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I was listening to a geology podcast describing how sedimentary rock forms and later turns into metamorphic rock. As a river lays down sediment, the lower layers can eventually be several kilometers below ground, where heat and pressure are significant.

How does this happen? Does the river bed get built higher and higher until the first layer of sediment is buried deeply, or does the bottom layer get pushed down into the earth? In other words, is the elevation of a given section of river increasing while this happens, or does it remain the same?

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  • $\begingroup$ Sedimentary rock is usually created at the bottom of (shallow) oceans. $\endgroup$
    – Erik
    Jan 16, 2020 at 8:21

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The rivers transport most of the sediments to the coastal or marine basins where they accumulate. An exception are lacustrine basins, but most of world rivers sediments are deposited on shallow or deep marine basins.

When the stack of sediments is large, listric faults develop, and the bottom layer get pushed down into the earth. This process is called subsidence.

enter image description here

Source: apg-pro.com

As new sediments accumulate, faults are activated and the basin becomes larger. Elevation of a given section of the basin remains approximately the same, as this is a slow process.

In the case the bottom layer reaches great depth, burial metamorphism can develop:

"Burial Metamorphism: occurs when sedimentary rocks that had undergone diagenesis are buried even deeper. Diagenesis grades into burial metamorphism, a relatively mild type of metamorphism resulting from the heat and pressure exerted by overlying sediments and sedimentary rocks. Although partial alteration of the mineralogy and texture may occur, bedding and other sedimentary structures are usually preserved".

Source: uh.edu

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  • $\begingroup$ @Universal_learner You're right ! I confused normal and transform. I believe what Jean-Marie Prival means (apart from the graben which belongs to a larger system together with the Rhine Graben) is a "foreland basin" (link in my answer below). $\endgroup$
    – user18607
    Jan 16, 2020 at 13:29
  • $\begingroup$ The podcast host described how the river sorts debris: big rocks deposited early on when the current is strong, smaller ones as it slows down, all the way to fine clay particles that only settle in calm ocean waters, with each of them (as I understood it) being buried and turning into different kinds of metamorphic rock. That's why I pictured at least some of this happening on land. Am I confused about that? $\endgroup$
    – Nathan Long
    Jan 16, 2020 at 14:19
  • $\begingroup$ @Nathan Long Most of sedimentary basins are on oceans not on continents. Only great basins reach burial metamorphism gradient, most of them marine basins. As JMP noticed, lacustrine basins can have a huge amount of sediments, say 4000 meters. But this don't happens at rivers. There migth be a continental sedimentary basin deep enought were burial metamorphim happens, if you heard about one I will be happy if you quote it :) $\endgroup$
    – user18590
    Jan 16, 2020 at 14:23
  • $\begingroup$ Hmm. I know dynamic, contact, regional, shock and hydrothermal metamorphism. Most sedimentary basins en.wikipedia.org/wiki/Sedimentary_basin are on continental crust. Oceans get subducted quickly. Sediments are deposited on the shelves, sometimes trubidites run down the slopes and reach on the abyssal plains, but the vastness has, if at all, only brief sediments lying upon it, cosmic dust and so. Or am I confusing just another thing ? (Edit: I see, I didn't get the question it seems. Nevermind :-)) $\endgroup$
    – user18607
    Jan 16, 2020 at 14:34
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    $\begingroup$ You have an upvote from my side :-) $\endgroup$
    – user18607
    Jan 16, 2020 at 15:42
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No, sediments do not sink into the earth because they are too light. They stay on the surface and over very long times they form the continents. But they can get buried under many km of other sediments as parts of the crust they're on subsides or is pushed over other parts. Thus they can get into pressure/temperatue conditions where diagenesis (see other answer) or metamorphism alter them.

On a very general level, plate tectonics keep the crust in motion. It can cause the surface to rise in some places and subside in others. When it rises it forms mountains, for example in an orogeny over a subduction zone or when two continents collide and the crust between them is forced upwards. Weathering and erosion starts in the rising zones, and they produce a lot of sediments which are transported by water, wind and gravity down into lower areas. Otoh, where the crust is stretched or subsides, e.g. in Graben or basins, the sediments accumulate, layer by layer, and tell the long story of what has happened in the past (see @Universal_Learner's answer).

Another, somewhat more exotic process that forms thick sediment packages, is the scraping off of marine sediments from a subducting plate to form an accretionary wedge.

These sediment packages can get many kilometers thick, the Pannonian basin holds the sediments of a mountain range in thicknesses between 1 and 5 km, >6km in the Molasse basin north of the alps, up to 16km in the Upper Rhine Graben, where they can be altered metamoprhically (apart from contact or hydrothermal metamorphosis in regions with volcanism). Metamoprhic conditions ("facies") can also be met in a continent-continent collision and the connected mountain ranges, like parts of the Himalaya.

Rivers (i am not an expert in this and kindly ask for correction) do not rise above the surrounding (gravity is against it ;-)), but they change shape and appearance with the slope and sediment load. For example from "alluvial fans" at the foot of a mountain over "braided rivers" in mountainous areas where slope is still high and grain size big over meander patterns in low slope areas with small grain size to deltas filled with mud and clay.

When a river reaches an area where it can't carry on because all around the land rises again, it can build a lake until it finds a new exit, or it simply fills up the area with its sediments.

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The podcast may have given the impression that sediment always gets buried or pushed down into the earth. This isnt the reality of the processes which are dynamic and dependent on geologic time as well as where these process can or can't happen. It really depends on the location and the processes happing there.

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  • $\begingroup$ The podcast described burial metamorphism as a process of piling sediments up until the lower layers are transformed by heat and pressure. The idea of them sinking into the earth was my own; I'm just trying to picture how a stream could sort sediments as it flows and have burial metamorphism happen to the different sizes of sediment. $\endgroup$
    – Nathan Long
    Jan 24, 2020 at 16:35

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