6.1 Relative & Absolute Dating

Why Dating Matters on the Regents

The History of Earth strand asks you to reconstruct the order of past events from rock and fossil evidence, and to attach numerical ages to that order. Roughly 11-20% of the Earth and Space Sciences Regents is built on this reasoning. You will face two kinds of tasks: relative dating (which event came first, with no numbers) and absolute dating (how many years ago, using half-life math). Both depend on reading rock layers, fossils, and the Radioactive Decay Data table in the 2024 Earth and Space Sciences Reference Tables (ESRT).

Think of a roadcut or cliff face as a stack of pages in a book. The principles below tell you the order the pages were written. The half-life math tells you the publication dates.

Relative Dating Principles

Relative dating orders events from youngest to oldest without assigning a number of years. Five principles drive almost every Regents question:

• Superposition - In an undisturbed sequence of sedimentary layers, the oldest bed is on the bottom and each layer above it is younger. This is the single most-tested idea in the strand.
• Original horizontality - Sediments are deposited in nearly flat, horizontal layers. So tilted or folded beds were originally flat and were deformed after deposition.
• Cross-cutting relationships - A feature that cuts across rock (a fault, a fracture, or an igneous intrusion such as a dike) must be younger than the rock it cuts.
• Inclusions - Fragments (inclusions) of one rock found inside another are older than the rock that surrounds them, because they had to exist first to be picked up.
• Unconformities - A buried erosional surface that represents a gap of missing time. Rock was deposited, uplifted, eroded away, then buried by new sediment, so part of the record is gone.

Reading an Outcrop

Work from the bottom up using superposition, then insert any fault, intrusion, or unconformity using the rule that the disturbance is younger than everything it cuts or erodes. A common trap: an intrusion can be younger than the layers above it only if it actually cuts them - check exactly which beds it crosses.

Correlation, Rock Type, and Index Fossils

Correlation matches rock layers of the same age in different locations. Two methods appear on the Regents:

1. Correlation by rock type (walking the bed) - Layers with the same color, grain size, composition, and position can be matched between nearby outcrops. This works well over short distances but fails when rock types change laterally.
2. Correlation by fossils - The most reliable method over long distances. Layers containing the same fossils are interpreted to be the same age.

Index Fossils

An index fossil is the best correlation tool. To be useful, an organism must be:

• Widespread geographically (found in many places),
• Abundant (easy to find),
• Short-lived as a species (existed during only a brief slice of geologic time), and
• Easily identified.

Because the species existed for only a short time, finding it pins the rock to a narrow age. The ESRT Geologic History of New York State chart lists many New York index fossils (such as the trilobite Phacops and the eurypterid Eurypterus) with the time interval each one marks. A long-lived organism makes a poor index fossil because it does not narrow the age.

Absolute Dating: Radioactive Decay and Half-Life

Absolute (numerical) dating uses radioactive decay. A radioactive parent isotope is unstable and decays into a stable daughter product at a constant rate. A half-life is the time it takes for half of the remaining parent atoms to decay. Decay is unaffected by heat, pressure, or chemistry, which is what makes it a reliable clock.

After each half-life the parent that is left is cut in half again:

ESRT Radioactive Decay Data

The Earth and Space Sciences Reference Tables give the half-life and decay product for four isotopes. Memorize that you must look these up - the Regents expects you to read this table:

Matching the clock to the age: C-14 has a short half-life, so it dates recent organic material (wood, bone, shells) up to roughly 50,000-60,000 years - never the age of the Earth. U-238, K-40, and Rb-87 have enormous half-lives and date very old rocks, including Earth's oldest minerals.

Worked Half-Life Calculations

Example 1 - Whole number of half-lives. A bone contains 1/8 of its original Carbon-14. How old is it?

• 1/8 = (1/2)^3, so 3 half-lives have passed.
• C-14 half-life = 5,700 years (ESRT).
• Age = 3 x 5,700 = 17,100 years.

Example 2 - Reading 'fraction remaining' from a graph. A rock sample contains 25% of its original Potassium-40. How old is the rock?

• 25% = 1/4 = (1/2)^2, so 2 half-lives have passed.
• K-40 half-life = 1.3 billion years.
• Age = 2 x 1.3 = 2.6 billion years.

Example 3 - Parent-to-daughter ratio. A rock has equal amounts of Uranium-238 and its daughter Lead-206 (a 1:1 ratio).

• Equal parent and daughter means half the parent has decayed, so 1 half-life has passed.
• U-238 half-life = 4.5 billion years.
• Age = 1 x 4.5 = 4.5 billion years.

Example 4 - Choosing the right isotope. To date the age of the Earth (about 4.6 billion years), you would NOT use C-14, because almost none would remain after just tens of thousands of years. You use a long half-life isotope such as U-238.

Calculation Checklist

1. Convert the fraction (or percent) of parent remaining into a power of 1/2.
2. The exponent = the number of half-lives.
3. Multiply the number of half-lives by the ESRT half-life for that isotope.
4. Sanity-check the scale: organic + young = C-14; ancient rock = U-238, K-40, or Rb-87.