Here are the 3 main techniques used to date rocks and fossils

Interesting Engineering summarizes all you need to know about how 'Earth' scientists date rocks, fossils, and geological events.
Sade Agard
Uplistsikhe rocks stock photo.
Uplistsikhe rocks stock photo.

ninochka/iStock 

  • Earth scientists, or more specifically, "geochronologists." have devised timelines for numerical ages and dates for materials and events using various techniques to date rocks, fossils, and geological events.
  • Radioisotope dating and paleomagnetism are just two of the many techniques used in dating events.
  • Assembling a historical record of our planet could be key to understanding what may occur in the future.

The Earth's surface has changed dramatically over the last 4.5 billion years. Mountains have been formed and eroded, continents and oceans have formed and moved great distances, and the planet has gone from being covered in molten magma to primarily covered in ice and then back to being relatively warm.

Not to mention the emergence of life and the record of mass extinctions, including the Cretaceous-Paleogene extinction event 66 million years ago, infamously known for ending the age of the non-avian dinosaurs.

These changes usually happen so slowly that they are entirely undetectable over the course of a human's lifespan. So, how do we know all this, then? How are we able to date, say, a newly discovered fossil, geological event, or, as highlighted above, the age of Earth?

Well, earth scientists, or more specifically, "geochronologists," have devised their own clock using various dating techniques to achieve this goal.

Interesting Engineering (IE) talks you through all you need to know about the three main approaches.

How do earth scientists date rocks and fossils?

Geologists use a wide variety of different techniques to address the question: "How old is this fossil/ rock?" These tend to fall into one of three general approaches that allow geologists to date rocks and fossils including the use of radioactive isotopes (absolute geochronology), and methods for relative dating such as measuring stable isotope ratios and paleomagnetism.

Simply put, relative dating arranges events or rocks according to their chronological order of occurrence. Absolute dating is more specific- it assigns more precise dates and times to fossils, rocks, or events. For example, lining up family members from oldest to youngest or guessing their ages based on appearance or other information is relative dating, while finding the exact age of each person is absolute dating.

What methods does relative dating use?

One common method for relative dating is stratigraphy or the study of rock strata. This involves applying a set of principles to sedimentary and volcanic rocks that are exposed at the Earth's surface in order to determine the relative ages of geological events preserved in the rock record.

Superposition

Here are the 3 main techniques used to date rocks and fossils
Sedimentary rock layers at Zabriskie Point, Death Valley, USA

One of the principles commonly used in stratigraphy is superposition. Generally, the top layers of a group of rocks ( 'formation') are younger than those below them. Naturally, this will only be accurate if the sedimentary layers have maintained their chronological order (in order). I.e., they have not been disrupted.

If sedimentary rocks are disturbed by events, such as fault movements, that cut across layers after the rocks were deposited, then the principle of cross-cutting relationships is used. This states that any geologic features that cut across strata must have formed after the rocks they cut through.

Examining these relationships allows the ordering of geological events in one location. However, it can't be used to determine the relative ages of rocks in different areas. In this case, fossils can be useful tools for understanding the relative ages of rocks.

Fossil traces have been found in rocks of all ages, with the simplest organisms discovered in the oldest rocks. The older the rock is, the more basic the organism is.

3.5 billion-year-old cyanobacteria from western Australian Archaean 'strata' (rocks) are the oldest fossils that have been found. This makes sense, considering that the oldest rocks are thought to be around 3.8 billion years old!

Fossil species that are used to distinguish one layer from another are called index fossils. Index fossils are usually fossil organisms that are common, easily identified, and found across a large area.

The principle of faunal succession states that if an unidentified fossil is found in the same rock layer as an index fossil, the two species must have existed at the same time period. Similarly, if the same index fossil is found in different areas, then it is likely that both strata were deposited at the same time. In this way, the relative age of fossils found at different sites can be determined.

Game: Can you see why A is the oldest and F is the youngest in the diagram below?

Here are the 3 main techniques used to date rocks and fossils
Diagram illustrating cross-cutting relations in geology.

What methods does absolute dating use?

Absolute dating, also called numerical dating, uses methods that provide chronological estimates of the age of geological materials or fossils.

Absolute dating techniques tend to work better for igneous and metamorphic rocks.

Absolute dating methods use a type of 'clock' to determine the date a rock or fossil was formed. A variety of radiometric dating methods are commonly used for this. These are based on the natural radioactive decay of certain elements, such as potassium and carbon. The rate of radioactive decay is constant over time and serves as a reliable clock.

Other methods used by geologists include electron spin resonance and thermoluminescence, which measure the effects of radioactivity on the electrons "trapped" in the crystal structure of a mineral.

Rock magnetism: 20,000 to billions of years.

Rock magnetism is another method that may be used to determine the age of a fossil. This technique uses the changing orientation of the Earth's magnetic field. The Earth acts like a massive magnet; it has a magnetic North and South Pole and a giant magnetic field that extends around it.

Throughout Earth's history, the magnetic poles have reversed, and geologists have developed a time scale to reflect when these have changes known as the "geomagnetic polarity time scale (GPTS)."

Here are the 3 main techniques used to date rocks and fossils
These rocks in Yoro River in Ichihara City, Chiba Prefecture, prove Earth's geomagnetism reversed about 770,000 years ago.

In the same way, as a compass needle will point toward magnetic north, magnetic minerals in rocks also point toward magnetic north. In this way, rocks record the orientation, or polarity, of the Earth's magnetic field.

Geologists cross-reference this with the Geomagnetic Polarity Time Scale (GPTS) — a record of the Earth's magnetic polarity — to generate ages between 20,000 to billions of years.

For example, a "geomagnetic reversal stratum" has been discovered in Ichihara City, Chiba Prefecture, along the Yoro River. It is a stratum that demonstrates how the N and S poles of Earth were reversed around 770,000 years ago (as pictured above).

However, because every magnetic reversal looks the same in the rock record, additional evidence is used to match the site to the GPTS. This includes information such as index fossils or radiometric dating to match a particular paleomagnetic reversal to a known reversal in the GPTS.

Radiometric Dating Method

Radiometric dating is an absolute or numerical dating technique based on the rate of natural radioactive decay of certain elements, such as potassium and carbon. It is a reliable geological clock for dating historical events or rock/fossil specimens. For instance, geologists use this technique to estimate how long ago rocks formed and to deduce the ages of fossils enclosed within those rocks.

The atoms of some chemical elements have different forms, called isotopes (isotopes contain equal numbers of protons but different numbers of neutrons in their nuclei, so they differ in relative atomic mass but not in chemical properties). These break down over time in a process that scientists call "radioactive decay." Each original isotope, called the parent, gradually decays to form a new isotope, called the daughter. For example, unstable Carbon-14 (parent isotope) decays into nitrogen-14 (daughter isotope) after emitting a beta particle.

Because this radioactive decay occurs at a stable rate over time, scientists can determine the amount of time that has elapsed since the formation of a rock by measuring the ratio of parent to daughter isotopes.

What equipment is used for radiometric dating?

Radiometric dating uses thermal ionization mass spectrometers to measure the isotopic ratios.

Here are the 3 main techniques used to date rocks and fossils
Thermal ionization mass spectrometer.

What are some radiometric dating methods?

The radiocarbon dating method: 1 - 70,000 years

The radiocarbon dating method relies on the uptake of a naturally occurring radioactive isotope of carbon, called carbon-14, by all living organisms.

When these organisms die, their carbon-14 uptake stops, and the radioactive clock is "set." Any dead organism intact with sedimentary deposits is a probable candidate for carbon-14 dating. However, the radiocarbon method is only valid for measuring things that were formed in the relatively recent geologic past up to 70,000 years. This is because, after 70,000 years or more, the amount of C-14 remaining will be too small to measure accurately.

This technique is suitable for dating organic materials such as bones, wood, charcoal, and shells.

Potassium-Argon dating: 1,000 - billions of years

Potassium-Argon dating is a method for determining the age of potassium-bearing minerals and rocks. It works by calculating the ratio of radioactive decay between isotopes of argon (Ar) and potassium (K-40) in the specimen. This method uses the decay of potassium-40 to argon-40 to date rocks older than 20,000 years up to billions of years.

K-Ar dating allows the dating of materials beyond the radiocarbon dating limit.

Uranium-Lead dating: 1,000 - 500,000 years

Here are the 3 main techniques used to date rocks and fossils
U-Pb dating revealed the geological age of mountains in Australia's Eastern Highlands.

Uranium-Lead dating involves the decay of uranium (U)-238 to lead (Pb)-206 for rocks older than 1 million years. This technique identifies the age of Uranium-bearing minerals. U-Pb dating is most often done on igneous rocks containing zircon. It has been used to determine the age of ancient hominids when combined with fission-track dating.

A previous IE article described small fragments of speleothem "rubble" taken from the Eastern Highlands of Victoria, Australia, for radiometric U-Pb dating. In doing this, scientists discovered that the mountains there were around five million years old - instead of 90 million years as previously thought.

Fission-track dating: 1,000 to billions of years.

Here are the 3 main techniques used to date rocks and fossils
Fission tracking: Tracks from 238U fission in a mineral.

Fission-track dating includes scanning the polished surface of a piece of rock and estimating the density of identifying marks or "tracks" left on it by the radioactive decay of U-238.

The uranium content of the sample is determined by placing a plastic film over the polished surface of the rock piece and bombarding it with low-energy neutrons. This bombardment generates new tracks, the number of which can be compared to the number of original tracks to calculate the age. Fission track dating can be used to reveal ages between 1,000 to billions of years.

Luminescence dating: A few decades to 100,000 years

Luminescence dating measures the quantity of light radiated from energy stored in specific igneous rocks to obtain a date range for a past event.

Here are the 3 main techniques used to date rocks and fossils
Luminescence dating is able to date ancient lava flows.

Luminescence dating methods cannot be technically labeled as radiometric, since they do not involve calculating ratios of radioactive isotopes. However, they do use radioactive material in samples. This material can be stimulated using light ('optically stimulated luminescence') or heat ('thermoluminescence'), which causes a signal to be released from the object.

This method is able to date archaeological specimens, such as ceramics, and minerals, like lava flows and limestones. It can date back a sample to a normal range of a few decades to 100,000 years.

Electron Spin Resonance (ESR): Up to 100,000 years old

Here are the 3 main techniques used to date rocks and fossils
EPR spectrometer set up at the Jagiellonian University.

Electron Spin Resonance (ESR) measures the accumulation of electrons in traps present in the crystal structure of the specimen. Suppose the amount of radiation directed at an object is constant. In that case, the number of electrons trapped in the imperfections in the crystal structure of the specimen will be proportional to the specimen's age.

This method dates rocks and fossils that are up to 100,000 years old. However, as specimens age, all of the "traps" in the crystal structures become occupied, and no more electrons can enter and accumulate.

"The past is key to the future"

Here are the 3 main techniques used to date rocks and fossils
Geologist Sir Charles Lyell.

These valuable techniques are critical to geologists studying rocks and fossils. Fundamentally, they underscore the reality of 'deep time' by revealing clues about what Earth looked like- long before the very first humans appeared.

Assembling a historical record of our planet from these methods helps us to understand the rate and nature of past geological events. Such knowledge could shed light on what may be of the future. As the famous geologist Sir Charles Lyell declared: "The past is key to the future."

Given the climate-conscious era we currently live in, don't you think we need as many clues as we can get for how Earth may respond to such changes?

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