The massive Hawaiian Islands rose from the Pacific as volcanoes periodically added layers of lava Fig. From modern-day eruptions, we know that active volcanoes grow by perhaps a meter every century.
The highest point on the big island of Hawaii is Mauna Kea at 4, meters above sea level. However, the volcano rises approximately 10, meters above the ocean floor, so a rough calculation gives its age:. The Hawaiian Islands are a chain of volcanoes, each formed from layer after layer of lava. A new island dubbed Loihi is now forming south of the big island. This is a rough estimate, to be sure, but it jives well with other methods that date the big island of Hawaii as about a million years old.
The other islands that string out to the northwest, each with now-dormant volcanoes, are progressively older and a new island, dubbed Loihi, is already forming as volcanoes erupt on the ocean floor southeast of the big island. You can do a similar calculation to date the Atlantic Ocean, which is about 3, kilometers wide and grows wider every year.
These continents were once joined into the supercontinent Pangaea; the Atlantic Ocean formed when Pangaea split down the middle and formed a divergent boundary, now marked by the Mid-Atlantic Ridge Fig. New crust forms along the Ridge, as Europe and Africa move away from the Americas. Exacting satellite measurements over the past two decades reveal an average spreading rate of 2.
The Atlantic Ocean formed when the supercontinent Pangaea began to split apart about million years ago. Source: USGS. This rough estimate of about million years is close to other measurements of the age of the Atlantic. It is remarkable to imagine that a great ocean, a seemingly permanent feature of our home planet, is so transient in the context of Earth history.
A third simple calculation reveals even longer time spans. The Appalachian Mountains are now gently rounded and relatively low—mostly below 3, meters high Fig. But geological evidence reveals that they once were the grandest mountain chain on Earth, rivaling the Himalayas in ruggedness and height with some peaks at more than 10, meters.
Ever so gradually, erosion has worn the Appalachians down to their present state, but how long might that process take? The gently rounded Appalachian Mountains a were once the tallest and most rugged mountain rage in the world, rivaling the modern Himalayas b. Hundreds of millions of years of erosion were required to achieve their present appearance.
The volume of this impressive mountain is thus:. Now, imagine a stream that flows down the side of this mountain. Mountain streams carry silt and sand downwards—a key factor in erosion. All of these sediments came from higher up the mountain, which is constantly being eroded away. To estimate how long a mountain might survive against erosion, consider a mountain with six principal streams.
A typical stream might carry an average of one-tenth of a cubic meter of rock and soil a few shovels full per day off the mountain, though the actual amount would vary considerably from day to day. Over a period of a year, the six streams might thus remove:. That means every year on the order of cubic meters of material, or about 20 dump trucks full of rock and soil, might be removed from a mountain by normal stream erosion.
If the mountain streams remove about cubic meters per year, then the lifetime of the mountain can be estimated as the total volume of the mountain divided by the volume lost each year:. This estimate is certainly rough and not directly applicable to any specific mountain.
Nevertheless, a few hundred million years is but a small fraction of a few billion years. How can we say Earth is 4. The physical process of radioactive decay has provided Earth scientists, anthropologists, and evolutionary biologists with their most important method for determining the absolute age of rocks and other materials Dalrymple ; Dickin Trace amounts of isotopes of radioactive elements, including carbon, uranium, and dozens of others, are all around us—in rocks, in water, and in the air Table 1.
The rest of the uranium will have decayed to , atoms of other elements, ultimately to stable i. Wait another 4. Radiometric dating relies on the clock-like characteristics of radioactive decay. In one half-life, approximately half of a collection of radioactive atoms will decay.
Source: NCSE. The best-known radiometric dating method involves the isotope carbon, with a half life of 5, years. Every living organism takes in carbon during its lifetime. At this moment, your body is taking the carbon in your food and converting it to tissue, and the same is true of all other animals. Plants are taking in carbon dioxide from the air and turning it into roots, stems, and leaves. But a certain small percentage of the carbon in your body and every other living thing—no more than one carbon atom in every trillion—is in the form of radioactive carbon As long as an organism is alive, the carbon in its tissues is constantly renewed in the same small, part-per-trillion proportion that is found in the general environment.
All of the isotopes of carbon behave the same way chemically, so the proportions of carbon isotopes in the living tissue will be nearly the same everywhere, for all living things. When an organism dies, however, it stops taking in carbon of any form. From the time of death, therefore, the carbon in the tissues is no longer replenished. Like a ticking clock, carbon atoms transmute by radioactive decay to nitrogen, atom-by-atom, to form an ever-smaller percentage of the total carbon.
Scientists can thus determine the approximate age of a piece of wood, hair, bone, or other object by carefully measuring the fraction of carbon that remains and comparing it to the amount of carbon that we assume was in that material when it was alive. If the material happens to be a piece of wood taken out of an Egyptian tomb, for example, we have a pretty good estimate of how old the artifact is and, by inference, when the tomb was built. The result: the two independent techniques yield exactly the same dates for ancient fossil wood.
Carbon dating often appears in the news in reports of ancient human artifacts. In a highly publicized discovery in , an ancient hunter was found frozen in the ice pack of the Italian Alps Fig.
The technique provided similar age determinations for the tissues of the iceman, his clothing, and his implements Fowler Aristotle thought the earth had existed eternally. Roman poet Lucretius, intellectual heir to the Greek atomists, believed its formation must have been relatively recent, given that there were no records going back beyond the Trojan War.
The Talmudic rabbis, Martin Luther and others used the biblical account to extrapolate back from known history and came up with rather similar estimates for when the earth came into being.
Within decades observation began overtaking such thinking. In the s Nicolas Steno formulated our modern concepts of deposition of horizontal strata. He inferred that where the layers are not horizontal, they must have been tilted since their deposition and noted that different strata contain different kinds of fossil.
This position came to be known as uniformitarianism, but within it we must distinguish between uniformity of natural law which nearly all of us would accept and the increasingly questionable assumptions of uniformity of process, uniformity of rate and uniformity of outcome. That is the background to the intellectual drama being played out in this series of papers. It is a drama consisting of a prologue and three acts, complex characters, and no clear heroes or villains.
We, of course, know the final outcome, but we should not let that influence our appreciation of the story as it unfolds. Even less should we let that knowledge influence our judgment of the players, acting as they did in their own time, constrained by the concepts and data then available.
One outstanding feature of this drama is the role played by those who themselves were not, or not exclusively, geologists. Most notable is William Thomson, ennobled to become Lord Kelvin in , whose theories make up an entire section of this collection. He was one of the dominant physicists of his time, the Age of Steam. His achievements ran from helping formulate the laws of thermodynamics to advising on the first transatlantic telegraph cable. Harlow Shapley, who wrote an article in on the subject, was an astronomer, responsible for the detection of the redshift in distant nebulae and hence, indirectly, for our present concept of an expanding universe.
And we know from chronologies found elsewhere in the Bible that Abraham lived about 2, years before the birth of Jesus Christ. Summing these lengths of time, we get about 6, years technically just a little more.
They rely on radiometric dating, though the story is a bit more complicated than it sounds. Some rocks contain trace amounts of radioactive atoms. Those radioactive atoms decay into stable atoms over time. By knowing the decay rate and measuring the amount of both kinds of atoms in a rock, scientists can compute the amount of time it took to produce the stable atoms.
Some assumptions are involved, however. Were some of the stable atoms present in the rock to begin with? Did some of either type of atom leave or enter the rock during the time being measured for decay? To make matters worse, measuring the age of a rock by different kinds of radioactive atoms such as uranium or rubidium often yields very different ages.
There are many examples of such discordant ages. But even if we accept these ages as correct, there are many other assumptions that cause even more problems. The earth is a very dynamic place, with volcanic eruptions and tectonic plate movements that constantly recycle old rocks into new rocks.
When rocks are recycled this way, it is believed that their radiometric dates are reset. Instead, scientists must look to other bodies in the solar system that are less active geologically. The search for primordial rocks was one of the scientific reasons we sent men to the moon a half-century ago. Scientists thought that since the moon has far less geological activity than the earth, its rocks would be older. Their source rocks have not yet been found. Meanwhile, scientists have also found 7-billion-year-old stardust on Earth.
The rocks and zircons set a lower limit on the age of Earth of 4. When life arose is still under debate, especially because some early fossils can appear as natural rock forms. Some of the earliest forms of life have been found in Western Australia, as announced in a study ; the researchers found tiny filaments in 3.
Other studies suggest that life originated even earlier. Hematite tubes in volcanic rock in Quebec could have included microbes between 3. Researchers looking at rocks in southwestern Greenland also saw cone-like structures that could have surrounded microbial colonies some 3.
In an effort to further refine the age of Earth, scientists began to look outward. The material that formed the solar system was a cloud of dust and gas that surrounded the young sun.
Gravitational interactions coalesced this material into the planets and moons at about the same time. By studying other bodies in the solar system, scientists are able to find out more about the early history of the planet. The nearest body to Earth, the moon, doesn't experience the resurfacing processes that occur across Earth's landscape. As such, rocks from early lunar history still sit on the surface of the moon.
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