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 molten magma which crystallises within the earth or is erupted onto the earth's surface
• Sedimentary - composed of material like sand and mud which has come from the wearing down of older rocks.
• Metamorphic - igneous or sedimentary rocks which have been transformed by high pressures, high temperatures, or both.

Billions of years' worth of radioactive decay?

Crystalline rocks usually give very old radioisotope dates, within a broadly consistent sequence from older to younger. There is also other evidence that a substantial amount of radioactive decay has gone on in the past:

• The right amount of decay products - just what we would expect from millions of years of decay at today's rates.
• Short-lived radioisotopes are absent - suggesting that long ages have passed to allow them to all decay away.
• Visible scars (radiohalos) left by decay - direct evidence for hundreds of millions of years' worth of decay at today's rates.
• Crystal damage (fission tracks) left by splitting atoms - indicating millions of years of decay at today's rates.
• The expected heat in rocks near the Earth's surface - left by millions of years of decay at today's rates.

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

Conflicts between radioisotope ages and the direct evidence

Radioisotope dating implies that the sedimentary layers that lie between crystalline rocks built up very slowly – commonly at a rate of a hair’s breadth per year. But other evidence suggests that events were more violent and the periods over which they occurred 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? The presence of fossils in so many sedimentary rocks points to the latter. Well-preserved fossils would have been much rarer if rates of deposition were of the order indicated by radioisotope dating.

If radioisotope dates are valid, the brief periods during which layers accumulated rapidly must have been compensated for by hugely disproportionate periods when no deposition took place. But the top layers at such junctures often have surprisingly little to show for such long gaps. The evidence which we can actually see and touch suggests that the planet’s age should be measured in tens of thousands of years rather than thousands of millions.

Towards a solution: fast radioactive decay on a young earth

The conflicting lines of evidence may be reconciled if radioactive decay rates were much faster in the past. Faster rates of decay may have been a consequence of a faster speed of light, for which there is independent evidence, including the distance-related (and hence time-related) redshift of stars.

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

Creationist scientists are currently researching this possibility. One of the strongest pieces of evidence for fast decay is that zircon crystals in Precambrian rocks contain unexpectedly high amounts of helium. Helium is a gas that forms as a by-product of radioactive decay, and in the case of granites of Precambrian age most of the helium should have escaped into the atmosphere. But this is not what we find. The amount of helium still locked in the granite suggests that the decay which produced the helium has been going on for a relatively short period.

For further information on the helium evidence, see the Creation Research Society article Helium diffusion age of 6,000 years supports accelerated nuclear decay. While we do not agree that Earth history can be condensed into 6,000 years or that this is the timescale required by Genesis, it does seem fair to conclude that the ages produced by radioisotope dating are greatly inflated.

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.