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:
|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|
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.