Geologists use two main methods to unravel Earth's history: relative and absolute dating. Relative dating puts events in order, like figuring out which layer of rock is older. Absolute dating gives specific ages, like determining a rock is 100 million years old.

These techniques help piece together Earth's timeline. Relative dating uses principles like superposition, while absolute dating relies on radioactive decay. Together, they reveal the fascinating story of our planet's past.

Relative and Absolute Dating Methods

Relative vs absolute dating methods

• Relative dating determines the order of events or relative age of rocks, structures, or fossils without providing specific numerical ages (superposition, cross-cutting relationships, index fossils)
• Absolute dating determines the specific numerical age of rocks, minerals, or fossils expressed in years or millions of years using techniques like radiometric dating (potassium-argon, uranium-lead, carbon-14 dating)

Principles of relative dating

• Principle of superposition states that in an undeformed sequence of sedimentary rocks, the oldest layers are at the bottom and the youngest layers are at the top
• Principle of original horizontality suggests that sedimentary layers are deposited in a nearly horizontal position, and if not horizontal, they have been tilted, folded, or deformed after deposition
• Cross-cutting relationships indicate that a geologic feature cutting across another feature must be younger than the feature it cuts (faults, intrusions, erosional surfaces)
• Inclusions are fragments of one rock included within another rock that must be older than the host rock, as the included fragments were already present when the host rock formed around them

Radiometric dating for absolute ages

• Radiometric dating is based on the decay of radioactive isotopes with unstable nuclei that decay at a constant rate measured by the half-life, the time required for half of the original amount of the isotope to decay
• The age can be calculated by measuring the ratio of parent isotope to daughter product in a mineral or rock sample using the equation: $t = \frac{1}{\lambda} \ln(\frac{N_t}{N_0} + 1)$
  1. $t$ represents the age of the sample
  2. $\lambda$ is the decay constant
  3. $N_t$ is the number of atoms of the daughter isotope at time $t$
  4. $N_0$ is the number of atoms of the parent isotope at time $t=0$
• Common radiometric dating methods include:
  • Potassium-argon (K-Ar) dating used for rocks older than 100,000 years
  • Uranium-lead (U-Pb) dating used for rocks older than 1 million years
  • Carbon-14 dating used for organic materials less than 50,000 years old

Limitations of dating methods

• Relative dating limitations:
  • Does not provide numerical ages
  • Can only determine the sequence of events, not the duration between events
• Radiometric dating assumptions:
  • The decay rate has remained constant over time
  • The system has remained closed with no loss or gain of parent or daughter isotopes
  • The initial amount of daughter product is known or can be determined
• Radiometric dating limitations:
  • Requires the presence of suitable minerals or organic material
  • The sample must not have been altered or contaminated since its formation
  • The half-life of the isotope must be appropriate for the age range being measured (short half-lives for young samples, long half-lives for old samples)