An Earth billions of years old?

Geological ages, which are expressed in terms of millions or billions of years, rely on dating methods based on radioactive decay.

Radioactivity

To understand how the method works, we must first understand something about atoms.

Every atom has a small nucleus surrounded by a cloud of particles called electrons (with a negative electrical charge). In the nucleus there are protons (with a positive charge) and neutrons (with no electrical charge).

Some atoms are unstable and decay by emitting a certain kind of particle. As they decay, they change from being an atom of one element into an atom either of another element or of the same element with a lighter nucleus. An element that exists in more than one form is called an isotope, and an unstable isotope is called a radioisotope.

The time taken for one half the atoms of a radioactive substance to decay into its daughter element is called its half-life. In simple terms, radioisotope dating works by measuring the ratio of parent to daughter in a rock and calculating how long it took for the one to decay into the other. It can be applied only to rocks that formed directly out of magma (igneous rocks). The age of sedimentary rocks is worked out indirectly by dating the igneous rocks associated with them.

Rock types

There are 3 main types of rocks:

• Igneous – formed from magma which melts within the earth or is erupted onto the surface.
• Sedimentary – composed either of material such as sand and mud eroded from older rocks, or of biologically generated material such as calcium carbonate (e.g. chalk).
• Metamorphic – igneous or sedimentary rocks which have been transformed by high temperature or pressure.

Billions of years' worth of radioactive decay?

Rocks usually give very old radioisotope dates, within a broadly consistent sequence from older to younger, and there is no reason to doubt the amount of radioactive decay inferred. Evidence includes:

• the right amount of decay products in relation to what is left of the parent elements
• the absence of short-lived radioisotopes – suggesting that sufficient time has passed for them to decay away.

Many people suppose that evidence such as this proves that the Earth is billions of years old.

Conflicts between radioisotope ages and the direct evidence

The radioisotope dates so arrived at imply that the sedimentary layers between dated igneous rocks built up very slowly – commonly at a rate of a hair’s breadth per year. But other evidence suggests that the periods over which they occurred were much shorter.

Geologists are increasingly recognising that Earth history has not always been a tranquil affair, but has been punctuated by asteroid impacts, vast outpourings of lava, large-scale inundations, catastrophic releases of methane, ice-dam bursts and other such catastrophes. Were these events totally exceptional, or do they indicate a generally heightened level of geological activity in the past? The presence of fossils in so many sedimentary rocks is one indication that rates of deposition may have been generally much higher in the past. Fossilisation is generally an indication of what we would not judge to be abnormal conditions.

If radioisotope dates are valid, however, the brief periods during which layers accumulated rapidly must have been compensated for by hugely disproportionate periods when no deposition took place. This is a difficult argument, for in many sequences sedimentation appears to have been continuous. The evidence which we can actually see and touch suggests that the planet’s age is orders of magnitude less than thousands of millions of years.

Towards a solution: fast radioactive decay on a relatively young earth

The conflicting lines of evidence may be reconciled if radioactive decay rates were much faster in the past, possibly linked to a faster speed of light. Based on the observation that there is more evidence for time in later than in earlier rocks, our view is that radioactive decay has been slowing down at a decelerating rate. It was very fast in the distant past and bottomed out towards the present rate in the historical period.

The 'fast decay' proposal means that, while radioisotope dates would correctly reflect the order of events in earth history, the total timescale would shrink drastically, in keeping with the evidence of the rocks themselves.

This evidence includes:

1. tidal rhythmites – sedimentary sequences that can be matched to tidal cycles, indicating sedimentation rates of metres per year
2. thick, rhythmic sequences of turbidites – beds that each form over hours or minutes as unstable sediments avalanche down shelf
3. the growth rates of fossil reef-building organisms, such as corals
4. ecological recovery patterns following mass extinctions: radioisotope dating indicates millions of years, but actual experience suggests, at most, hundreds

For an in-depth discussion focusing on how long chalk beds took to form, see How old is the Earth? on the Earth History website.