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How many years must pass before something can be measured using a geological time scale? “This
day wi11 never end.” “My grandparents were once my age?!” Are these measured as geoIogic time?
Thousands and milHons ofyears are used on a geoIogical time scale. GeoIogic time is the chronoIogy
Of the Earth’s fomation, Changes, development, and existence. Grand Canyon began foming 5
million years ago. The Chinle Formation in the Painted Desert began forming over 200 million years
ago. The Earth is 4.6 billion years old.皿ese events are measured on a geoIogical time scale.
Scientists do not measure geoIogic time on a cIock or calendar. They use a linear timeline based on
the age ofrocks and their corresponding fossils, aS We11 as the change in life that occurred over
mi11ions ofyears. The GeoIogic Timeline on the next page shows an accepted timeline for the Earth
based on current science. The timeline is subject to change as new discoveries are made.
能EC州さ綱川 t帥E marks the origin ofthe Earth at about 4,500 million (4.5 billion) t血ough 543
million years ago. The oldest rocks and fossils found on Earth to date are within Precambrian Time.
Ybu can see some ofthe oldest rocks on Earth at the bottom of Grand Canyon. The metamorphic
Vishnu Schist that creates the imer gorge ofthe canyon is 2,000 million (2 billion) years old. The
fossil record of the Proterozoic Era shows that life was very primitive, COnSisting of photosynthetic
bacteha, Primitive marine plants, and single-Celled animals.
The P調州雷ROZO重く EO軸consists ofthe Paleozoic, Mesozoic, and Cenozoic Eras. The P▲LEOZO重C
駅▲ marks the fomation and movement of a supercontinent geoIogists call Pangaea, fi-Om 543
mi11ion through 248 million years ago. A11 continents presently on血e surface ofthe Earth had their
OngmS Within this single land mass. During the six periods ofthe Paleozoic, life developed rapidly
from marine plants and invertebrates, tO宜sh and spore-bearing plants, tO amPhibians. At the begin-
ning ofthe Paleozoic, Pangaea was Iocated cIoser to the South Pole and covered by glaciers, but by
the end ofthe era, it had migrated to the equator, Where its glaciers melted, reSulting in global climate change. The end ofthe Paleozoic was a time ofmass extinction amongst Earth’s life forms.
The細旺SOZO￨〔離▲ maks the development of life foms of increasing complexity, including
reptiles, dinosaurs, Small mammals, birds, COnifers, CyCads, and flowering plants, from 248 million
t血ough 65 mi11ion years ago. I}皿ing血e first period ofthe Mesozoic, the Triassic Period, the rocks
now within Petrified Forest National Park and the greater Painted Desert region were deposited.
During the Jurassic and Cretaceous Periods, dinosaurs developed into giants, dominating the animal
WOrld. The end ofthe Mesozoic Era is marked by a mass extinction ofthe dinosaurs, 1eaving only
their fossilized remains to te11血eir story.
We are currently in the C鮒OZO重く 駅▲, dating from 65 m皿on years ago through today. This era
includes the development of large maIrmals and ofhunan beings. It also includes several periods of
COntinental glaciations, Or ice ages, that played a role in the formation of geoIogical features visible
On Earth today.