framework for 4.54 billion years of Earth history, divided into hierarchical units
eons: Hadean, Archean, Proterozoic, Phanerozoic
eras: Paleozoic, Mesozoic, Cenozoic (within Phanerozoic)
periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Paleogene, Neogene, Quaternary
epochs: subdivisions of periods (Holocene, Pleistocene, Miocene)
boundaries defined by extinction events, tectonic shifts, climate transitions
radiometric dating (uranium-lead, potassium-argon, carbon-14) provides absolute ages
deep time: the recognition that Earth's age dwarfs human history by six orders of magnitude
stratigraphy reads rock layers as a record of planetary entropy and renewal