Lecture Notes – Geologic Eras

Geologic Timescale The 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 Timescale The 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 Timescale The 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 Timescale The four eras are divided up into 15 periods: 3 Pre-Cambrian Periods (PC 4.55 bya to 542 mya Hadean (pA) – 4.00 bya no life forms Archean (A) – 2.50 bya bacteria Proterozoic (PL) – bya (542 mya) – simple life forms

Precambrian Era: b.y.a Hadean – 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 bya algae, jellyfish and worms appear 1.5 – 1.0 bya

Proterozoic period – 2.5 bya bya  Gondwana forms from Africa, South America, Australia, India and Antarctica mya  First signs of oxygen build up in the atmosphere 2.5 bya. o Caused the first major extinction.