Geologically how old is the earth

The third, which by the end was the most acute, presented a problem that outlasted the controversy itself. He did not need to wait long. In Sir Arthur Eddington came up with the answer: the fusion of hydrogen into helium. One referred to the depth of the sediments and the time they would have taken to accumulate; the other referred to the salinity of the oceans, compared with the rate at which rivers are supplying them with sodium salts.

In hindsight, both theories were deeply misguided, for similar reasons. They assumed that current rates—of sediment deposition and of salt transport by rivers—were the same as historical rates, despite the evidence they had that our own age is one of atypically high geologic activity. Worse, they measured inputs but ignored outputs. The rock cycle, as we now know, is driven by plate tectonics, with sedimentary material vanishing into subduction zones.

And the oceans have long since approached something close to a steady state, with chemical sediments removing dissolved minerals as fast as they arrive.

Nevertheless, by the late 19th century the geologists included here had reached a consensus for the age of the earth of around million years.

Having come that far, they were initially quite reluctant to accept a further expansion of the geologic timescale by a factor of 10 or more. And we should resist the temptation to blame them for their resistance.

Radioactivity was poorly understood. Different methods of measurement such as the decay of uranium to helium versus its decay to lead sometimes gave discordant values, and almost a decade passed between the first use of radiometric dating and the discovery of isotopes, let alone the working out of the three separate major decay chains in nature.

The constancy of radioactive decay rates was regarded as an independent and questionable assumption because it was not known—and could not be known until the development of modern quantum mechanics—that these rates were fixed by the fundamental constants of physics. It was not until , when under the influence of Arthur Holmes, whose name recurs throughout this story the National Academy of Sciences adopted the radiometric timescale, that we can regard the controversy as finally resolved.

The oldest — and by far the longest — is called the Precambrian. The Cenozoic started about 65 million years ago. Each of these Eras, in turn, are divided into increasingly smaller divisions known as Periods, Epochs and Ages.

That means changes happen in spurts, rather than at some slow and steady pace. Take the Precambrian Era. That burst marked the beginning of the Paleozoic Era. Sea creatures like trilobites and fish emerged and came to dominate. Then, million years ago, the Mesozoic Era burst into being. It marked the biggest mass extinction of all. It also kicked off the spread of life on land. This era then ended abruptly — and famously — But they did understand relative ages, based on a simple, yet powerful principle.

That principle is called the Law of Superposition. It states that in an undisturbed stack of rock layers, the oldest layers will always be on the bottom, and the youngest on top. The Law of Superposition allows geologists to compare the age of one rock or fossil to another. It makes the sequence of geologic events more clear. After all, they lived millions of years apart. The oldest meteorites have ages between 4. The asteroids, and these oldest meteorites, are thought to be pieces of planetesimals.

Planetesimals are small, solid bodies that formed early in solar system history, most of which combined together to become the planets.

Asteroids are the remains of planetesimals that did not condense into a larger planet, and thus have not changed much since that early stage of solar system history.

One type of meteorite, known as chondrites, is thought to represent the first type of solid that condensed during the initial formation of the solar system, and chondrites have ages close to 4.

Other types of meteorites appear to represent stages of planetesimal formation, with ages tapering down to about 4.

The age of the oldest Moon rocks. The Apollo astronauts brought back samples of moon rocks thought to have formed soon after the Moon originated, as it solidified from a largely molten state. The oldest Moon rocks are about 4. The theoretical age of the Sun. Stars are, in a sense, a lot less complicated than planets such as earth, because earth is made of solids, liquids, and gases, whereas the Sun is essentially just a big ball of gas. Also, scientists recently reported 4.

Narryer and the Jack Hills in Western Australia Nutman , which suggests the erosion of granite-like crust older than the oldest known preserved crust Cooper Thus, we can be confident that the minimum age for the Earth exceeds 4 billion years by examining Earth materials. So how do we get from 4.