Methods of dating fossils and rocks

Two methods for dating fossils – Festival Vallenato

methods of dating fossils and rocks

There are two main methods determining a fossils age, relative dating and Absolute dating is used to determine a precise age of a rock or fossil through. This activity on determining age of rocks and fossils is intended for 8th or 9th 5) To use radiometric dating and the principles of determining relative age to. Dating of animal and between parts of dating in geologic time. Different methods show that the radiometric dating of the sedimentary rocks. Left and fossils as.

Certain naturally occurring elements are radioactive, and they decay, or break down, at predictable rates. Chemists measure the half-life of such elements, i.

  • Accuracy of Fossils and Dating Methods

Sometimes, one isotope, or naturally occurring form, of an element decays into another, more stable form of the same element. By comparing the proportions of parent to daughter element in a rock sample, and knowing the half-life, the age can be calculated. Older fossils cannot be dated by carbon methods and require radiometric dating. Scientists can use different chemicals for absolute dating: The best-known absolute dating technique is carbon dating, which archaeologists prefer to use.

However, the half-life of carbon is only years, so the method cannot be used for materials older than about 70, years. Subtle differences in the relative proportions of the two isotopes can give good dates for rocks of any age. Scientists can check their accuracy by using different isotopes. The first radiometric dates, generated aboutshowed that the Earth was hundreds of millions, or billions, of years old.

Since then, geologists have made many tens of thousands of radiometric age determinations, and they have refined the earlier estimates.

Age estimates can be cross-tested by using different isotope pairs.

methods of dating fossils and rocks

Results from different techniques, often measured in rival labs, continually confirm each other. Every few years, new geologic time scales are published, providing the latest dates for major time lines.

Older dates may change by a few million years up and down, but younger dates are stable. For example, it has been known since the s that the famous Cretaceous-Tertiary boundary, the line marking the end of the dinosaurs, was 65 million years old. Repeated recalibrations and retests, using ever more sophisticated techniques and equipment, cannot shift that date. It is accurate to within a few thousand years.

The strict rules of the scientific method ensure the accuracy of fossil dating. Conclusion The fossil record is fundamental to an understanding of evolution. Fossils document the order of appearance of groups and they tell us about some of the amazing plants and animals that died out long ago. Fossils can also show us how major crises, such as mass extinctions, happened, and how life recovered after them.

If the fossils, or the dating of the fossils, could be shown to be inaccurate, all such information would have to be rejected as unsafe.

Geologists and paleontologists are highly self-critical, and they have worried for decades about these issues. Repeated, and tough, regimes of testing have confirmed the broad accuracy of the fossils and their dating, so we can read the history of life from the rocks with confidence. The same rock formation also contains a type of trilobite that was known to live to million years ago. Since the rock formation contains both types of fossils the ago of the rock formation must be in the overlapping date range of to million years.

Studying the layers of rock or strata can also be useful.

Dating Fossils – How Are Fossils Dated?

Layers of rock are deposited sequentially. If a layer of rock containing the fossil is higher up in the sequence that another layer, you know that layer must be younger in age. This can often be complicated by the fact that geological forces can cause faulting and tilting of rocks.

Absolute Dating Absolute dating is used to determine a precise age of a rock or fossil through radiometric dating methods. This uses radioactive minerals that occur in rocks and fossils almost like a geological clock. 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.

methods of dating fossils and rocks

The atoms in some chemical elements have different forms, called isotopes. These isotopes break down at a constant rate over time through radioactive decay.

Continue through a total of 4 to 5 timed intervals. That is, each team should stop according to their TIME paper at the end of the first timed interval 2 minutesor at the end of the second timed interval 4 minutesand so on. After all the timed intervals have occurred, teams should exchange places with one another as instructed by the teacher.

The task now for each team is to determine how many timed intervals that is, how many half-lives the set of pieces they are looking at has experienced. The half life of U is million years.

methods of dating fossils and rocks

Both the team that turned over a set of pieces and the second team that examined the set should determine how many million years are represented by the proportion of U and Pb present, compare notes, and haggle about any differences that they got. Right, each team must determine the number of millions of years represented by the set that they themselves turned over, PLUS the number of millions of years represented by the set that another team turned over.

Pb atoms in the pegmatite is 1: Using the same reasoning about proportions as in Part 2b above, students can determine how old the pegmatite and the granite are. They should write the ages of the pegmatite and granite beside the names of the rocks in the list below the block diagram Figure 1. This makes the curve more useful, because it is easier to plot it more accurately. That is especially helpful for ratios of parent isotope to daughter isotope that represent less than one half life.

For the block diagram Figure 1if a geochemical laboratory determines that the volcanic ash that is in the siltstone has a ratio of U If the ratio in the basalt is 7: Students should write the age of the volcanic ash beside the shale, siltstone and basalt on the list below the block diagram.

Why can't you say exactly what the age of the rock is? Why can you be more precise about the age of this rock than you could about the ages of the rock that has the trilobites and the rock that contains acritarchs and bacteria?


Based on cross-cutting relationships, it was established that the pegmatite is younger than the slate and that the slate is younger than the granite. Therefore, the slate that contains the acritarch and bacteria is between million years and million years old, because the pegmatite is million years old and the granite is million years old. The slate itself cannot be radiometrically dated, so can only be bracketed between the ages of the granite and the pegmatite.

The trilobite-bearing limestone overlies the quartz sandstone, which cross-cuts the pegmatite, and the basalt cuts through the limestone. Therefore the trilobites and the rock that contains them must be younger than million years the age of the pegmatite and older than million years the age of the basalt. The limestone itself cannot be radiometrically dated, so can only be bracketed between the ages of the granite and the pegmatite.

methods of dating fossils and rocks

The Triceratops dinosaur fossils are approximately 70 million years old, because they are found in shale and siltstone that contain volcanic ash radiometrically dated at 70 million years. Any Triceratops found below the volcanic ash may be a little older than 70 million years, and any found above may be a little younger than 70 million years. The age of the Triceratops can be determined more closely than that of the acritarchs and bacteria and that of the trilobites because the rock unit that contains the Triceratops can itself be radiometrically dated, whereas that of the other fossils could not.