GEOLOGIC TIME - EARTH'S HISTORY
ABOUT THE GEOLOGIC TIME SCALE
PRE-ARCHEAN (or HADEAN) EON
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
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
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
~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.
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)
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.)
EON - 570 Million years to the Present
The eon of complex life
ERA - 570 Million years to 245 Million years
The era of ocean life; land animals appear toward the end of this era
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
Period - 408 to 360 Million years
The first insects and first amphibians/tetrapods; "age of fishes"; first abundant forests on land
Period - 360 to 320 Million years - (see also Carboniferous
Abundant amphibians and the appearance of the first reptiles
Period - 320 to 286 Million years
305 MYA -- The first mammal-like reptiles
Period - 286 to 245 Million years
Reptiles spread and diversify; evaporate deposits; glaciation in the Southern Hemisphere
ERA - 245 Million to 65 Million years
The era of reptiles; some times called the "age of the dinosaurs"
Period - 245 to 208 Million years
First appearance of dinosaurs in the fossil record
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
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.
ERA - 65 Million years to the Present
The era of mammals.
Period - 65 Million to 1.6 Million years ago
Epoch - 65 Million to 58 Million years ago
Began with extinction of the dinosaurs
Mountain building in Europe and Asia
Epoch - 58 Million to 37 Million years ago
Horses (around 53 MYA), whales, and monkeys first appear in the fossil record
Epoch - 37 Million to 24 Million years ago
Elephants and apes first appear in the fossil record
Epoch - 24 Million to 5 Million years ago
Hominids first appear in the fossil record
Epoch - 5 Million to 1.6 Million years ago
2 MYA -- First humanlike animals
Period - 1.6 Million years to the Present
Epoch -- 1.6 Million to 10,000 years ago
The modern ice age; first modern humans appear
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"