What Is The Age Of The Earth?
For about as long as humans have had the capacity to ponder, we have sought to understand the fundamental questions of our existence. Who are we? How did we get here? Where did our world come from?
Over much of our history, we have answered these questions with theology - inventing deities and creation myths to explain how our world came to be. From the Genesis story of the Old Testament to the diving water beetle of the Cherokee creation myth to the birth of Gaia in ancient Greek mythology, humans have demonstrated remarkable creativity in explaining how our world came to be.
Most of these myths have included an estimate of the age of the world, usually on the order of a few thousand to a few tens of thousands of years. For most of our history, these myth-based timelines were sufficient for understanding the age of our planet. Apart from the occasional catastrophic flood, the world was considered static and unchanging.
The scientific revolution of the 18th and 19th centuries changed all this. As naturalists began to understand the principles of geology, the old creation timelines no longer fit.
Discovering The Old Earth
One of the most influential figures in the scientific revolution was a Scottish naturalist, chemist, physician, and agriculturalist named James Hutton. After completing his medical degree and founding a successful chemical manufacturing firm, the young Hutton inherited his father's farmland in Berwickshire, Scotland in the 1750s. He began spending much of his time studying the various features of the land. Hutton was particularly keen on observing the ditches and streams of the farmland, noting how the flow of water eroded the land in places and deposited silt in others.
Studying rock formations in the region, he noted that the rocks seemed to be composed of the same materials - including pieces of organic material from decomposed animals and plants. From these observation, he began to arrive at a conclusion that was quite revolutionary for its time - the same forces of erosion and deposition at work on his farms today had formed Scotland's sedimentary rock formations in the distant past. The creation of the world had not been an instantaneous act, but a long and gradual process.
In 1785, after three decades of observation and study, Hutton completed his Theory of the Earth; or an Investigation of the Laws observable in the Composition, Dissolution, and Restoration of Land upon the Globe and presented his findings to the Royal Society of Edinburgh. The paper introduced the scientific world to the idea of deep time, and an Earth that was millions of years old rather than thousands. And although Hutton never used the term in his work, he also introduced the concept of uniformitarianism - the idea that geologic processes had happened at uniform rates throughout time.
The principle of uniformitarianism was a highly influential one at the time. Geologists of the 19th century began using the idea to arrive at estimates of the age of the Earth, measuring present rates of salt deposition or the cooling of rock or the Moon's tidal friction, and then using these rates to calculate when these processes would have began. These methods yielded dates varying from 20 million to 400 million years old for the beginning of the world.
The wide variance of ages demonstrated the need for a more accurate method of dating. The key to this method arrived with the discovery of radioactivity.
At the dawn of the 20th Century, scientists studying the properties of radioactive materials discovered that they decayed over time, changing from heavier elements to lighter ones by shedding protons and neutrons. This decay was found to occur at a constant rate. This rate is known as the half-life - the amount of time necessary for half of a radioactive parent isotope to decay into stable daughter isotope.
This discovery provided geologists with the tool they needed to precisely measure the ages of the rocks and fossils in the geologic column. By extension, provided a method for measuring the age of the Earth, as the planet could not be any younger than any of its rocks. This tool was radiometric dating.
Certain minerals in igneous rocks were found to contain trace amounts of radioactive elements in their crystal structures, along with trace amounts of the daughter isotopes. A good example of this is the mineral zircon, often found in granite. Pure zircon is a crystal of the elements zirconium, silicon, and oxygen with the formula ZrSiO4. However, since uranium and zirconium have similar electron configurations, uranium atoms will often take the place of zirconium atoms in the lattice as the crystal forms - usually at concentrations of about 10 parts per million.
Lead, the end product of uranium's radioactive decay, has a different electron configuration and does not naturally bond with the zircon crystal. As magma cools and crystallizes to form rock, any lead in the mixture is rejected by the zircon crystals as they form - a process that has been demonstrated by laboratory experiment. This means that any lead found in a zircon crystal had to have been the product of uranium decay.
Thus, measuring the ratio of uranium to lead in a zircon crystal sample provides a precise estimate of the age of the rock in which the crystal formed. Since uranium has two common isotopes, uranium-235 and uranium-238, the uranium-lead dating method also provides geologists with a check on their work. Both isotopes of uranium decay at different rates, so the measured ratios of each isotope to its daughter isotope should provide similar dates for the rock's formation.
The Search For The Oldest Rocks
Even with the accuracy of radiometric dating, establishing the age of our planet using samples of Earth rocks is not an easy task. The difficulty is in finding the oldest rocks. Since our planet is tectonically active, it has recycled its crust over time as continents drifted apart and crashed together.
This process has destroyed many of the oldest rock samples, leaving very few rocks that date back near the time Earth formed. To date, the oldest rocks found on Earth are in the Jack Hills formation of Western Australia. Radiometric dating of rock samples from this formation has yielded dates of 4.4 billion years.
In order to date the Earth's formation most precisely, scientists have turned to rocks that are undisturbed by billions of years of geologic processes because they spent most of that time in space - asteroids. Since asteroids are left-over chunks of rock from the formation of the solar system, dating their formation also provides an approximate date for the formation of the Earth. Since bits of asteroid fall to Earth as meteorites, there is an ample supply of asteroid material available for dating.
One example is the Canyon Diablo meteorite, a fragment of the space rock which created the Barringer Crater in Arizona some 50,000 years ago. Uranium-lead dating of samples from this meteorite dated its formation to 4.55 billion years ago. Another example is the Allende meteorite, which fell in Mexico in February 1969. Argon-argon testing of samples from this meteorite has yielded dates of 4.48 to 4.57 billion years old.
Repeated tests of these and other meteorites, using different parent-daughter isotope pairs, have all demonstrated similar formation dates. This body of work puts the upper limit for the age of the Earth at 4.57 billion years, an answer that nicely checks off one of the great questions of our existence.
How old is the Earth?
Sources and Further Information
- The Age of the Earth
The Earth is accepted by scientists to be around 4.5 billion years old. How do we know the Earth is this old? Some of the lines of evidence for an ancient Earth are presented.
- Radiometric Dating: How Does It Work?
Since radioactive elements have a measurable half-life - the period of time needed for an amount of the substance to decrease by half - they can be used to measure the ages of rocks containing them.
- James Hutton - Theory of the Earth
Hutton's Theory was first read at the March 7 and April 4, 1785, meetings of the Royal Society of Edinburgh. The reader on March 7 was the chemist Dr. Joseph Black, a good friend of Hutton's. Hutton finished the reading himself on April 4.
- The incorporation of Pb into zircon
E.B Watson, et al. Chemical Geology: Volume 141, Issues 1–2, 29 August 1997, Pages 19–31
- 4.2 Ga zircon xenocryst in an Acasta gneiss from northwestern Canada: Evidence for early continental
Evidence for the existence of continental crust older than 4.06 Ga has so far been obtained only from zircons in the Yilgarn Craton of Western Australia. In this paper we report the first occurrence of a very old zircon with a U-Pb age of 4.2 Ga...
- Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gy
Here we report, based on a detailed micro-analytical study of Jack Hills zircons, the discovery of a detrital zircon with an age as old as 4,404 plus/minus 8 Myr—about 130 million years older than any previously identified on Earth.
- Uniformitarianism (World of Earth Science) - eNotes.com
In order to understand uniformitarianism, one must examine its roots in the Enlightenment era (c. 1750-1850) and how the term has been distorted in meaning since that time.