Characteristics of Sedimentary Rocks

The geology of the earth can be a fascinating subject for people to study. Formed from the remains of other eroded rocks, sedimentary rocks have been studied ever since geology began. Only about 5% of the earth’s lithosphere is sedimentary but because they are close to the surface, 75% of the rocks seen are sedimentary. 

There are five main characteristics of sedimentary rocks. The first of these is layering or stratification. Stratification is a layer of rock that looks different from the band of rock above or below it. In other words, the rock will look like it has banded stripes of colour in it. This can be caused by changes in what rocks are laid down over the years or pauses in time that allow a chemical change to occur in the already placed rock before another layer is placed over it. The most common cause of this though is considered to be a variation in whatever transported the rock to the deposit in the first place, for example a change in wind strength or ocean tide. 

Fossilization is next, as only sedimentary rocks have been found with fossils in them. Unlike igneous rocks which form from magma (which would destroy the fossils) and metamorphic rocks which are changed by high temperature and pressure, sedimentary rocks are formed at just the right temperature and pressure to allow fossils to survive intact. Additionally the layering characteristic of sedimentary rocks also helps to keep fossils intact.

Porosity is how great a capacity to hold water a rock has. Sedimentary rocks are all considered porous because the grains of the sediments are usually of completely different sizes, allowing for a lot of space in the rock for water to seep into.  All sedimentary rocks will be porous to some degree, and will be more porous than either igneous or metamorphic rocks. 

Rapid erosion is something that sedimentary rocks go through. Erosion is the process of breaking down surface material and transporting the grains to somewhere else through the mediums of wind, water or ice. Rapid erosion is when this happens over the course of short span of time, geologically speaking. Sedimentary rocks will show this process due to them being made up of the sediment of other already eroded rocks. 

Marks and imprints are often found in sedimentary rocks. These marks are called ripple marks and are often caused by the effect of water or wind on sedimentary rock. Water ripple marks will often be symmetrical while wind will produce more varied patterns in the rock. 

Even though sedimentary rocks make up just a small portion of the Earth’s crust their importance cannot be ignored. The study of these rocks has become the main source of knowledge for scientific fields like paleogeography and paleoclimatology. It also plays a large part in other Earth sciences, making sedimentary rock vitally important to our understanding of how life not only evolved, but also lived on this planet before humans.