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Earth and Space Science Unit 4: Earth's history

Rock layers and radioactive elements let scientists read Earth's 4.6-billion-year history.

Covers: Relative and absolute dating, superposition, fossils, radioactive decay and half-life, geologic time scale

Study guide

Geologists use two kinds of dating. Relative dating puts events in order without exact ages. The law of superposition says that in undisturbed sedimentary layers, the bottom layers are oldest. The principle of cross-cutting relationships says a fault or intrusion is younger than the rock it cuts through. Unconformities are gaps where layers were eroded away. Index fossils, from species that lived briefly but were widespread, help match and date layers in different places.

Fossils are preserved remains or traces of ancient life, including bones, shells, imprints, molds and casts, footprints and amber. They form most often in sedimentary rock, and they show how life and environments have changed.

Absolute dating gives an age in years using radioactive decay. Radioactive isotopes (parents) decay into stable daughter isotopes at a constant rate. The half-life is the time for half of the parent atoms to decay. After one half-life, 1/2 remains; after two, 1/4; after three, 1/8. Comparing the amounts of parent and daughter gives a rock's age. Carbon-14, with a half-life of about 5,730 years, dates once-living material up to about 50,000 years old, while uranium and potassium isotopes date much older rocks.

The geologic time scale divides Earth's 4.6-billion-year history into eons, eras, periods and epochs, often marked by mass extinctions. Most of Earth's history is the Precambrian. The Paleozoic Era saw the explosion of complex life, the Mesozoic Era was the age of dinosaurs and ended with an asteroid impact, and the Cenozoic Era, the age of mammals, continues today. Humans appeared very recently.

Earth's early atmosphere came mostly from volcanic outgassing of water vapor, carbon dioxide and nitrogen, and it had almost no free oxygen. Photosynthesizing cyanobacteria gradually added oxygen, beginning more than 2 billion years ago, which made today's oxygen-rich atmosphere and ozone layer possible. A volcanic ash layer settles over a huge area in a very short time, so it marks the same moment wherever it's found. Geologists use these layers, like index fossils, to match rock layers between distant outcrops, and ash can also be dated radiometrically.

Sediments are originally deposited in flat, horizontal layers. So layers that are tilted, folded or offset along faults show that the crust moved after the rocks formed, and those deformations are younger than the layers they affect.

Key ideas

• Superposition: in undisturbed layers, the bottom is oldest.
• Cross-cutting relationships: a fault or intrusion is younger than the rock it cuts.
• Index fossils date layers because the organisms lived briefly but widely.
• Radioactive isotopes decay at a steady rate; a half-life is the time for half to decay.
• The geologic time scale divides history into eons, eras, periods and epochs.

Formulas and rules

Common mistakes

• Thinking a half-life means all the isotope is gone after two half-lives. A quarter remains.
• Mixing up relative dating (order) with absolute dating (years).

Words to know

Law of superposition

Bottom layers are oldest (if undisturbed).

Half-life

Time for half of a radioactive sample to decay.

Index fossil

A fossil that dates rock layers: lived briefly, widespread.

Age of Earth

About 4.6 billion years.

Relative vs absolute dating

Relative gives order; absolute gives an age in years.

Relative dating

Putting events in order without exact ages.

Cross-cutting relationships

A fault or intrusion is younger than the rock it cuts.

Unconformity

A gap in the rock record where layers were eroded.

Absolute dating

Giving an age in years using radioactive decay.

Parent / daughter isotope

The decaying isotope / the stable product.

Carbon-14 dating

Dates once-living material up to ~50,000 years (half-life ~5,730 years).

Fossil

Preserved remains or traces of ancient life.

Geologic time scale

Eons, eras, periods and epochs dividing Earth's history.

Mass extinction

A large loss of species in a short time, often marking era boundaries.

Cenozoic Era

The age of mammals, continuing today.

Sample practice questions

19 more questions like these, from level 1 to 5, are in the practice set.