Because the exact amount of time this accretion process took is not yet known, and the predictions from different accretion models range from a few million up to about 100 million years, the exact age of Earth 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.

Unlike the recent history of the earth, the very distant past is not on written record.

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.

Studies of strata, the layering of rocks and earth, gave naturalists an appreciation that Earth may have been through many changes during its existence.

These layers often contained fossilized remains of unknown creatures, leading some to interpret a progression of organisms from layer to layer.

Lloyd Anderson, far right, takes home-schooled children on a tour of the land around Mount St.

Helens, pointing out geologic structures formed by the volcano’s eruption in 1980 that young-Earth creationists say support their beliefs.“Look! ” Lloyd Anderson springs ahead of the small group of visitors he has been leading through Gifford Pinchot National Forest.

First, let’s define a theory in a better way than the textbook describes it.

“A theory has its genesis in a hypothesis, which is a working assumption as to why we observe something—an educated guess.

In the mid-18th century, the naturalist Mikhail Lomonosov suggested that Earth had been created separately from, and several hundred thousand years before, the rest of the universe. In 1779 the Comte du Buffon tried to obtain a value for the age of Earth using an experiment: He created a small globe that resembled Earth in composition and then measured its rate of cooling.

This led him to estimate that Earth was about 75,000 years old.

To test this assumption, scientists conduct experiments that either disprove or correlate with the hypothesis. If new technology allows better experimentation, for example, a theory may need to be discarded.