Page 22 - Laboratory Manual in Physical Geology

THE GEOLOGIC TIME SCALE
A chart showing the sequence, names, and ages of Earth’s rock layers (oldest at the bottom)
Eon of time Era of time Period of time Epoch of time Millions Some notable fossils
Eonothem Erathem of rock System of rock** Series of rock of years in named rock layers
of rock ago (Ma)
Holocene First Homo fossils,
Quaternary (Q) .0117 70–100% extant mollusks +
Pleistocene
2.6
Neogene Pliocene First humans (Hominidae),
(N) Miocene 5.3 15–70% extant mollusks +
Tertiary Oligocene 23
Cenozoic:
Cenozoic:
Cenozoic: Paleogene Eocene 34 More mammals than reptiles,
(new
life)
(new life) (P G ) 56 <15% extant mollusks +
(new life)
Age
Age of Mammals Paleocene
Age of Mammals
Mammals
of
66
Mesozoic:
Mesozoic:
Mesozoic: Last dinosaur fossils:
(middle life)
(middle life) Cretaceous (K) including Tyrannosaurus rex
(middle life)
Reptiles
Age
of
Age of ReptilesAge of Reptiles
145
First bird fossil:
Jurassic (J)
Archaeopteryx
201 First dinosaur, mammal,
Phanerozoic Permian (P) 252 turtle, and crocodile fossils
Triassic (T)
R
Last (youngest) trilobite fossils
Paleozoic:
(old life) 299
Carboniferous (C)* Mississippian (M) 323 First fossil conifer trees
Age of Trilobites Pennsylvanian (P ) P First reptile fossils
359
First amphibian, insect,
Devonian (D)
tree, and shark fossils
419
Silurian (S) First true land plant fossils
443
Ordovician (O) First fossils of coral and fish
485
First trilobite fossils
–
Cambrian (C)
First abundant visible fossils
541
Proterozoic
2500
Archean
Precambrian: Oldest fossils of visible life (stromatolites) 3500 Oldest fossils: mostly
An informal name for all 4000 microscopic life,
of this time and rock. visible fossils rare
Acasta Gneiss, northwestern Canada 4030
Nuvvuagittuq greenstone belt, Quebec, Canada 4280
Hadean
Zircon mineral crystals in the Jack Hills 4400
Metaconglomerate, Western Australia
Oldest meteorites 4550
*European name
**Symbols in parentheses are abbreviations commonly used to designate the age of rock units on geologic maps.
+
Extant mollusks are mollusks (clams, snails, squid, etc.) found as fossils and still living today.

FIGURE 1.3 The geologic time scale. Absolute ages in millions of years ago (Ma) follow the International Commission on Stratigraphy,

2013. See their website for more detailed versions and recent updates of the international geological time scale ( http://www.stratigraphy.org/
index.php/ics-chart-timescale ).
Thinking Like a Geologist ■ 7