# Age of the Earth

This is a learning project for exploration of scientific methods that have been used to measure the age of the Earth.

## Background

Until the Scientific revolution there was no way for people to systematically explore the age of the Earth. People in some cultures imagined that the Earth was very old (maybe even infinitely old) and others imagined that it was young, possibly only a few thousand years old.
Even after modern science began to develop in Western Europe, exploration of topics such as the age of the Earth was inhibited by cultural momentum. When Galileo was led by his astronomical observations to "radical" ideas such as the Earth moving around the Sun, he was ordered to abandon his heliocentric ideas. This religiously-motivated ban on advocacy of heliocentrism lasted more than 100 years. Ernst Mayr suggested that new discoveries such as recognition by astronomers of the large distance to the stars was important in allowing Europeans to begin to think about both vast space and long durations of time. In response to astronomical observations made using telescopes, some philosophers such as Immanuel Kant (1755, Universal Natural History and Theory of Heaven) eventually began to discuss cosmological theories in which the universe might have "infinite extent, both in space and time".

### study project

Which cultures around the world have favored either short or long ages for the Earth?

## Geology

During the 1700s geologists began to interpret evidence such as sedimentary rock strata as being consistent with vast periods of time during which erosion produced layers of sediments that were compacted into rock. Buffon thought it likely that the Earth was hundreds of thousands of years old, but such estimates were based on indirect arguments linking observations of recent geological processes to interpretations of observable rock strata.

### study project

What are the deepest known sedimentary rocks?
See also: Geological structures of fossiliferrous bearing material.

## Biology

Along with careful analysis of sedimentary rock strata came recognition that different types of fossil organisms are found in the various rock strata. Charles Darwin's intuitions about the time required for biological evolution made him comfortable with an interpretation of the geological evidence consistent with Earth being billions of years old.

### study project

When were the first microscopic fossils recognized?
Reading: Solution to Darwin's dilemma: Discovery of the missing Precambrian record of life by J. William Schopf See also: Bacterial fossils

## Physics

In Darwin's time, physical scientists such as William Thomson performed calculations on physical processes such as the cooling of the Earth and the burning of chemical fuel by the sun. They concluded that the Earth might only be tens of millions of years old. These estimates, based on seemingly irrefutable physical principles deeply troubled Darwin. How could the geological record and a gradual evolution of the great diversity of living organisms be made compatible with such a young age for the Earth?
Marie Curie and her collaborators eventually discovered radioactivity. Radioactivity provides a source of heat for the Earth that was unknown during Darwin's life time. Nuclear fusion was eventually recognized as a natural energy source that could keep the sun burning far longer than any chemical fuel.

### study project

Assume that the Sun is made of coal. Estimate how much longer the Sun could burn at its current rate of energy release.

## Radiometric dating

Radiometric dating makes use of the existence of radioisotopes. An example of a useful radioisotope for dating very old rocks is uranium. Uranium-238 is the most common isotope of uranium, accounting for more than 99% of uranium atoms in rocks on Earth. Uranium-238 nuclei are not stable: uranium-238 has a half life of about 4.5 billion years. Uranium-238 decays to lead-206. There are some minerals such as zircon which incorporate uranium but not lead. Most of the lead found in zircon comes from decay of uranium, so a determination of the ratio of uranium to lead allows estimation of the age of zircon-containing rock. This method was used as early as 1907 by Bertram Boltwood, who found that some Earth rocks appeared to be well over a billion years old.

### Nucleosynthesis and the age of the universe

Since the best estimates of the age of the Earth come from studies using radiometric dating, it is worth asking where radioactive isotopes such as uranium-238 come from. Cosmologists estimate that the universe is about 14 billion years old (see: age of the universe). About 14 billion years ago, most of the atoms of our universe formed and have remain unchanged as hydrogen and helium atoms. Most heavier atoms such as uranium apparently formed inside stars during the past 14 billion years (see: Supernova nucleosynthesis). Nucleosynthesis of uranium inside stars was followed by release of that uranium when stars exploded. The Sun is classified as a third generation star because of its relatively large amount of high atomic number atoms. The vast majority of atoms in the universe are hydrogen atoms. Stars such as the Sun fuse hydrogen atoms and produce helium atoms. An estimate of the age of the Sun can be made based on the ratios hydrogen and helium. The large amount of helium in the Sun (about 25%) is consistent with the solar system being several billion years old.

#### Study project

At the current rate of hydrogen --> helium fusion, how long will it take to increase the helium content of the Sun from 25% to 50%?

### Limitations on the use of radiometric dating of Earth rocks

There are some serious limitations on the use of radiometric dating of Earth rocks as a way to estimate the age of the Earth. There may have been an extended period of time during which the Earth existed without solid minerals and rocks. It is widely assumed that when the Earth was young it was too hot to allow formation of a solid crust. Estimation of the age of the Earth by radiometric dating of Earth rocks relies on the formation of minerals under relatively cool conditions and the persistence of those minerals at cool temperatures until the time they are processed for radiometric dating.
An additional problem is that the Earth remains geologically active. Rocks and minerals near the surface of the Earth are at risk of being subjected to subduction, heating and removal of any once accessible radiometric record of the past.
Radioactive Isotopes decay at an average rate. Therefore, an accurate half life is unknown for small samples, and thus age estimates are not as reliable as are often claimed.
Does it matter if we can precisely date the Earth? One reason to try to find and date the oldest rocks is to compare when they formed to the age of the rocks with the oldest fossils.

### Radiometric dating of meteorites

When the solar system formed, mineral condensations formed in space millions of years before the Sun ignited. Meteorites have been dated by radiometric methods, providing estimates of how long ago the Earth probably began to form by a process of accretion.

## Time

Tests with atomic clocks have shown that the flow of time is related to the speed of an object. Current theories suggest that the universe is either accelerating or decelerating (depending on who you believe). If this is true then it is safe to say that the Universe is expanding at a different speed than it was 1 billion years ago, i.e. as the earth is in a non-central position in the universe, it is moving at a different speed than 1GYa. This would lead to the conclusion that the flow of time (i.e. the length of 1s) was different 1GYa which leads to decay rates being different and, ultimately the age of the Earth being different that that which can be calculated. Once the varying speeds of the progression of the Galaxy through the Universe, the varyiance in the rotational speed of the Galaxy as well as those of the local cluster and solar system, the actual age of the Earth could be very different from the 4.54GYa quoted.Kendroche 07:56, 28 July 2010 (UTC)

### Study project on time dilation

Motion of the Earth: what is the current best estimate of the velocity of the Earth? Use the equations of time dilation to calculate the difference in time, had the Earth been stationary during its entire history. See Time dilation.

## Reading

* Age of the Earth - Wikipedia article (needs more citations to good sources)
* The Age of the Earth by Brent Dalrymple. Stanford University Press 1994. ISBN 0804723311
* Bursting the Limits of Time by Martin J. S. Rudwick. University of Chicago Press 2005. ISBN 0-226-73111-1

## Activities

* Develop b:Geology