Activity 8.4 numerical dating of rocks and fossils

activity 8.4 numerical dating of rocks and fossils

How can isotopic dating be used to date fossils?

In most cases, we cannot use isotopic techniques to directly date fossils or the sedimentary rocks they are found in, but we can constrain their ages by dating igneous rocks that cut across sedimentary rocks, or volcanic ash layers that lie within sedimentary layers.

When was it possible to determine the age of a fossil?

It was only in the early part of the 20th century, when isotopic dating methods were first applied, that it became possible to discover the absolute ages of the rocks containing fossils.

How do geologists determine the age of rocks?

Fossils alone cannot provide us with numerical ages of rocks, but over the past century geologists have acquired enough isotopic dates from rocks associated with fossil-bearing rocks (such as igneous dykes cutting through sedimentary layers) to be able to put specific time limits on most fossils.

How to constrain the age of a rock based on fossils?

If the rock we are studying has several types of fossils in it, and we can assign time ranges to those fossils, we might be able to narrow the time range for the age of the rock considerably. An example of this is given in Figure 8.11. Figure 8.11 The application of bracketing to constrain the age of a rock based on several fossils.

How is absolute dating used to determine the age of fossils?

Absolute dating is used to determine a precise age of a fossil by using radiometric dating to measure the decay of isotopes, either within the fossil or more often the rocks associated with it. ... So, often layers of volcanic rocks above and below the layers containing fossils can be dated to provide a date range for the fossil containing rocks.

How do scientists date rocks and fossils?

Scientists use two approaches to date rocks and fossils. Relative age dating is used to determine whether one rock layer (or the fossils in it) are older or younger than another base on their relative position: younger rocks are positioned on top of older rocks.

What is the basis of isotopic dating?

Isotopic dating of rocks, or the minerals in them, is based on the fact that we know the decay rates of certain unstable isotopes of elements and that these rates have been constant over geological time. It is also based on the premise that when the atoms of an element decay within a mineral or a rock,...

Is it possible to use isotopes to date sedimentary rocks?

It might be possible to directly date some chemical sedimentary rocks isotopically, but there are no useful isotopes that can be used on old chemical sedimentary rocks. Radiocarbon dating can be used on sediments or sedimentary rocks that contain carbon, but it cannot be used on materials older than about 60 ka.

Listen - most geologists can do the simple stuff, like - ‘if it’s in a rock layer under that other one, then the layer underneath is older’. We can ALL do that. But accurate ages? Take it to people who have the right tools, which are specialists.

How do we determine the age of rocks and fossils?

It wasnt until well into the 20th century that enough information had accumulated about the rate of radioactive decay that the age of rocks and fossils in number of years could be determined through radiometric age dating. This activity on determining age of rocks and fossils is intended for 8th or 9th grade students.

Why might the age of a rock be several million years?

That range might be several million years because some organisms survived for a very long time. If the rock we are studying has several types of fossils in it, and we can assign time ranges to those fossils, we might be able to narrow the time range for the age of the rock considerably. An example of this is given in Figure 8.11.

Why is the time scale given by fossils only a relative scale?

But the time scale given by fossils is only a relative scale, because it does not give the age of the rock in years, only its age relative to other layers. Long after the relative time scale was worked out from fossils, geologists developed methods for finding the absolute ages of rocks, in years before the present.

How are fossils used to correlate rocks between regions?

This technique uses fossil assemblages (fossils of different organisms that occur together) to correlate rocks between regions. The best fossils to use are those that are widely spread, abundant, and lived for a relatively short period of time. Yet another technique, chronostratigraphic correlation, is to correlate rocks that have the same age.

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