Chapter 22 – The Precambrian Earth Notes 22.1 – Early Earth

The Age of Earth Early earth was hot, volcanically active, and had no continents Rocks did not exist Scientists know very little about Earth’s first 700 million years.

Oldest existing rocks From Archean Eon Earliest life-forms were simple, unicellular organisms Multicellular organisms did not appear until the end of the Proterozoic

Crustal Rock Evidence Absolute-age dating has revealed that the oldest crustal rocks are between 3.96 and 3.8 billion years in age Earliest rocks were made of Zircon (ZrSiO4)

Zircon Stable and common mineral Can survive erosion and metamorphism Often used to age-date old rocks Zircon found in Precambrian rocks in Australia are dated to be at least 4.4 billion years old Earth must be at least that old

Solar System Evidence Scientists agree all parts of the solar system formed at the same time Therefore Earth and meteorites are approximately the same age Meteorites = small fragments of orbiting bodies that have fallen on Earth’s surface Meteorites have fallen all throughout Earth’s history Most meteorites have been dated between 4.7 & 4.5 billion years old

Solar System Evidence Moon Rocks = oldest samples have been dated at 4.45 billion years old Scientists believe Moon formed after Earth when a massive solar system body collided with Earth Based on all the evidence, scientists agree that Earth is about 4.56 billion years old.

Early Heat Sources Earth was extremely hot after it formed Three likely sources of heat Earth’s gravitational contraction Radioactivity Bombardment by asteroids, meteorites, and other solar system bodies

Gravitational contraction Earth formed by the gradual accumulation of small, rocky bodies in orbit around the Sun As Earth accumulated these small bodies, it grew in size and mass. As Earth’s mass increased, gravity increased Gravity increased caused Earth’s center to squeeze together with so much force that the pressure raised temperature

Radioactivity Radioactive decay generates heat There were more radioactive isotopes in early Earth So more heat was generated Making Earth hotter than it is today

Bombardment Asteroids = metallic or silica-rich objects between 1 km and 950 km in diameter For the first 500 to 700 million years of Earth’s history, asteroids struck Earth much more frequently than they do today. The impacts generated a tremendous amount of thermal energy The debris caused a blanketing effect, which prevented the newly generated heat from escaping to space

Earth Cooling Evidence suggests that Earth cooled enough for liquid water to form within its first 200 million years The cooling process continues today As much as half of Earth’s internal heat remains from Earth’s formation

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