why do we find fossils in sedimentary rocks
Well, as you know there are three types of rock. There are,
and. Let's think about all of them and why some can contain, but the other can't. But first of all what is a fossil? Fossils are organic products which are preserved in the Earth's crust. This means that the fossil is usually formed on or under (but not too deep! ) the or under the water. In other words in the biosphere (think about the biosphere limits). Then this organic material is covered by debris and etc. and becomes the fossil material. Now the igneous rocks, these rocks are usually formed from extremely hot magma deep inside the crust or with severe (not always) lava explosions. Obviously, magma and lava can't contain any lifeforms, because they come from the deepest and hottest places on the Earth where life is impossible. That's why fossils can't be found in the igneous rocks. The metamorphic rocks also can't contain any fossil as you'll see below. How does the metamorphic rock form? Simply the igneous rocks (Which CAN NOT originally contain any FOSSIL) or the sedimentary rocks (Which CAN originally contain some FOSSILS) undergo extremely high pressures or/ and temperatures.
But in these conditions rock's, even if they originally contained some organic material, structure will be altered. Finally, the sedimentary rocks are the one type which can contain fossils because these rocks are formed on the Earth surface, under the water etc. In other words, the biosphere can only interact with the sediments. The most common minerals in detrital sedimentary rocks are clay minerals and quartz but they can also consist of feldspars and micas. Detrital rock are distinguished by particle size as you can see in the table above. Apart from distinguish different sedimentary rocks the particle size can also tell us useful information about the environment where the particle once deposited. Big particle size needs stronger currents to move them and so big particle size implies that the particle was settled in an stronger current.
Less energy is required to transport small particles as clay and sand so then we know that fine sand can be transported by wind and smaller water currents and it takes a very calm environment for clay to settle and deposit. This is very interesting and by thinking about it we can figure out where a sedimentary rock was created. Shale Shale is a very common sedimentary rock that consists of clay and silt sized particles. Since the particles are so small they can not be seen without magnification. The particle size is very small and this means that it must have been deposited in a relatively calm environment such as deep-ocean basins or in lakes with not so strong currents. Other places where shale can form are lagoons and river floodplains. The special with shale is that this sedimentary rock has the ability to split into thin layers. This is because silt and clay particles in shale are so closely packed and the particles are also positioned parallel alignment to each other.
Although shale is the most common of sedimentary rock it is not so well known as the sandstone. The reason for that is probably that shale isn't so visible and many time the shale are covered with soil or are overgrown by vegetation. The soil comes from the shale itself since shale decompose easily. This is very obvious in places where shale and sandstone are present. In such places you can see sandstone that have dramatic forms with steep edges and the shale that has much less steep slopes and shale is also often the areas where vegetation are visible. Sandstone Sandstone is a rock that contains of sand sized grains and sandstone is the second most common sedimentary rock on earth and probably the most known. The history and origin of a sandstone can often be told by the sorting of the grains, by the particle size, the particles roundness and mineral composition.
For example, if the grains are rounded we can tell that the particle has been transported some distance by water. There are many different types of sandstones and the difference between them is due to the minerals in the stone. Grand Canyon, Arizona is a place where both Sandstone and Shale can be seen. Conglomerate and Breccia Conglomerate consists mostly of gravels. It can consist of large boulders and smaller gravel. The particles with large grain size can be seen visually and the spaces between the gravel are often filled with sand and mud. With the aid from the big particle size in conglomerate we can tell that there are indications of that the deposition have occurred in an environment with strong currents and/or slopes. Breccia is pretty much the same as a conglomerate but in a breccia the particles have angular shape instead of rounded shape. The angular shape tells us that the gravel hasn't been transported very long distance from where it was deposited.
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