Geologic Time Scale: Quick Notes

Geologic Time Scale: Core Concepts

• The geologic time scale divides Earth's history into manageable units based on the rock record.
• Major units: $ext{Eons}, ext{ Eras}, ext{ Periods}, ext{ Epochs}$.
• Precambrian time represents about $80\%$ of Earth's history; includes the Hadean, Archean, and Proterozoic Eons.
• Phanerozoic Eon includes the Paleozoic, Mesozoic, and Cenozoic Eras.

Fossils and Fossil Types

• Fossils: preserved remains or traces of past life.
• True form fossil: entire organism is preserved.
• Mold fossil: hollow impression of the organism in rock.
• Cast fossil: minerals fill a mold.
• Trace fossils: imprints of activity (e.g., footprints, nests).

Dating Fossils: Relative vs Absolute

• Two main approaches: Relative dating and Absolute dating.
• Relative dating places fossils in a sequence without exact ages.
• Absolute dating provides numeric ages using radiometric methods.

Relative Dating Principles

• Law of Superposition: younger rocks on top of older rocks when layers are undisturbed.
• Law of Original Horizontality: deposition is horizontal; tilting/folding happens after deposition.
• Law of Cross-Cutting Relationships: intrusions/faults are younger than the rocks they cut.
• Law of Inclusion: fragments within a rock are older than the host rock.
• Faunal Succession: fossils evolve through time and correlate strata regionally.
• Index (guide/zone) fossils: short-lived organisms used to define geologic intervals.

Absolute Dating Principles

• Absolute dating uses radiometric dating to determine actual ages.
• Key isotopes: $^{14}\mathrm{C}$ (Carbon-14) for relatively recent ages; $^{40}\mathrm{K}$ (Potassium-40) for older ages.
• Half-life concept: the time required for half the parent isotope to decay to daughter.
• The decay products are stable (daughter) atoms.
• Expressed as $t_{1/2}$, the half-life.

The Geologic Time Scale: Eons, Eras, and Periods

• Phanerozoic Eon comprises three major Eras: Paleozoic, Mesozoic, Cenozoic.
• Precambrian time precedes Phanerozoic and includes Hadean, Archean, Proterozoic Eons.
• The timeline is based on rock records and fossil evidence.

The Precambrian Time

• Represents ~$80\%$ of Earth's history.
• Eons: Hadean, Archean, Proterozoic.
• Often called the "Hidden Life" of Earth due to scarce fossil evidence.

Hadean Eon

• Early, hostile conditions; meteorite bombardment; formation of oceans.
• Atmosphere and early hydrologic cycle develop.

Archean Eon

• Anaerobic life; no ozone; photosynthetic prokaryotes (cyanobacteria) begin releasing oxygen.
• Life remains primarily single-celled.

Proterozoic Eon

• Oxygen rises to ~$3\%$ of the atmosphere.
• Multicellular life appears; ozone layer forms.

Phanerozoic Eon: Paleozoic, Mesozoic, Cenozoic

• Paleozoic: early animal fossils (sponges, corals, trilobites); early fishes; forests with giant ferns.
• Mesozoic: "Age of Reptiles"; dinosaurs dominate; first birds and flowering plants appear; end of era marks major extinctions.
• Cenozoic: "Age of Mammals"; mammals diversify; ice ages occur; origin of humans.

Mass Extinctions

• Extinction is normal background through time; mass extinctions are periods when many species die out.
• Dinosaurs’ extinction linked to dramatic climate changes, possibly due to a meteorite impact or volcanic activity.
• Mass extinctions are followed by rapid diversification of surviving groups.