Early Earth, Page 20: [Previous Page] [Next Page] [Class Home Page]

The Geological Periods and Attaching Ages to Them

Below is the full-blown description of the Geological Periods. Rather than breaking up the Periods as even mathematical bins, each Era, Period, Epoch, etc., is always attached to a certain characteristic of the prevalent species at that time. The exact dates of these eras then becomes an academic matter, consequential, but open for debate and refinement without altering the original definition of the Periods.

EON	ERA	PERIOD			EPOCH	DATES	AGE of
Phanerozoic	Cenozoic	Quaternary			Holocene	0-2	Mammals	Humans
					Pleistocene
		Tertiary	Neogene		Pliocene	2-5
					Miocene	5-24
			Paleogene		Oligocene	24-37
					Eocene	37-58
					Paleocene	58-66		Extinction of dinosaurs
	Mesozoic	Cretaceous				66-144	Reptiles	Flowering plants
		Jurassic				144-208		1st birds/mammals
		Triassic				208-245		First Dinosaurs
	Paleozoic	Permian				245-286	Amphibians	End of trilobites
		Carboniferous		Pennsylvanian		286-320		First reptiles
				Mississippian		320-360		Large primitive trees
		Devonian				360-408	Fishes	First amphibians
		Silurian				408-438		First land plant fossils
		Ordovician				438-505	Invertebrates	First Fish
		Cambrian				505-570		1st shells, trilobites dominant
Proterozoic	Also known as Precambrian					570-2,500		1st Multicelled organisms
Archean						2,500-3,800		1st one-celled organisms
Hadean						3,800-4,600		Approx age of oldest rocks 3,800




Note: Dates are in millions of years Reference: Graham R. Thompson, Jonathan Turk, Modern Physical Geology, Saunders College Publishing

Radioactive Dating
When a mineral grain forms, the elements contained are locked in. If it contains radioactive elements, these will have slowly decayed with time. We can see their 'waste' accumulating in the rock the longer they have been locked in, giving us an estimate of when the rock formed. This method works best on igneous rocks. Important radioactive elements (parent-daughter) include: Argon-Potassium, Rubidium-Strontium, and Uranium-Lead. For more info click HERE and HERE