Why Fossils are Found in Sedimentary Rock
Most fossils are found in sedimentary rock because the fossils are not destroyed by high temperatures and high pressure. Fossils will be entombed in sediment which preserve them as the sedimentary rock forms through time.
Heat and pressure will have completely destroyed the fossils in igneous and metamorphic rocks. Sedimentary rock will not be of a part of a full cycle within the rock cycle if there are to be any fossils available within it. In some rare cases metamorphic rocks may contain some remnants of fossils as these rocks do not undergo a full melting point back to liquid magma.
Note that a majority of fossils will start by being captured in sediment which will then be compressed into sedimentary rock. Not all fossils form this way. There are other means where fossils can be obtained such as in amber and in frozen environments.
Sedimentary rocks are very important because fossils can be found within them. These fossil bearing rocks can help create a timeline in which important data is collected for dating the sedimentary rock and the fossil.
Fossils in Rock Layers
Rock layers are considered youngest to oldest starting from the earth’s surface and not oldest to youngest. As we dig down, rock layers will get older making any fossils contained within them, older as well.
When fossils in rock layers are discovered in sedimentary rock, the sedimentary rock is typically close to the surface of the earth unless you have been doing some digging. This means that some earthly event could have happened that pushed these sedimentary rocks to the surface instead of deeper within to be destroyed.
At times some very old layers containing some very old fossils end up toward the surface of the earth. The fossils contained within can help date how old the rock is and help geologists come to some conclusions about the area they are in.
Rock layers containing numerous fossils can be called fossil layers. This grouping of fossils trapped in a fossil layer of sedimentary rock may have been due to a disastrous event. The event destroyed the life all at a moment in time trapping them in a fossil layer.
Fossils Preserve Well in Sedimentary Rock
Fossils are preserved in sedimentary rock by finer sediment encapsulating the life. This preserves the fossil from elements such as air and water. Without the outside elements to completely decay the fossil, it can remain preserved for thousands of years.
Larger grained sediment is not very good at preserving trapped remains in sedimentary rock. This makes the sedimentary rock porous. A porous rock will allow in liquid more easily and it is also not as air tight.
A very good example of a porous sedimentary rock is sandstone. Though some fossils can be preserved in sandstone, the fossil does not look as full and defined like you would see in some limestones.
Why Fossils in Sedimentary rocks and not in Metamorphic Rocks
It is unlikely fossils will be found in metamorphic rocks because metamorphic rocks will undergo great amounts of pressure and heat which will destroy the fossil. Fossils, which are organic in nature, will have little chance surviving such harsh conditions to be embedded in a metamorphic rock.
That does not mean that it is not possible. I did use the word unlikely. Imagine a much bigger fossilized animal, or shell animal, captured and pushed deep within the earth. The heat and the pressure may be enough to alter rock into metamorphic rock and not entirely destroy the remains of a fossil.
In this case the heat and pressure would have to be on the lighter side of things and not for a long period of time.
Why Fossils in Sedimentary Rocks and not in Igneous Rocks
Fossils are not found in igneous rocks because for an igneous rock to be, it has to be melted to magma first. If fossils are in the rock when melted to magma, they cannot survive. They will not exist in fossil form when the rock cools as an igneous rock.
What kinds of fossils are found in sedimentary rocks?
The kinds of fossils found in sedimentary rocks are primarily water type fossils. Though all kinds of fossils can be found within sedimentary rocks, water creatures such as horn coral, fish, trilobites, shell fish, crinoid’s, and other sea life will be more abundant.
Plant life and mammals can also be found in sedimentary rocks. By what means that they get there can be many. You can think of mudslides, flash floods, or any manner of earthly violent activity that can drive these forms of life to end up in a sedimentary layer of rock.
If you are looking for more information on sedimentary rocks in the rock cycle, we have a great article “Diagram of the Rock Cycle Explained”.
For more information about sedimentary rocks you can read “What are Sedimentary Rocks”.
