HOW OLD IS THE EARTH, AND HOW CAN WE CALCULATE IT?
How long is “a very long time”? More precisely, when did the Earth come into being?
In his “Theory of Earth,” published in 1788, James Hutton made revolutionary observations about the Earth’s lifespan and volatility: Our planet has been kneaded by ancient geological processes for a very long time. To the Scottish geologist, these processes have “no vestige of a beginning, no prospect of an end” [1].
That’s a fairly accurate statement, but it’s still very ambiguous. How long is “a very long time”? More precisely, when did the Earth come into being?
Previously, there had been many attempts to address these questions. Based on Egyptian mythology, Herodotus, the first historian, dated the world to around 12–13,000 BC [2]. The Archbishop of Ireland, James Ussher, arrived at another estimation: analyzing all the events recorded in the Bible, he proposed that the Earth had been created at 6 p.m. on October 26, 4004 BC, a Saturday evening [3]. Young Earth theorists often claim that our planet is only about 10,000 years old [4]. Still, what these timelines have in common is that they are all based on information found in ancient books, which can only store information that spans several thousand years of human civilization. Not to mention that at the time, people still believed that humans appeared at or near the time of Earth’s birth. Given such a history, defining this planet’s actual age will require more specialized evidence and scribes.
Earth, along with Mercury, Venus, and Mars, is one of the Solar System’s inner planets. They are all terrestrial planets with the distinguishing structural feature of a solid surface layer surrounding a molten metal core [5]. The Earth’s crust is primarily composed of silicate minerals, the common name for the rocks that have formed the planet’s diverse landscapes and topography, ranging from mountains and canyons on the continent to volcanoes and oceanic fissures. This rocky crust, which covers the entire planet and endures over time, has recorded all the important facts about the Earth’s origin. Beneath its roughness, each stone or pebble narrates its own stories. Let us follow the clues and decipher the rock stories to find out when this planet was born.
1. Rock, the unassuming historian
We’ve probably heard something about calculating the age of a tree by counting the circles on the trunk’s cross-section. Cell division produces new woody substances that enlarge the trunk and form visible ridges over time. It is possible to determine how many seasons a tree has experienced in the rainy season based on the number of rings and the number of growth spurts per year. The thickness and color of the various layers of wood even tell the past weather conditions or the amount of nutrients the tree had during each growth period. In some places, the Earth’s rocky crust also goes through kindred “growth” processes.
Among the three main types of rock (volcanic, sedimentary, and metamorphic), the middle one is formed by the accumulation and petrification (lithification, or compaction and cohesion) of minute minerals polished by wind, running water, ice, or animals [6] on the surface. Soil and rock, for example, are eroded into small particles and washed away by the flow of water, settling to the bottom, remaining in the river bed and accumulating over time. The layers beneath are gradually compacted as a result of the pressure and mass of the upper layers, increasingly held together by chemical reactions or other natural agents and eventually forming clay layers that lie on top. Thus if given the number of layers of rock formed, geologists can easily determine the age of a sedimentary structure.
The shale structure beneath glacial lakes in Sweden, for example, has over 13,500 layers, which is equivalent to all those years of lake bed accretion. Similarly, the ice in the frigid polar regions is formed by the accumulation of annual snowfalls, and is even older. The ice cores of Antarctica and Greenland contain over 160,000 layers of snow [7], but beneath it are thousands of meters of ice, not to mention an older layer of rock at the bottom. Nonetheless, this accumulation must have occurred much earlier, and the Earth’s age is undoubtedly many times greater. The order of sedimentary rock layers can be a colorful and informative calendar, but is still insufficient to tell the vast scope of our planet’s history. Still, those are only the surface clues.
On the surface, stones can appear to be a hard and durable form of matter; deep within which, there are nonetheless processes of constant change. Each rock sample is a collection of different elements at the micro level (or substances). The elements, in particular, can exist as radioactive isotopes. Atoms of these isotopes are unstable: they decay continuously, changing the number of protons or neutrons (or both) in the nuclear structure and thus transforming the original element into another (for example, uranium becomes lead, or potassium becomes argon…). The decay rate of radioactive isotopes, in particular, remains constant over time. In other words, since its formation, each atom of a radioactive isotope has decayed at the same rate — by regular intervals like clock ticks.
Since the discovery of this property of nuclear radioactivity, scientists have used it to date matter more precisely, through the concept of “half-life”, which is the time it takes for half of the original radioactive isotopes to decay to become a new element. For example, the half-life of the isotope carbon 14 to nitrogen 14 is 5,730 years. If a bone has a measured amount of carbon 14 equal to nitrogen 14, it has undergone one half-life. If the carbon/nitrogen ratio is 1/3, 2 half-lives have elapsed equal to 11,460 years. Just by determining the ratio of the two substances, the age of the specimen reveals itself.
Radiocarbon dating is a fairly common method frequently used on organic specimens such as human, animal, or wood bones. Its half-life of 5,730 years appears to be long, but in fact pales in comparison to uranium, whose half-life ranges from millions to billions of years depending on the isotope. Still, the oldest known matter on Earth are the minerals in a zircon crystal found in the Jack Hills in Western Australia, dating back 4.375 billion years [8]. At the time, the rock was formed as lava from volcanic eruptions cooled. It is tough enough to last for billions of years and has uranium in its component. Due to the unique crystal structure, the uranium in zircon can’t be contaminated with lead during its formation whilst trapping those generated by the radioactive decay that follows. In other words, calculating the age of this zircon crystal based on the amount of uranium and internal lead will yield an extremely accurate result.
Still, that isn’t the true age of the Earth. Other rocks can be impacted and spoiled by geological processes that occur on a continuous basis, i.e. weathering or erosion. Though zircon, as a sturdy rock, is not affected by such processes, it can hardly resist being submerged and melted beneath the magma by the cyclic cycle of all rocks on the lithosphere [9]. In other words, all matter, no matter how solid and stable, majestic and ancient, appeared after the birth of the Earth. Rock can indeed tell us a lot, but we need to look further to ferret out the origin of this planet.
2. A gift from the universe
The final piece of this puzzle are meteorites. In essence, they are the solar system’s oldest and most primordial bodies of matter, were formed at the same time as our planet but have escaped the latter’s harsh geological processes. Those are time tablets that contain the definitive answer to the Earth’s age.
The Canyon Diablo meteorite, a rock with a massive iron core, was discovered in Arizona, USA, in 1891. Scientists estimate that this meteorite was suspended in orbit for 4.5 billion years until landing into the Earth’s atmosphere approximately 50,000 years ago [10]. Numerous subsequent discoveries confirm this figure, providing a strong foundation for the age of not only the Earth, but also the entire solar system.
When compared to the age of the zircon specimen above, this figure is about 200 million years older, which likewise fits perfectly with theories about the Earth’s formation. According to the geologic time scale (GTS), the period from the formation of the Earth until approximately 5–700 million years later is referred to as the Hadean Eon, named after Hades, the Greek mythological deity of Hell. The reason for this moniker is that the young Earth at the time was very different from the blue globe today, resembling what we often refer to as Hell. Its surface was ablaze with lava flows erupting from active craters, the ideal environment for the formation of the oldest zircon crystals. Meteors were falling continuously from the sky, bombarding the land and leaving big empty holes. In this event of the Earth being pummeled by meteorites for hundreds of millions of years (also known as Late Heavy Bombardment), Theia, an object around the size of Mars, collided with our planet. The impact was so powerful that it separated a portion of the Earth’s mass, shot into space, and eventually became the Moon, our lovely satellite [11]. Apollo 14’s Moon sample 14321, a rock sample identified as being of Earth origin, corroborates this hypothesis [12].
However, meteorites bred not only death and ruins. Numerous hypotheses propose that over Earth’s lengthy history of collisions with meteorites, rocks from space may have brought the required elements for life to the planet. Metaphorically, they added ‘spices’ to the ‘primordial soup’ and brought about life’s initial sprouts [13]. Also, scientists have lately postulated that meteorites might have played a role in the development of water on Earth [14]. If these hypotheses are right, beautiful and vibrant organic lives on this planet have evolved from coarse, abrasive inorganic stuff.
And if the rocks in space are the seeds of life, the rocks on Earth are the pages of a book that record the evolution of which via fossils. From the microbial mats in the ocean to the enormous dinosaurs that once controlled the globe, the rocks have meticulously retained the remains of animals that once inhabited this planet. Additionally, they provide the missing connections to the overall evolutionary process of the living universe, and humans in particular.
More recently, rock has served as a testimony to human civilization’s advances. From crudely hewn tools to beautiful pyramids, there is something for everyone. From cave drawings to intricate bas-reliefs. From the regal’s treasured jewel to the charred brick lying beneath the excavation trench. Each stone has its own unique story to tell.
3. A big Bang to nescience
Until the dawn of the scientific era, humans lived in a more closed, mystical, and sacred world. Everything appears to revolve around humanity’s fate. The planet and the universe are both man’s, and they were created to serve man, in accordance with the Creator’s will. However, the advancement of science has provided a series of foundations, evidence, and means for people to expand into an infinitely large (and still expanding) space and immerse themselves in an infinite timeline. With each passing epoch, people begin to assert that “our history is longer” and that “we live in a larger space than we believe”. The fact that Archbishop James Ussher stated in 1654 that “the Earth was born in 4004 BC” was shocking because it was an extremely long-term prediction in comparison to the lifespan of an individual, a dynasty, or even a country. Following that, people stretched it further, but they were all extremely small in comparison to the scientifically confirmed 4.54 billion years.
According to Paul S. Braterman’s biography in Scientific American [14], gaining an understanding of the Earth’s lifespan necessitated the efforts and discoveries of numerous individuals, specifically the following:
- Nicolas Steno proposed horizontal stratigraphic deposition in the seventeenth century, educating people on the significance of the stack of layers of soil and rock on top of one another.
- In the 18th century, canal construction flourished to facilitate economic and political development while also allowing scientists to access deeper geological layers at literally no cost (for the scientists).
- At the end of the 18th century, James Hutton developed a specific theory about the cycles of accretion layer formation but arrived at incomplete conclusions, as mentioned at the beginning.
- The advancement of physics, particularly thermodynamic studies in support of the industrial revolution and the steam engine, demonstrated that the Earth, like any other energy in the universe, must have a beginning and an end. Thermodynamic principles have established a rigid framework for determining the Earth’s age.
- Physicists discovered radioactivity and began using it in mineral analysis.
- At the end of the nineteenth century, after much debate and research, scientists agreed that the Earth was 100 million years old, then 3.3 billion years old. However, radiometric measurements remained problematic and imprecise.
Getting to know the topic of the Earth’s age, scientists were ecstatic. After discovering that uranium decays into lead, scientists set out to determine the Earth’s original lead composition. Clair Cameron Patterson undertook this task and was horrified to discover that every specimen he collected was contaminated with lead. Although the age of zircon was known at the time, Patterson’s measurements were always skewed due to the presence of too much lead, rendering it impossible to determine the age of the Earth. Patterson accomplished a great deal, from building his own laboratory which is so secure that even air can’t enter to cleaning all objects with acid. Within what is regarded as the cleanest lab ever, Patterson came up with the exact datum as we recognize it today: 4.544 billion years. The scientist compared lead-contaminated samples in routine laboratory studies to this new finding and discovered a discrepancy in the results, indicating that the air was seriously contaminated with lead. As a result, he concentrated his research on lead pollution, collecting samples beneath the ice sheets of Greenland and Antarctica, and eventually concluding that since industrial lead tetraethyl became widely used in fuel, lead levels in the air (and indeed, everywhere) have steadily increased at alarming rates.
For lead has been the primary cause of a wide variety of serious human health problems, Patterson spent the remainder of his life studying it in greater detail. In the end, he issued both a warning and an appeal to the world that lead should be removed from the burgeoning industry of fuel additives. This effort jeopardized his career and possibly his life, as it directly impacted the Ethyl Corporation and a number of other large fuel additive companies at the time. However, he persisted in his fight against environmental pollution and eventually succeeded. The United States mandated the use of unleaded gasoline in certain new car models in 1975, and the country established a set of standards in 1986 to eliminate lead from all gasoline. This effort promptly averted a catastrophe for humanity, as new studies revealed that blood lead levels in Americans had decreased by 80 percent from their pre-war levels in 1990. Knowing that previously, the parameters used to measure blood lead levels in people had been considered “normal”, despite the fact that they had been nearly twice as high.
E92 gasoline was withdrawn from the market due to this reason, along with a series of new standards for the paint industry, food containers, and plumbing. The story of the discovery of the Earth’s age is still associated with Patterson and his discovery of lead pollution [16].
On the one hand, science advancements brought about the industrial revolution, which in turn gave birth to new scientific discoveries and aided people in their quest for a better understanding of the world. On the other, they have nonetheless assisted us in recognizing how rich and primitive industries have been destroying the world and humans, subsequently saving us from tragedies inherent to development.
After all, human development, science, and understanding would all be meaningless if they couldn’t save humanity.
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References:
[1] James Hutton. Theory of the Earth; or an investigation of the laws observable in the composition, dissolution, and restoration of land upon the Globe, 1788. Transactions of the Royal Society of Edinburgh
[2], [3] Wikipedia Contributors, “Dating creation,” Wikipedia, 19-Oct-2021. [Online]. Available: https://en.wikipedia.org/wiki/Dating_creation. [Accessed: 28-Oct-2021]
[4] Wikipedia Contributors, “Young Earth creationism,” Wikipedia, 28-Oct-2021. [Online]. Available: https://en.wikipedia.org/wiki/Young_Earth_creationism. [Accessed: 28-Oct-2021]
[5] “Terrestrial | Planet Types — Exoplanet Exploration: Planets Beyond our Solar System,” Exoplanet Exploration: Planets Beyond our Solar System, 22-Mar-2021. [Online]. Available: https://exoplanets.nasa.gov/wha.../planet-types/terrestrial/. [Accessed: 28-Oct-2021]
[6] “rock | Definition, Characteristics, Formation, Cycle, Classification, Types, & Facts | Britannica,” Encyclopædia Britannica. 2021 [Online]. Available: https://www.britannica.com/science/rock-geology. [Accessed: 28-Oct-2021]
[7] “Ice Cores and the Age of the Earth,” Icr.org, 2011. [Online]. Available: https://www.icr.org/article/ice-cores-age-earth/. [Accessed: 28-Oct-2021]
[8] B. Oskin, “Confirmed: Oldest Fragment of Early Earth is 4.4 Billion Years Old,” livescience.com, 23-Feb-2014. [Online]. Available: https://www.livescience.com/43584-earth-oldest-rock-jack.... [Accessed: 28-Oct-2021]
[9] National Geographic Society, “The Rock Cycle,” National Geographic Society, 21-Nov-2019. [Online]. Available: https://www.nationalgeographic.org/encyclopedia/rock-cycle/. [Accessed: 28-Oct-2021]
[10] “Meteoritical Bulletin: Entry for Canyon Diablo,” Usra.edu, 2021. [Online]. Available: https://www.lpi.usra.edu/meteor/metbull.php?code=5257. [Accessed: 28-Oct-2021]
[11] M. Wall, “Ancient impact that formed Earth’s moon was likely a one-two punch,” Space.com, 24-Sep-2021. [Online]. Available: https://www.space.com/moon-forming-impact-one-two-punch..... [Accessed: 28-Oct-2021]
[12] M. Greshko, Science, 25-Jan-2019. [Online]. Available: https://www.nationalgeographic.com/.../earths-oldest-rock.... [Accessed: 28-Oct-2021]
[13] JR Minkel, “Were Meteorites the Origin of Life on Earth?,” Scientific American, 16-Jun-2008. [Online]. Available: https://www.scientificamerican.com/.../were-meteorites.../#. [Accessed: 28-Oct-2021]
[14] K. Shah, “Meteorites may have brought water to Earth in the recent past,” New Scientist, 2021. [Online]. Available: https://www.newscientist.com/.../2264333-meteorites-may.../. [Accessed: 28-Oct-2021]
[15] P. S. Braterman, “How Science Figured Out the Age of Earth,” Scientific American, Oct. 20, 2013. https://www.scientificamerican.com/.../how-science.../ (accessed Oct. 28, 2021).
[16] Wikipedia Contributors, “Clair Cameron Patterson,” Wikipedia, Oct. 10, 2021. https://en.wikipedia.org/wiki/Clair_Cameron_Patterson (accessed Oct. 28, 2021).
