Alpha and beta decay are generally slower processes than gamma decay.
Half-lives for beta decay range upward from one-hundredth of a second and, for alpha decay, upward from about one one-millionth of a second.
For example, about 1.5 percent of a quantity of Uranium 238 will decay to lead every 100 million years.
Despite seeming like a relatively stable place, the Earth's surface has changed dramatically over the past 4.6 billion years.
The age of the planet, though, was important to Charles Darwin and other evolutionary theorists: The biological evidence they were collecting showed that nature needed vastly more time than previously thought to sculpt the world.
A breakthrough came with the discovery of radioactivity at the beginning of the 1900s.
But for humans whose life span rarely reaches more than 100 years, how can we be so sure of that ancient date? Even the Greeks and Romans realized that layers of sediment in rock signified old age.
But it wasn't until the late 1700s -- when Scottish geologist James Hutton, who observed sediments building up on the landscape, set out to show that rocks were time clocks -- that serious scientific interest in geological age began.