Lecture Notes – Geologic Eras. Geologic Timescale The geologic timetable is divided into 4 major...

Lecture Notes –

Geologic Eras

Geologic TimescaleThe geologic timetable is divided into 4 major eras: The oldest era is called the Pre-Cambrian Era (PC).

o There are not many fossils from the Pre-Cambrian

o but it makes up 80 %

of the earth’s history.

Geologic TimescaleThe three more recent eras the Paleozoic, Mesozoic and Cenozoic Eras:

“-zoic” comes from the Greek word “zoe” which means “life”.

Geologic Timescale“Paleo-” means “ancient”, so Paleozoic means “ancient life”.“Meso-” means “middle”, so Mesozoic means “middle life”.“Ceno-” means “recent”, so Cenozoic means “recent life”.

Geologic TimescaleThe geologic timescale is not like a historical timescale with decades and centuries. There is not a set number of years per time division. Rather, each time division is distinguished by events that happen to signal the “end of an era”.

Geologic TimescaleThe four eras are divided up into 15 periods:3 Pre-Cambrian Periods (PC 4.55 bya to 542 mya

Hadean (pA) - 4.55 – 4.00 bya no life forms

Archean (A) - 4.00 – 2.50 bya bacteria

Proterozoic (PL) - 2.50 – 0.542 bya (542 mya) –

simple life forms

Precambrian Era: 4.6 - .542 b.y.aHadean – 4.55 bya – 4.00 bya • Earth and solar system still forming.• No rocks, except meteorites, are this old.•Great Iron Catastrophe - Molten iron sinks to the core; crust solidifies. This event marks the end of the Hadean Period.

Update: Geologists have found a

4.28 by old rock in northern Quebec. So now there is one rock this old!!

Archaeozoic Period – 3.8 bya – 2.5 bya Earliest fossils appeared – bacterial, 3.5 bya

Life forms did not need oxygen - anaerobic

Oxygen was produced by stromatolites: photosynthetic colonies of bacteria.

3.8 bya: oldest known rocks

2.8 bya: 2/3 continents created

Proterozoic period – 2.5 bya - .542 bya• algae, jellyfish and worms appear 1.5 – 1.0 bya

Proterozoic period – 2.5 bya - .543 bya Gondwana forms

from Africa, South America, Australia, India and Antarctica - 650 mya

First signs of oxygen build up in the atmosphere 2.5 bya.o Caused the first

major extinction.