The Earth is very old 41/2 billion years or more according to recent estimates. This vast span of time, called geologic time by earth scientists, is difficult to comprehend in the familiar time units of months and years, or even centuries. How then do scientists reckon geologic time, and why do they believe the Earth is so old? A great part of the secret of the Earth’s age is locked up in its rocks, and our centuries-old search for the key led to the begin￾ning and nourished the growth of geologic science. Mankind’s speculations about the nature of the Earth inspired much of the lore and legend of early civilizations, but at times there were flashes of in￾sight. The ancient historian Herodotus, in the 5th century B.C., made one of the earliest recorded geological observations. After finding fossil shells far inland in what are now parts of Egypt and Libya, he correctly inferred that the Mediterranean Sea had once extended much farther to the south. Few believed him, however, nor did the idea catch on. In the 3rd century B.C., Eratosthenes depicted a spherical Earth and even calculated its diameter and circumference, but the concept of a spherical Earth was beyond the imagina￾tion of most men. Only 500 years ago, sailors aboard the Santa Maria begged Columbus to turn back lest they sail off the Earth’s “edge.” Similar opinions and prejudices about the nature and age of the Earth have waxed and waned through the centuries. Most people, however, appear to have traditionally believed the Earth to be quite young that its age might be measured in terms of thousands of years, but certainly not in millions. The evidence for an ancient Earth is concealed in the rocks that form the Earth’s crust and surface. The rocks are not all the same age or even nearly so but, like the pages in a long and complicated history, they record the Earth￾shaping events and life of the past. The record, however, is incomplete.

Many pages, especially in the early parts, are missing and many others are tattered, torn, and difficult to decipher. But enough of the pages are preserved to reward the reader with accounts of astounding episodes which certify that the Earth is billions of years old. Two scales are used to date these episodes and to measure the age of the Earth: a relative time scale, based on the sequence of layering of the rocks and the evolution of life, and the radiometric time scale, based on the natural radioactivity of chemical elements in some of the rocks. An explanation of the relative scale highlights events in the growth of geologic science itself; the radiometric scale is a more recent development borrowed from the physical sciences and applied to geologic problems.

At the close of the 18th century, the haze of fantasy and mysticism that tended to obscure the true nature of the Earth was being swept away. Careful studies by scientists showed that rocks had diverse origins. Some rock layers, containing clearly identifiable fossil remains of fish and other forms of aquatic animal and plant life, originally formed in the ocean. Other layers, consisting of sand grains winnowed clean by the pounding surf, obviously formed as beach deposits that marked the shorelines of ancient seas. Certain layers are in the form of sand bars and gravel banks rock debris spread over the land by streams. Some rocks were once lava flows or beds of cinders and ash thrown out of ancient volcanoes; others are portions of large masses of once￾molten rock that cooled very slowly far beneath the Earth’s surface. Other rocks were so transformed by heat and pressure during the heaving and buck￾ling of the Earth’s crust in periods of mountain building that their original features were obliterated. Between the years of 1785 and 1800, James Mutton and William Smith advanced the concept of geologic time and strengthened the belief in an ancient world. Mutton, a Scottish geologist, first proposed formally the fun￾damental principle used to classify rocks according to their relative ages. He concluded, after studying rocks at many outcrops, that each layer represented a specific interval of geologic time. Further, he proposed that wherever un￾contorted layers were exposed, the bottom layer was deposited first and was, therefore, the oldest layer exposed; each succeeding layer, up to the topmost one, was progressively younger. Today, such a proposal appears to be quite elementary but, nearly 200 years ago, it amounted to a major breakthrough in scientific reasoning by establishing a rational basis for relative time measurements. However, unlike tree-ring dating in which each ring is a measure of 1 year’s growth no precise rate of deposition can be determined for most of the rock layers. Therefore, the actual length of geologic time represented by any given layer is usually unknown or, at best, a matter of opinion.

Relative dating is used to arrange geological events, and the rocks they leave behind, in a sequence. The method of reading the order is called stratigraphy (layers of rock are called strata). Relative dating does not provide actual numerical dates for the rocks.

Fossils and relative dating

Fossils are important for working out the relative ages of sedimentary rocks. Throughout the history of life, different organisms have appeared, flourished and become extinct. Many of these organisms have left their remains as fossils in sedimentary rocks. Geologists have studied the order in which fossils appeared and disappeared through time and rocks. This study is called biostratigraphy.

Fossils can help to match rocks of the same age, even when you find those rocks a long way apart. This matching process is called correlation, which has been an important process in constructing geological timescales.

Some fossils, called index fossils, are particularly useful in correlating rocks. For a fossil to be a good index fossil, it needs to have lived during one specific time period, be easy to identify and have been abundant and found in many places. For example, ammonites lived in the Mesozoic era. If you find ammonites in a rock in the South Island and also in a rock in the North Island, you can say that both rocks are Mesozoic. Different species of ammonites lived at different times within the Mesozoic, so identifying a fossil species can help narrow down when a rock was formed.

Correlation can involve matching an undated rock with a dated one at another location. Suppose you find a fossil at one place that cannot be dated using absolute methods. That fossil species may have been dated somewhere else, so you can match them and say that your fossil has a similar age. Some of the most useful fossils for dating purposes are very small ones. For example, microscopic dinoflagellates have been studied and dated in great detail around the world. Correlation with them has helped geologists date many New Zealand rocks, including those containing dinosaurs.

Our planet did not come with a guidebook. It has taken generations of scientists building upon one another’s ideas and research to compile our current understanding of the earth’s systems and history. However, our planet did come with clues to its long history, and it is the work of geologists, archaeologists, and others to sleuth through these clues and put the pieces together. Methods for accurately dating historical events, situating moments of Earth’s history in place and time are an essential part of earth science. In this paper, I will discuss one clue that scientists have leveraged to solve the mysteries of Earth’s past. This is the isotope carbon 14, which can be used to date objects and artifacts up to 50, 000 years old (MacDougall, 2008:190).

I will first give some essential background on the methods that scientists use to date organic and inorganic materials, and discuss geochronology as a discipline. I will recount the story of the discovery of carbon 14’s usefulness for dating, and how our knowledge regarding the uses of this isotope has changed over time. I will explain how and why carbon 14 works as a recorder of time, and describe some notable cases when it has proven useful for dating human artifacts and other relics. Lastly, I will review some case studies of interesting uses of radiocarbon dating in research today. The investigation of earth’s past has a fascinating social history. James Hutton, working as a geologist during the Industrial Revolution, believed that the earth worked rather like a machine. Surrounded by industry, and Darwin and science’s understanding of evolution, Hutton hypothesized that Earth history progressed in cycles, repeating the same patterns over and over again on an inconceivably long time scale (MacDougall, 2008:8-11). This concept was widely accepted until some years later, when Lord Kelvin cast doubt upon it. Based upon research in deep mines, Kelvin believed that planet earth was hot inside, and cooling over time. As a physicist, Kelvin believed that if he could estimate the earth’s original temperature, and determine the rate at which the planet was cooling, then he could mathematically determine the age of the earth (MacDougall, 2008:14).

While neither Hutton nor Kelvin was completely correct in his strategy for dating the earth, their theories were hugely impactful for science. Hutton’s theory contributed the idea that the earth goes through “cycles”, and that geologic time is extremely long (MacDougall, 2008:11). Kelvin’s research was ingenious, but incomplete. In fact, if Lord Kelvin had added just a few more figures to his equation, it would have worked perfectly to date the earth. But no one had yet discovered the convection of the mantle, or the existence of radioactive materials within it (MacDougall, 2008:16). There were many other thinkers who undertook to develop a strategy for dating the Earth around Kelvin’s time, and although these strategies and the theories behind them were creative, they did not contribute as much to science. George Darwin, for example, believed the speed of the earth’s rotation to be slowly decreasing, estimated its original speed and rate of slowing, and claimed that the earth was 50 to 60 million years old (MacDougall, 2008:15). John Joly believed that the earth’s oceans were getting saltier over time, and that he could determine its age using the amount of salt in the sea and the rate at which rivers deposited it over time (MacDougall, 2008:15). These theories may sound mad to us now, but they were mostly based on sound logic, and the knowledge that existed at the time.

The age of the Earth is estimated to be 4.54 ± 0.05 billion years (4.54 × 10⁹ years ± 1%).^[ This age may represent the age of the Earth’s accretion, of core formation, or of the material from which the Earth formed. This dating is based on evidence from radiometric age-dating of meteorite material and is consistent with the radiometric ages of the oldest-known terrestrial and lunar samples.

Following the development of radiometric age-dating in the early 20th century, measurements of lead in uranium-rich minerals showed that some were in excess of a billion years old. The oldest such minerals analyzed to date—small crystals of zircon from the Jack Hills of Western Australia—are at least 4.404 billion years old.^] Calcium–aluminium-rich inclusions—the oldest known solid constituents within meteorites that are formed within the Solar System—are 4.567 billion years old, giving a lower limit for the age of the Solar System.

It is hypothesised that the accretion of Earth began soon after the formation of the calcium-aluminium-rich inclusions and the meteorites. Because the time this accretion process took is not yet known, and predictions from different accretion models range from a few million up to about 100 million years, the difference between the age of Earth and of the oldest rocks is difficult to determine. It is also difficult to determine the exact age of the oldest rocks on Earth, exposed at the surface, as they are aggregates of minerals of possibly different ages.

The Crust

The crust is everything we can see and study directly. The thinnest layer of the Earth, the crust still measures about 40 km on average, ranging from 5–70 km (~3–44 miles) in depth. But at the scale of the planet, that’s less than the skin of an apple.

There are two types of crust: continental and oceanic crust. Oceanic crust can be found at the bottom of the oceans or below the continental crust; it is generally harder and deeper, consisting of denser rocks like basalt, while continental crust contains granite-type rocks and sediments. The continental crust thicker on land.

The crust is not one rigid thing, but it’s split into several tectonic plates. These tectonic plates are not stationary, but are in relative motion one from another. Depending on the relationship and geologic setting, there are three types of tectonic plate boundaries: convergent (moving one toward the other), divergent (moving away from the other) and transformant (moving laterally).

These plates “float” on the soft, plastic upper mantle.

The Mantle

The mantle extends down 2,890 km, making it the thickest layer of Earth. It makes up about 84% of Earth’s volume. Everything we know about the mantle we know indirectly, as no human study managed to go beyond the crust. Most of the things we know about the mantle we know from seismologic studies (more on that later).

The mantle is also divided into several layers, based on seismologic properties. The upper mantle extends from where the crust ends to about 670 km. Even though this area is regarded as viscous, you can also consider it as formed from rock – a rock called peridotite to be more precise. Below that, the lower mantle extends from 670 to almost 2900 kilometers below the surface.

The Core

We sometimes refer to the core as one thing, although the inner core and the outer core are fundamentally different – not layers of the same thing. The  “solid” inner core has a radius of ~1,220 km, while the “liquid” outer core extends up to a radius of ~3,400 km.

Wait, if we couldn’t go to the mantle, how could we possibly know one is solid and one isn’t? Well, as before, the answer is the same: seismic waves (we’re almost there).

The Inner Core

The temperatures and pressures of the inner core are absolutely extreme, at approximately 5,400 °C (9,800 °F) and 330 to 360 gigapascals (3,300,000 to 3,600,000 atm).

It’s generally believed that the inner core is growing very slowly – as the core cools down, more of the outer core solidifies and becomes a part of the inner core. The cooling rate is very low thought, at about 100 degrees Celsius per billion years. However, even this slow growth is thought to have a significant impact in the generation of Earth’s magnetic field by dynamo action in the liquid outer core.

The Outer Core

The outer core is a low viscosity fluid (about ten times the viscosity of liquid metals at the surface) – “liquid” is a rather improper term. Because it has a very low viscosity, it is easily deformed and malleable. It is the site of violent convection. It is also thought to suffer very violent convection currents – hey, and guess what? The churning of the outer core and its relative movement is responsible for the Earth’s magnetic field.

The hottest part of the outer core is actually hotter than the inner core; temperatures can reach 6,000° Celsius (10,800° Fahrenheit)—as hot as the surface of the sun.

Continental drift is the theory that the Earth’s continents have moved over geologic time relative to each other, thus appearing to have “drifted” across the ocean bed. The speculation that continents might have ‘drifted’ was first put forward by Abraham Ortelius in 1596. The concept was independently and more fully developed by Alfred Wegener in 1912, but his theory was rejected by many for lack of any motive mechanism. Arthur Holmes later proposed mantle convection for that mechanism. The idea of continental drift has since been subsumed by the theory of plate tectonics, which explains that the continents move by riding on plates of the Earth’s lithosphere.

Developed from the 1950s through the 1970s, plate tectonics is the modern version of continental drift, a theory first proposed by scientist Alfred Wegener in 1912. Wegener didn’t have an explanation for how continents could move around the planet, but researchers do now. Plate tectonics is the unifying theory of geology, said Nicholas van der Elst, a seismologist at Columbia University’s Lamont-Doherty Earth Observatory in Palisades, New York.

“Before plate tectonics, people had to come up with explanations of the geologic features in their region that were unique to that particular region,” Van der Elst said. “Plate tectonics unified all these descriptions and said that you should be able to describe all geologic features as though driven by the relative motion of these tectonic plates.”

How many plates are there?
There are nine major plates, according to World Atlas. These plates are named after the landforms found on them. The nine major plates are North American, Pacific, Eurasian, African, Indo-Australian, Australian, Indian, South American and Antarctic.

The largest plate is the Pacific Plate at 39,768,522 square miles (103,000,000 square kilometers). Most of it is located under the ocean. It is moving northwest at a speed of around 2.75 inches (7 cm) per year.

There are also many smaller plates throughout the world.

How plate tectonics works
The driving force behind plate tectonics is convection in the mantle. Hot material near the Earth’s core rises, and colder mantle rock sinks. “It’s kind of like a pot boiling on a stove,” Van der Elst said. The convection drive plates tectonics through a combination of pushing and spreading apart at mid-ocean ridges and pulling and sinking downward at subduction zones, researchers think. Scientists continue to study and debate the mechanisms that move the plates.

Mid-ocean ridges are gaps between tectonic plates that mantle the Earth like seams on a baseball. Hot magma wells up at the ridges, forming new ocean crust and shoving the plates apart. At subduction zones, two tectonic plates meet and one slides beneath the other back into the mantle, the layer underneath the crust. The cold, sinking plate pulls the crust behind it downward.