The geologic timescale

Silurian Period to mya The first jawed fishes and uniramians like insects, centipedes and millipedes appeared during the Silurian over million years ago. Warming conditions saw the short-lived Ediacarian biota and finally the appearance of first metazoa.

Early history[ edit ] In Ancient GreeceAristotle BCE observed that fossils of seashells in rocks resembled those found on beaches — he inferred that the fossils in rocks were formed by living animals, and he reasoned that the positions of land and sea had changed over long periods of time.

The reducing anaerobic atmosphere enabled Archaea anaerobic microbes to develop, and plate tectonics followed a regime of continental drift different to that of the Proterozoic and later.

Until the discovery of radioactivity in and the development of its geological applications through radiometric dating during the first half of the 20th century, the ages of various rock strata and the age of Earth were the subject of considerable debate.

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Deep Time: The Geological Timescale

Late in the Proterozoic a runaway icehouse effect meant that the preceding warm conditions were replaced by a " Snowball Earth " with ice several kilometers deep covering the globe.

During this era, one type of organism, the Cyanobacteria blue-green algae produced oxygen as a metabolic by-product; the eventual build-up of this highly reactive gas was to eventually prove fatal to many life-forms, and converted the atmosphere from.

Brachiopods, crinoidscorals.

Geologic Time Scale

This was done by making a linear time line on the left side of the time columns. He also formulated the law of superposition, which states that any given stratum is probably older than those above it and younger than those below it.

First tetrapods appear toward the end of the period. This theory, known as " Plutonism ", stood in contrast to the "Neptunist" flood-oriented theory. Ordovician Period to mya Primitive plants appear on land. Gradually the molten Earth cools, outgassing of first atmosphere and oceans, bombardment by left-over planetesimal and debris.

Ends in huge extinction, due to glaciation. Many marine invertebrates marine animals with mineralized shells: Detailed geologic time scale: Archean Eon Lasting more than twice as long as the Phanerozoic eon, the Archean was a time when diverse microbial life flourished in the primordial oceans, and the continental shields developed from volcanic activity.

The supercontinent Rodinia began to break into smaller continents no correspondence to modern-day land masses. Our geologic time scale was constructed to visually show the duration of each time unit.

GSA Geologic Time Scale

Eras Eons are divided into smaller time intervals known as eras. The Hadean eon was characterized by extensive volcanism and formation of the first crust. Paleozoic Era Early in the million year history of the Paleozoic, atmospheric oxygen reached its present levels, generating the ozone shield that screens out ultraviolet radiation and allows complex life to live in the shallows and finally on land.

If two strata however distant in space or different in composition contained the same fossils, chances were good that they had been laid down at the same time.The first geologic time scale was proposed in by the British geologist Arthur Holmes ( - ).

This was soon after the discovery of radioactivity, and using it, Holmes estimated that the Earth was about 4 billion years old - this was much greater than previously believed.

rows · The geologic time scale (GTS) is a system of chronological dating that relates geological strata (stratigraphy) to time. It is used by geologists, paleontologists, and other Earth scientists to describe the timing and relationships of events that have occurred during Earth's history.

Walker, J.D., Geissman, J.W., Bowring, S.A., and Babcock, L.E., compilers,Geologic Time Scale v.

Geologic time scale

Geological Society of America, Online exhibits. Geologic time scale. Take a journey back through the history of the Earth — jump to a specific time period using the time scale below and examine ancient life, climates, and geography. GSA Geologic Time Scale. View printable PDF Buy the poster (20"x26") [in press] updated August Other versions: | | | Read about the changes to v () Commentary GSA Bulletin Article (Walker, et al.

) Files require Acrobat Reader to view and print. Geologic Ages of Earth History by Jeff Poling - a detailed and up to date chart showing every era, period, epoch and age of the geological time scale, together with time when began and during in millions of years.

The part of the chart marked in red shows the periods when dinosaurs were around.

The geologic timescale
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