Why Devonian could not be considered an index fossils?
How can the occurrence of fossils and their known age ranges be used to date rocks?
Need help by understanding the question.
What does your text say about dating rocks, index fossils, and the Devonian period?
Fossil Occurrence and the Age of Rocks:
Groups of fossil organisms occur throughout the geologic record for specific
intervals of time. This time interval is called the fossil’s range. Knowing the
range of the fossils of specific organisms or groups of organisms can be used
to relatively date rocks and sequences of rocks. In this laboratory exercise,
you will use such information to assign a date to a hypothetical unit of rock.
The text above^^ is for this question: How can the occurrence of fossils and their known age ranges be used to date rocks?
Fossils died at the same time the rocks were formed.
I still don't understand.
how? why?
Please read about the three different kinds of rocks.
http://www.kidsloverocks.com/html/types_of_rocks.html
okay, and how about this question. Why Devonian could not be considered an index fossils?
I'm sorry, but I don't know. Perhaps if you read this site carefully, you'll find the answer.
http://www.britannica.com/science/Devonian-Period
1. Why Devonian could not be considered an index fossil?
To understand why the Devonian period may not be considered an index fossil, we need to first clarify what index fossils are. Index fossils are commonly used in stratigraphy to date the age of rock layers or geological formations. They are typically species that existed for a relatively short period but had a widespread geographic distribution.
The Devonian period, which lasted from approximately 419.2 million to 358.9 million years ago, does contain several species that could be considered index fossils, such as trilobites, ammonites, and brachiopods. However, the designation of an index fossil depends on certain criteria.
One crucial criterion is the species' occurrence and representation in a relatively narrow time range. Although the Devonian period represents a large span of time, index fossils are preferred to have a more precise temporal range. While some species from the Devonian period can be useful indicators for dating rocks within the period, their presence alone may not be specific enough to accurately determine the age of a rock layer.
Furthermore, the presence of index fossils is often used to establish correlations between rock layers in different locations. If an index fossil from a specific time range is found in multiple locations, it suggests that the rocks containing those fossils are of similar age. Since the Devonian period covers a vast expanse of time and diverse environments, it may not provide the consistency needed to serve as a reliable index fossil for correlations across different regions.
2. How can the occurrence of fossils and their known age ranges be used to date rocks?
Fossils and their known age ranges are valuable tools for relative dating of rocks. Relative dating involves determining the order of events or objects in relation to one another but does not provide an absolute age in years.
When using fossils for relative dating, several principles are applied. These include the Principle of Superposition, which states that in undisturbed rock layers, the oldest rocks are found at the bottom and the youngest at the top. The Principle of Faunal Succession is used to correlate rocks of similar age based on the presence of particular fossil assemblages characteristic of specific time periods.
By comparing the fossils found in different rock layers, geologists can establish a relative chronology of the rock formations. If a fossil is found in multiple rock layers, it is likely that those layers were deposited during the same geological time period when that organism was alive. This allows for the determination of the relative age of the rock layers above or below each other.
However, it's important to note that relative dating methods cannot provide an exact age in years. To obtain an absolute age, scientists would need to use radiometric dating techniques that measure the decay of radioactive isotopes in rocks. These methods provide numeric ages rather than just establishing relative order.