GEOLOGIC TIME - EARTH'S HISTORY

ABOUT THE GEOLOGIC TIME SCALE


PRE-ARCHEAN (or HADEAN) EON
ARCHEAN EON
PROTEROZOIC EON
PHANEROZOIC EON
PALEOZOIC ERA
MESOZOIC ERA
CENOZOIC ERA
TERTIARY PERIOD
QUARTARNARY PERIOD
PLEISTOCENE EPOCH
HOLOCENE EPOCH


ABOUT THE GEOLOGIC TIME SCALE

Geologic time is the 4.6 Billion year history of Earth, from its origin to the present, as inferred from the rock record, both on Earth and the Moon, and the geochemical make-up of those two bodies. Geologic time is sometimes called "deep time".
Geologic time is divided into a four-level hierarchy of time intervals:
EONS -- The first and largest division of geologic time.
ERAS -- The second division of geologic time; each era has at least two periods.
PERIODS-- The third division of geologic time. Periods are named for either location or characteristics of the defining rock formations.

Location = the region where the period's characteristic rocks were first studied.

Characteristics = the nature of the unique system of rocks and rock formations that define the Period.

EPOCHS -- The fourth division of geologic time; represents the subdivisions of a period.
The time of the transition from one interval of geologic time to the subsequent one is usually marked by a relatively abrupt change in fossil types and numbers.

is not known precisely, with the uncertainty increasing with increasing age since fossils become rarer and harder to identify in the distant past.

There are four eons: Pre-Archean or Hadean; Archean; Proterozoic, Phanerozoic. The first three eons account for most of Earth's existence; collectively these three are called the Pre-Cambrian.

PRE-ARCHEAN EON (or HADEAN EON) - 4.6 to 3.8 Billion years
~4.6 BYA -- Formation of Earth and Moon (as indicated by dating of meteorites and rocks from the Moon)

~4 BYA -- Likely origin of life

This is the "hidden" portion of geologic time as there is little evidence of this time remaining in Earth's rocks.



ARCHEAN EON - 3.8 to 2.5 Billion years
The eon of first life

~3.8 BYA -- Oldest known rocks

~3.5 BYA -- Oldest known fossils (single celled organisms resembling bacteria)

3.2 BYA -- First known plants (algae)



PROTEROZOIC EON - 2.5 Billion to 570 Million years
The eon of the first multicelled life

1.2 BYA -- First known animal (jellyfish)
(End of the Pre-Cambrian -- a period at least five times longer than all the geologic time that follows.)



PHANEROZOIC EON - 570 Million years to the Present The eon of complex life

PALEOZOIC ERA - 570 Million years to 245 Million years

The era of ocean life; land animals appear toward the end of this era

Cambrian Period - 570 Million years to 505 Millions years

Onset marked by the appearance of first shellfish and corals; sometime called the "age of marine invertebrates"

Abundant fossils appear in the rock record for the first time

End of the Cambrian denoted by the appearance of fish

Ordovician Period - 505 Million years to 438 Million years

Between 510M and 505MYA - Fish first appear in the fossil record; these are the first vertebrates

Silurian Period - 438 Million to 408 Million years

Appearance of the first land plants; mountain building in Europe

Devonian Period - 408 to 360 Million years

The first insects and first amphibians/tetrapods; "age of fishes"; first abundant forests on land

Mississippian Period - 360 to 320 Million years - (see also Carboniferous Period)

Abundant amphibians and the appearance of the first reptiles

Pennsylvanian Period - 320 to 286 Million years

305 MYA -- The first mammal-like reptiles

Permian Period - 286 to 245 Million years

Reptiles spread and diversify; evaporate deposits; glaciation in the Southern Hemisphere

MESOZOIC ERA - 245 Million to 65 Million years

The era of reptiles; some times called the "age of the dinosaurs"

Triassic Period - 245 to 208 Million years

First appearance of dinosaurs in the fossil record

Jurassic Period - 208 to 145 Million years

First appearance of mammals (around 222 MYA); dominance of the dinosaurs; mountain building in North America

150 MYA -- First birds

Cretaceous Period - 145 to 65 Million years

Flowering plants appear and spread rapidly; continued increase in dinosaurs.

Climate warmer than at present, with sea level higher

This period (and also the Mesozoic Era) end abruptly with death of the dinosaurs.

CENOZOIC ERA - 65 Million years to the Present

The era of mammals.

Tertiary Period - 65 Million to 1.6 Million years ago

Paleocene Epoch - 65 Million to 58 Million years ago

Began with extinction of the dinosaurs

Mountain building in Europe and Asia

Eocene Epoch - 58 Million to 37 Million years ago

Horses (around 53 MYA), whales, and monkeys first appear in the fossil record

Oligocene Epoch - 37 Million to 24 Million years ago

Elephants and apes first appear in the fossil record

Miocene Epoch - 24 Million to 5 Million years ago

Hominids first appear in the fossil record

Pilocene Epoch - 5 Million to 1.6 Million years ago

2 MYA -- First humanlike animals

Quaternary Period - 1.6 Million years to the Present

Pleistocene Epoch -- 1.6 Million to 10,000 years ago

The modern ice age; first modern humans appear

Holocene Epoch - 10,000 years ago to present day

Began with the end of the most recent glaciation

RETURN TO FUNDAMENTAL CONCEPT "TIME AND SPACE SCALES"