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The Geologic Time Scale


    1.    Explain how the geologic time scale summarizes Earth's history.

    2.    Explain some of the major changes that have occurred on Earth over time and give the reasons for the changes.



The geologic time scale is a summary of Earth's past as it is recorded in the rock layers of Earth's surface.  Geologists depend on the Law of Superposition and rock correlation to sequence and date major events and assign a time for their occurrence.  The time divisions are as follows:

(earthsci.org/palaeo/ palaeo.html)

eon - longest division
Archeon - 1st eon of Earth ~3.9 to 2.5 billion years ago
Proterozoic - lasted for the next 2 billion years
Phanerozoic - most resent with evidence of life
era - there are three eras per eon
Paleozoic - ~543 million years ago "Age of Invertebrates"
fossils of both land and plants
Mesozoic - ~248 million years ago "Age of Reptiles"
Cenozoic - ~most recent "Age of Mammals"
appearance of humans
Precambrian - all periods before the paleozoic era
rocks lack index fossils
fossil evidence is contained in stromatolites - layers of bacteria and algae
content is high in valuable metals - Au, Ag, Cu, ...
trillobites is the most common index fossil ~500 million years ago
graptolites are the most common index fossil - lived in colonies
Silurian - appearance of terrestrial animals
Devonian - Age of the Fishes - 1st fossils of lungfish found
Carboniferous - appearance of reptiles and vertebrates
Permian - formation of Pangaea - nearly 1/2 of known fossils extinct by the end of the period 
marks the end of the Paleozoic era
Triassic - Jurassic - Cretacious are periods of the Mesozoic
rise and fall of the dinosaurs
Paleogene, Neogene, & Quarternary are periods of the Cenozoic
formation of modern climate features and the rise of man

(earth.geol.ksu.edu/ sgao/g100/plots/)

The geologic history gives the history of how organisms have changed over time.  A term that was later called evolution described how organisms change over time as the environment changes.  The phrase natural selection refers to an organisms ability to adapt to its surroundings.  The individuals that are better adapted leave more offspring and therefore dictate the new phenotype (outward characteristics) of a species.





Last modified: March 29, 2004