ROCK CLOCK
PANEL TEXT
Absolute geologic time describes a rock's age in actual years. It is computed by measuring the radioactive elements in the rock.
Some rocks contain naturally radioactive elements. Called "parent" elements, they decay (release energy) at constant rates until they stabilize as different "daughter" elements. Radiometric dating measures the relative amount of parent and daughter elements in a rock to compute how many years ago it crystallized.
The time it takes for a parent element to release half of its radioactive energy is a known constant, called the "half-life" of that element. Potassium 40, which decays into Argon, has a half-life of 1.3 billion years and is the element most commonly used to determine the age of rocks.
Radiometric dating is used mostly for igneous rocks, which formed from molten material, such as magma. Most fossil-bearing rock is sedimentary; it formed from particles that eroded from other rocks. Sedimentary rock can be dated -- in years -- only by its position relative to other rocks, above and below it, that can be dated radiometrically, such as lava flows.
A process called radiocarbon dating can be used to calculate the age of organic fossils (wood, bone, teeth, etc.) in rocks younger than 50 thousand years. All living things acquire two forms of carbon; one is a naturally radioactive form called Carbon 14. When an organism dies, its Carbon 14 decays at a constant rate. The relative amount of Carbon 14 remaining in the fossil is measured to determine how long ago the organism died.
PICTURE CAPTIONS:
- Parent elements
- Daughter elements
- Half-Life 1.3 billion years