Detrital or clastic sedimentary rocks are composed of rock fragments. They are different than chemical sedimentary rocks, which are composed of mineral crystals.
Learn how these sedimentary rocks differ in their formation and composition.
Sedimentary rocks are like clues that geologists use to piece together the past. For example, sedimentary rocks may hold fossils that reveal past life forms. These rocks can also contain cemented-together fragments of all different shapes and sizes that reveal bits of information geologists use to weave together the history of an area.In this lesson, we will take a closer look at two types of sedimentary rocks: detrital, which contain these cemented rock fragments, and chemical, which, as you might guess, are rocks that form due to a chemical process.
Detrital Sedimentary Rocks
Let’s start by looking at detrital sedimentary rocks. Detrital rocks are sometimes referred to as clastic sedimentary rocks because they are made up of clasts or rock fragments.
The word ‘detrital’ actually means ‘rubbing away,’ and we see that detrital rocks form when pre-existing rocks are rubbed away or weathered by forces such as water, ice and wind, leaving behind smaller rock fragments.So, you can say that detrital sedimentary rocks are composed of rock fragments that have been weathered from pre-existing rocks. Because these rocks are created by the breakdown of other rocks, they are the most common rocks we see on the surface of the earth.
As these pieces of rocks get chipped away, they get carried by nature and roll and tumble and break up even more until they find a resting place in a low-lying area, like a valley or the basin of a lake or ocean where the grains of sediment accumulate and get cemented together, forming layers or strata.
Categorizing Detrital Sedimentary Rocks
Detrital sedimentary rocks are mainly classified by the size of their grains. The grains are different sizes because of the amount of weathering they were exposed to. For example, if you have a fragment of rock located near the pre-existing rock it broke away from, it might be fairly large, say, the size of a basketball or larger. This would be considered a boulder.If that boulder got carried away from the rock it originated from by forces like water, for instance, then it might get broken down even more to the size of a softball. At this point, it would be called a cobble, like you might see in a cobblestone street of an old-time village.
The pieces could continue to weather and leave us with a grape-sized grain, or pebble, followed by grains of sand, like you would find on the beach, and then silt, which is very fine sand, and finally clay, which is the finest of them all and looks and feels like flour.
Kinds of Detrital Sedimentary Rocks
These grains of sediments are what get cemented together to form sedimentary rocks. So, if you have clay-sized grains cemented together, you will get shale.
If you ever want to impress someone by telling them that you can break rocks with your bare hands, then I suggest grabbing a piece of shale, because it deposits in fragile layers that are easily broken.If you have silt-sized grains cemented together, you have siltstone, which is somewhat like a gritty form of shale but without the layers.If you have sand-sized grains cemented together, you have sandstone.
Sandstone is a porous and permeable rock, which allows water to easily move through it. Because of this, sandstone makes a great aquifer for the underground storage of water.If you have rock that contains grains larger than sand, you have conglomerate if it contains large rounded grains, and you have breccia, meaning ‘broken,’ if the rock contains angular grains.
Rounded grains would be the result of sediments that were carried and smoothed by water, and angular or broken fragments could result from an event such as a rockslide down the side of a mountain.
Chemical Sedimentary Rocks
Chemical sedimentary rocks are a different type of sedimentary rock because they are not made up of weathered sediment grains; instead they are composed of mineral crystals that form out of solution. In other words, chemical sedimentary rocks contain crystals and other elements that have been dissolved out of water. One way this can happen is if a body of water becomes so saturated with minerals that the minerals no longer fit and there are too many for all of them to remain dissolved.
The result is that some of the minerals precipitate, meaning the solid mineral crystals separate out of the solution.This is similar to what happens when moisture builds up in a cloud to the point where the cloud is so saturated that it can no longer hold the water and the water is released as rain. We see some types of limestone formed in this way when the abundant mineral calcite precipitates out of solution.We can also see the formation of evaporites, which are mineral deposits left behind after water evaporates. If you have a body of water that has a high mineral content and that water evaporates, the minerals will be left behind to form evaporite minerals and rocks.
Rock salt or halite is an example of a mineral found in rocks that forms from the evaporation of water. Rock salt forms from the evaporation of salty water. When the water evaporates, the salt is left behind in the crystal form that we call rock salt, which has many uses.In fact, you might sprinkle some on your front porch steps in the wintertime to prevent the steps from icing over. You can also get rocks made out of other evaporite minerals like gypsum, which is the main ingredient in drywall or sheetrock that probably makes up the walls of your home.
Let’s review…Detrital sedimentary rocks, also called clastic sedimentary rocks, are composed of rock fragments that have been weathered from pre-existing rocks. They are the most common rocks on the surface of the earth.
Detrital sedimentary rocks are mainly classified by the size of their grain. The largest grain is a boulder, followed by a cobble, a pebble, sand, silt and finally the finest grain, which is clay.These grains of sediments are what get cemented together to form sedimentary rocks. So if you have clay-sized grains cemented together, you will get shale.
If you have silt-sized grains cemented together, you have siltstone. If you have sand-sized grains cemented together, you have sandstone. If you have rock that contains grains larger than sand, you have conglomerate if it contains large rounded grains and breccia if it contains angular grains.Chemical sedimentary rocks are composed of mineral crystals that form out of solution.
The minerals precipitate, meaning the solid mineral crystals separate out of the solution, much like what happens when a cloud fills with moisture and releases rain. We can also see the formation of evaporites, which are mineral deposits left behind after water evaporates. Rock salt or halite and gypsum are examples of minerals found in rocks that form from the evaporation of water.
After you have finished with this lesson, you should be able to:
- Describe detrital sedimentary rocks
- Identify different detrital rocks as classified by the size of their grain
- Explain how chemical sedimentary rocks form
- Define evaporites