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Sedimentary Rocks

The Making of Sedimentary Rocks


Graphic of erosion breaking rock up into smaller pieces

Breaking rocks up into smaller pieces. There are lots of processes that cause rocks to break apart into smaller pieces. No matter what causes the rock to break, we call the smaller pieces "sediment". A rock that forms from these smaller pieces of sediment is called a "sedimentary rock"! Keep in mind that even though sediments are 'smaller' pieces of rock, they might be still be the size of a house if they broke off from the side of a massive mountain. Regardless of whether they are the size of a pea or the size of a car, we call all pieces of rock that break off from other rocks "sediment".


Graphic of sediment transport

Moving those pieces.... Water, wind, and gravity are the main processes that move pieces of rock from place to place. Sediment might get transported thousands of miles by one of the world's major rivers, or it might just go from the top of the hill to the bottom during a landslide or as a result of natural "creep". During this journey, a lot can happen to the sediment. For example, it can continue to erode into smaller pieces during transport. This happens because a piece of rock bumps into other rocks during transport and breaks into smaller pieces (erosion!). So transport can also include some erosion, and the longer it takes to transport a rock, the more chance it has to erode.


Image of truck illustrating rock deposition

...until you stop moving the pieces. Once the sediment stops being transported, you have a bunch of pieces of sediment in one place. A pile of sand is an example; you might find such a pile at a sandbar along the edge of a river. You can also find sediment that has been deposited at the bottom of steep hills, at the beach, in sand dunes, and many other places on earth. Larger pieces are harder to transport, so they tend to stop moving (deposition) before smaller pieces. For example, a huge boulder cannot move in a tiny trickle of water, but can move during a raging flood. Similarly, a muddy river flowing into the ocean dumps most of the larger pieces of sediment near the coast in what we call a "delta," but the really tiny pieces stay suspended in the water and can travel far out into the open ocean before finally settling to the bottom.

You can try this in the classroom -- throw a scoop of sand and soil into a glass bottle. The largest grains of sand will fall out quickly, but the water at the top will remain muddy for hours. If you leave the bottle long enough, even the fine grains will settle and leave clear, clean water at the top of the bottle.


Graphic of cementation and lithification

Turn the individual small pieces back into a solid rock by cementing them together. So far, we've broken a rock into tiny pieces and moved the pieces from one place to another so that we have a pile of loose sediments -- not a hard rock. The evolution of sediment into rock typically takes thousands of years or longer in nature. Most sedimentary rocks are held together by the minerals calcite and quartz, which act like a cement. The combination of high temperatures and pressures speeds the process of cementation. If sediment continues to be deposited in the same place, newer layers of sediment will bury older sediment. The added weight of the newer sediment increases the pressure on the older sediment and squeezes the bottom layers. The layer of newer sediment also acts like a blanket insulating the lower layers, which causes an additional increase in heat. So as sediments get buried by other sediments, they can eventually become "as hard as a rock!"

The fact that burial is so important in the last stage of making sedimentary rocks also helps explain why sedimentary rocks tend to form in layers. Layers most often reflect individual pulses of deposition -- like individual floods, wet seasons, or even climatic periods lasting millions of years. If a river, lake, or ocean stays around for many years so that it experiences lots of deposition events, there will be layer on top of layer on top of layer in the same spot. Because the newest layers always form on the top (burying older layers), geologists can read these layers like the pages of a history book.


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