Class notes
Unit 3 - Geology
- Course
- SES4U (SES4U)
- Institution
- White Oaks Secondary School
In class notes from the third unit of grade 12 earth and space science, geology.
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Unit 3 - Earth & Space Science Notes, Geologic Time
● Have you ever related past events to specific dates? Can you deduce the approximate time of
an event from other events? For example, you remember a snowstorm the last time you visited
Toronto - therefore you probably went during the winter. It is important for scientists to
organize events based on when they occurred and ideally to have specific dates of time
periods associated with each event. This organization allows them to identify patterns in
evolution.
● When referring to recorded historical events, the time is measured in years and centuries;
however geologic time is recorded in millions and billions of years because the Earth is
believed to be approximately 4.55 billion years old. Similar to how you record events on the
yearly calendar, a geologist can record events on a geologic calendar which is divided into
long time periods based on observable changes to life forms, geologic landforms, mass
extinctions, change in climate patterns, and continent positions.
Geologic Time Scale:
● The ages given are in millions of years before the present abbreviated as mya.
● The geologic time scale is divided into four eons. Precambrian time is composed of three
eons. The Hadean began 4550 mya, the Archean eon 3800 mya, and the Proterozoic eon
began in 2500 mya. The current Phanerozoic eon began 570 mya.
● The current eon is divided into eras and periods.
● The Paleozoic era began 570 mya and ended 245 mya. The periods of the Paleozoic era with
the beginning dates are, Cambrian 570 mya, Ordovician 505 mya, Silurian 436 mya, Devonian
406 mya, Mississippian 360 mya, Pennsylvanian 320 mya, and the Permian 286 mya.
, ● The Mesozoic era began 245 mya and ended 66.4 mya. The three periods and start dates are
the Triassic 245 mya, Jurassic 206 mya, and Cretaceous 144 mya.
● The Cenozoic era began 66.4 mya and is still going on. The Tertiary period has been recently
divided into two different periods. The Paleogene began 66.4 mya, and the Neogene 23.7
mya, the final period is the Quaternary, which began 1.6 mya.
● The epochs of the Tertiary period are the Paleocene 66.4 mya, Eocene 57.8 mya, Oligocene
36.6 mya, Miocene 23.7 mya, and the Pliocene 5.3 mya. The Quaternary period is divided into
two epochs the Pleistocene 1.6 mya and the Holocene 0.01 mya.
● The geologic time scale is like a calendar extending from Earth’s formation to the present. The
scale is divided into eons, eras period and epochs.
○ Eon - The largest group: billions of years long.
○ Era - Mass extinctions mark the boundaries between the eras: hundreds of millions of
years long
○ Period - tens of millions of years long
○ Epoch - divisions of the most recent periods; several million years long.
● Eons are the longest division of time based on the abundance of organisms. The Precambrian
Supereon is divided into three eons, the Hadean Eon (the beginning of Earth to the first living
organisms-bacteria); the Archean eon and the Proterozoic eon are the time periods when the
bacteria diversified and the atmospheric composition was altered with photosynthetic
cyanobacteria (also known as blue-green algae) converting carbon dioxide to oxygen. The
Phanerozoic eon is the current eon and is characterized by a diversification of multicellular
living organisms.
● Eons are subdivided into eras at mass extinction events. The Phanerozoic eon is composed of
three eras:
○ The Paleozoic era concluded 248 million years ago with a mass extinction of more than
90% of marine organisms.
○ The Mesozoic era known as the age of the dinosaurs concluded with a mass extinction
event caused by an asteroid impact.
○ The Cenozoic era (the current era) is characterized as the age of the mammals.
● The division between the Mesozoic and Cenozoic eras is illustrated in these two illustrations.
Also known as the K-T boundary, it is the mass extinction event between the two eras and is
the transition stage between the dinosaurs that dominated the Cretaceous period and the
mammals that dominate the Tertiary period.
● Note: mammals first appeared in the Mesozoic in the Triassic period, however there were only
small rodent-like species.
● Eras are subdivided into periods that are characterized with the appearance of different groups
of organisms. For example the Cambrian period with the first trilobites, the first fish in the
Ordovician period, and the first flowering plants in the Cretaceous period.
How Do You Remember All of the Periods?
● The following mnemonic could be used to help you in remembering the correct order of the
periods from the beginning of the Paleozoic to the end of the Cenozoic (Cambrian, Ordovician,
Silurian, Devonovian, Mississippian, Pennsylvanian, Permian, Triassic, Jurassic, Cretaceous,
Tertiary, Quaternary).
● “Cold Oysters Seldom Develop Many Precious Pearls, Their Juices Congeal Too Quickly”
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