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Chapter 22. Section 22.1 – Early Earth. The Age of Earth. When Earth first formed it was hot, volcanically active, and no continents existed on its surface Early on rocks were not present Scientists know very little about Earth’s first 700 million years. Oldest rocks are from Archean Eon
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Chapter 22 Section 22.1 – Early Earth
The Age of Earth • When Earth first formed it was hot, volcanically active, and no continents existed on its surface • Early on rocks were not present • Scientists know very little about Earth’s first 700 million years
Oldest rocks are from Archean Eon • Earliest life-forms were simple, unicellular organisms • Multicellular organisms did not appear until the end of the Proterozoic Eon
Crustal Evidence • Oldest crustal rocks are between 3.96 and 38 billion years old • Zircon (ZrSiO4) is a stable common mineral often used to age-date old rocks • Oldest Zircon found in Precambrian rocks in Australia that are at least 4.4 billion years old
Solar System Evidence • Meteorites = small fragments of orbiting bodies that have fallen on Earth’s surface • Meteorites have fallen to Earth throughout Earth’s history, but most have been dated at between 4.7 and 4.5 billion years old • Scientists agree all parts of solar system formed at same time • Therefore, the Earth and meteorites are approximately the same age
Solar System Evidence • Oldest rock samples from the Moon have been dated at 4.45 billion years old • Scientists think that the Moon formed very early in Earth’s history 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 • Gravitational contraction • Radioactivity • Bombardment by asteroids, meteorites, and other solar system bodies
Gravitational Contraction • Earth formed by gradual building up of small, rocky bodies in orbit around the sun • As earth gathered more small bodies, it grew in size and mass • As mass increased, gravity increased and caused Earth’s center to squeeze together • Massive force caused internal temperature to rise
Radioactivity • Radioactive decay generates heat • More radioactive isotopes were found in early Earth, therefore generating more heat and making early Earth hotter
Asteroid & Meteorite Bombardment • Asteroids = metallic or silica-rich objects between 1 km and 950 km in diameter • For the first 500-700 million years of Earth’s history, scientists believe that Earth was struck by asteroids with high frequency • The impacts generated tremendous amounts of thermal energy
Cooling of Earth • Within first 200 million years of existence, Earth cooled enough for liquid water to form • Cooling continues to this day • As much as half of Earth’s internal heat remains from Earth’s formation.
