to granite (continental crust, which is more variable). Then the mantle rock underneath is something known as peridotite, the biggest difference being that chemistry wise it has more magnesium and iron and less silica than the overlying crust, and mineralogically speaking it is mostly olivine and pyroxene, minerals which are largely absent from the crust as they either don’t form there very much in the first place or they get weathered into other minerals when they do occur in the crust.

This makes the boundary between crust and mantle quite a clear one, both in terms of seismic profiles (there is a change in speed of seismic waves when they go from crust to mantle, the boundary is known as the [Moho](https://en.wikipedia.org/wiki/Moho_discontinuity)) and in terms of the appearance which is visible in a few places where sections of the oceanic [lithosphere](https://en.wikipedia.org/wiki/Lithosphere) has been forced up and onto the continents in places geologists call [ophiolites](https://en.wikipedia.org/wiki/Ophiolite). In such places, the oceanic crust is dark, almost black rock known as basalt or gabbro which then turns into the peridotite rock of the mantle which can be quite a striking green colour due to the mineral olivine. The fresher the rock surface of peridotite, ie. the less time it has spent exposed to the elements, then the more striking green it will look. [Here is a particularly vivid example of peridotite from mantle rock exposed in an ophiolite](https://i.imgur.com/Lr3N1VU.jpg).

If you’ve managed to wade through all those “-ite” terms, then it’s worth pointing out that nothing I’ve said has included sedimentary rocks, which essentially just form a thin veneer over the top of much of the crust, thicker over certain parts of the continental crust. Sedimentary rocks in particular are associated with layering because that’s how they form — sediment (loose minerals weathered out from rocks elsewhere) makes its way into some sedimentary basin (could be a low point on land, or on the coast, or just offshore) where it accumulates and if it gets buried enough then it will turn into rock. There are distinct layers because either there is a change in the type of stuff getting dumped in the basin (so it might look different colours), or there is a change in the way that it gets layered. The latter leads to effects like [cross bedding](https://en.wikipedia.org/wiki/Cross-bedding) where you can see how different wind or water currents have lain down the sediment at different angles due to changes in the prevailing currents.