HOW WE KNOW THAT THE EARTH IS CLOSE TO 4.5 BILLION YEARS OLD.

Wikimedia Commons
Before we proceed lets take a look at radioisotopes

Radioisotopes are simply radioactive isotopes(radiation-emitting isotopes).

What are isotopes?
Isotopes are any two or more forms of an element where the atoms have the same number of protons but different number of neutrons in their nuclei.

A hydrogen atom has one proton and one electron. However, hydrogen has an isotope called deuterium,which has one proton,one neutron and one electron.Heavy water refers to the water that each of its hydrogen atom contains one neutron.It is referred to as 'heavy' because the presence of the neutron has increased its atomic weight.

There are some isotopes that are stable,which means they are 'happy' with the number of electrons,protons and neutrons that they have.An example is the deuterium isotope. There are some isotopes that are unstable,which means they are 'unhappy' with the number of electrons, protons and neutrons that they have.

When an atom is unstable, the different numbers of neutrons interact with other atomic components in such a way that some of the bits go flying off and over time, the isotope disintegrates and changes into some other atom.
When these unstable isotopes change into some other atom,they emit radioactivity,that's why they're called radioisotopes.

An important thing about radioisotopes is that scientists can accurately describe the average probability of the transition(disintegration into some other atom) and express the probability as a number called 'half-life'.
Half-life is the time required for an isotope to lose one-half of its radioactivity.
In the first half-life, half of the atoms disintegrate,in the second half-life, half of the remaining atoms disintegrate,leaving one-quarter of the original parent material,in the third half-life, half disintegrate again leaving one-eighth and so on.

N/B: Because half of the isotopes decay in the first half-life does not mean that the remaining half will decay in the second half-life. Only half decay every half-life.

Here are a few isotopes and their first half-life decay rate:

• Iodine-131=8.1 days.
• Phosphorus=14.3 days.
• Cobalt-60=5.26 years.
• Radium-226=1,620 years.

Half-lives vary and can be measured with devices such as a scintillation counter.

There are several other dating methods i'd like to point out;

• Relative dating(Biostratigraphy,Paleomagnetism,Tephrochronology,Relative chronology).
• Radiometric dating(Radioarbon dating,Single crystal fusion,Uranium series dating).
• Trapped charge dating( Thermoluminesence,Optically stimulated luminesence and Electron spin resonance).

When scientists are trying to determine the age of a material, they use any or more of these methods.
After dating,there are results that are good(acceptable),bad(not really acceptable) and ugly(outright absurd).
For scientists to have confidence in their own results,the dating has to use at least two different methods repeated extensively and independently in laboratories and later cross-checked. If the result was produced by only one result, it reduces their confidence in the result.

Let's look at the dating methods, shall we?;

   RELATIVE DATING

• Biostratigraphy: This is the layer upon layer (strata) study of fossil distribution. Biostratigraphy was one of the first dating methods used by scientists and it is very easy to understand. It works this way --Rock layers are superimposed upon each other. And all we know about sedimentation tells us that the layers beneath are older than the layers on top i.e. age increases DOWN the rock layers.If a fossil is found in a particular layer,the age range of the fossil can be estimated by giving a range culled from the known ages of the fossils in different strata beneath it and above it.
  The strata above and beneath can also be dated using other methods like paleomagnetism and tephrochronology --to give them confidence in their results.

• Faunal association is like a sub branch of biostratigraphy,paleontologists can determine the age of fossils based on the known age of fauna found in the same layer of rock.

• Paleomagnetism: This is the study of the earth's magnetic field as it has changed over time in rocks, sediment or archaeological material. The earth's magnetic field dances around every 100,000 to 600,000 years and when certain materials in rocks form,they "lock and record" the direction and intensity of the earth's magnetic field during the time of their forming.

The record provides information on the past location of tectonic plates and past behavior of earth's magnetic field.
Magnetostratigraphy(the accumulative record of geomagnetic reversals preserved in volcanic and sedimentary rock sequences provides a time scale that can be used as a gerontological tool. Paleomagnetism is used as rough method to cross-check dates of archaeological materials.

• Tephrochronology: This is a method that uses distinct layers of Tephra --volcanic ash from a single eruption-- to create a time framework in which archaeological and Paleo environmental events can be placed.

Hours or days after a volcanic eruption, Tephra is deposited in a single layer with a unique Geo chemical fingerprint.
Paleontologists can apply an absolute dating method to that layer and use it as a time marker --using it to relatively date nearby layers.

e.g, the New Zealand Taupo Volcano erupted in AD 232.
Any layer below the Taupo tephra is older than 232 AD, while any layer above it is younger than 232 AD.

• Relative Chronology(stylistic seriation): This method has to do with progressive complexity or stylistic evolution.

In a culture, their artwork is dated based on its complexity; music,drawings, pottery,etc,carvings,sculptures,etc. For example, egyptologists created a relative chronology of pre-pharaonic Egypt based on increasing complexity of cereamics found at burial sites.

 RADIOMETRIC DATING. 

• Radiocarbon dating: Also called carbon-14 dating,is used to date organic materials --plants and animals.
  Plants and animals take in carbon throughout their lifetime until they die. When they die,the radioactive carbon-14 decreases at a steady rate --has a half-life of about 4,700 years.
  Radiocarbon dating is accurate but it has limits. If a material is older than 40,000 years,radiocarbon dating becomes unreliable,reason being that the amount of carbon-14 becomes too small and it's easily contaminated. Ergo,it will generate an age unexpected or implausible.

• Single crystal fusion: Also called Ar-Ar (Argon-Argon) dating or single crystal Argon dating, is a refinement of the famed K-Ar dating. Both of them measure rocks not organic materials like radiocarbon dating and the older the sample, the better the results. Researchers use it to measure rocks older than 500,000 years.

What differentiates Ar-Ar dating from K-Ar dating is the individual destruction rate during the measuring process.
While K-Ar dating requires destroying large samples of rocks to measure Potassium and Argon levels differently, Ar-Ar dating can measure both isotope levels in a single, small rock sample.

• Uranium series dating: Uranium series dating consists of different methods based on the different uranium isotopes' decay rates.
  For example, Uranium-Thorium method is helpful for dating finds between 40,000 to 500,000 years.
  Which is too old for radiocarbon dating and too young for Ar-Ar or K-Ar dating.

  TRAPPED CHARGE DATING

There are certain organic and inorganic substances that trap electrons from sunlight and from cosmic rays that bombard the earth. The age of these substances can be determined by measuring the amount of trapped electrons, factoring in how much radiation the substance was exposed to yearly.
The technique is accurate for measuring from a few thousand to 500,000 years. Though it's debatable if the strength diminishes significantly after 100,000 years.

• Thermoluminesence: Thermoluminesence is the release of previously absorbed radiation upon being heated.
  The technicality is in the "heating".

• Quartz, a silicate rock is good at absorbing electrons. When researchers who work with prehistoric tools made from flint a hardened form of quartz --want to date the tool,they use thermoluminesence(TL).
  This is where the "heat" technicality comes in,when tools are made using flint,they're dropped into fire after carving and shaping,this heating releases previously trapped electrons from the flint,therefore resetting the "clock".
  So, in essence, scientists date the flint tool,not the flint rock.

• Optically Stimulated Luminesence(OSL): OSL is similar to TL, OSL measures when last certain quartz crystals saw sunlight. Exposure to sunlight resets the crystals' clock to zero. When it is buried, the trapped electrons accumulate a "luminesence signal" that can be measured in a laboratory when exposed to certain wavelengths of light that free these electrons,just enough for them to emit a photon. This light(luminesence signal) can be used to calculate when the sample was last exposed to sunlight.

• Electron Spin Resonance(ESR):
  ESR studies materials with unpaired electrons.
  It measures the trapped electrons using magnetic fields.
  Similar to the MRI scans.

Since it tracks the activity(spin) of the electrons without freeing them,it can be done repeatedly.
ESR has a long range,some scientists say it can measure up to one million years.
But, due to it's complications, it is susceptible to error.

Now, back to business!

To determine the age of a material, researchers compare the ratio of the parent and daughter products that were initially in the sample with the ratio of these products at the current time.
By doing so, they can calculate how much time has passed. The atomic clock is a very accurate time keeping apparatus calibrated by the precise regularity of radioactive decay.

One of the systems that has been both accurate and successful when dating fossils is the Potassium-Argon dating(K-Ar dating) and also the Argon-Argon dating(Ar-Ar dating).
Although most potassium isotopes aren't radioactive, one of them is,and one of its decay products is the gas Argon.
K-Ar dating relies on the fact that while potassium is a solid,Argon is a gas.
When rock is melted(example: lava),all the Argon in the rock escapes and when the rock solidifies again, only potassium is left.

The melting of the rock and the escaped Argon sets the K-Ar clock at Zero(0).
As time passes,Argon accumulates in the rock as a result of radioactive potassium decay.
When scientists analyse these rocks and compute the ratio of Argon to potassium, they know how long it has been since the rock cooled.

When scientists date the oldest rocks from our solar system in cross-reference with the oldest meteorites, the oldest date they find is 4.5 Billion years.

Radioactive dating has been perfected to the point that scientists can get within a few percentage points to the actual date. This is known because of the dating of lava flows that happened recent enough for their dates to be known historically.

Evidence for the radiometric dating method:

In 79AD,a volcano erupted that led to the formation of Mount Vesuvius in Pompeii. We know this from the eye-witness account of the historian Pliny the younger. Without going into much detail, the point is: This volcanic eruption happened during a pre-scientific period and there were people who witnessed and luckily,recorded the event.

When scientists from the Berkeley Geochronology center went to perform stratigraphical analysis and date Mount Vesuvius,their result was in congruence with the historical records.

This is how we know that the earth is close to 4.5 Billion years old.

References:

1. 2.3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

IMAGE SOURCES
All images used here are from free sources and are eligible for commercial use licensed under Creative Commons.