Geologic Time

Radioactivity and radiometric dating

Atomic structure reviewed

Nucleus Protons: + charge Neutrons: neutral charge Electrons: – charge

An atom's atomic number is the number of protons in the atom's nucleus, while it's mass number is the number of protons + the number of neutrons in the nucleus. An isotope is a variant of the same parent atom; for example, an atom can have a different number of neutrons and and therefore a different mass number. Some isotopes are stable – Isotopes that never change (¹³C & ¹⁴C), while others are radioactive – Isotopes that spontaneously decay.

Scientists can use radioactive isotopes to determine when a mineral (and therefore the rock it's in) formed. All isotopes have a half- life, the time it takes for half of the parent (original) atom to decay to the new and hopefully more stable daughter product.

Some isotopes only need to go through the decay process once, as the daughter product is stable. Sometimes the daughter product is not stable and a second (and sometimes third) decay process is necessary.

Radiometric dates give the time a mineral began to preserve all atoms of parent and daughter isotopes. Igneous rocks yeild the best results as once the magm cools to form minerals the only changes that occur are weathering. Metamorphic rocks can yeild good ages, but they record when the rock was metamorphosed and not when the original rock formed. (the radiometric “clock” reset during the metamorphic process). Sedimentary rocks are not good to use, as the minerals that comprise them are from older rocks that have weathered to form the new sedimentary rock.

Putting it all together: Relative and Radiometric Dating

Events can be ordered based upon Relative Dating Principles, and the radiometric dating of plutons, sills, and dikes yield dates. Thus, the ages for sedimentary rocks can be “bracketed”. Example: there are five layers of sedimentary rocks, layer 1 is the youngest and layer 5 the oldest. If a magma chamber, dated to 5 million years old, intrudes into layers 5 and 4, and a 2 million year old sill intrudes inbetween 3 and 2, then layer 3 can be no older than 5 million years and no younger than 2 million years.