EAS-100-51: Introduction to Earth Science

ORIGINS AND EARLIEST HISTORY OF THE EARTH

1.    THE BIG BANG OCCURS - the cataclysmic explosion and expansion of the original matter and energy of the Universe, estimated to have occurred about 10 to 15 billion years ago. Evidence: the stars appear to be receding (moving away) from Earth; the most distant stars are receding at the highest speeds.

2.    THE SOLAR SYSTEM IS CREATED - about 5 billion years ago, a cloud of gas (mostly hydrogen) and cosmic dust began to gather and contract together (coalesce) slowly, through gravitational attraction. As the contraction increased, it began to heat up and rotate. The central mass of this cloud heated up to about 2 million degrees Centigrade, starting nuclear reactions which created the Sun.

3.    THE PLANETS (INCLUDING EARTH) FORMS - the remaining gaseous material (planetesimals) of the proto-cloud condensed into smaller spheres, and became the planets. Smaller bits of leftover rocks and dust became asteroids, meteorites, and comets.

4.     EARLY EARTH HEATS UP - over the first several hundred million years of the Earth's earliest history, the planet's interior began to heat up, from these 3 sources:

a.     bombardment and absorption of planetesimals (meteorites, asteroids, and comets)
b.     energy of motion (energy from the spinning of the early dust cloud)
c.     radioactive decay of uranium and other radioactive elements

5.     THE GREAT IRON CATASTROPHE - Due to tremendous bombardment by huge meteorites, asteroids, and comets, the earth's earliest crust and perhaps early oceans may have been repeatedly destroyed and re-melted. A stable crust may not have developed until about 3.8 billion years ago, the age of the oldest known rocks on Earth. Originally, the elements of the Earth were probably distributed more or less evenly (homogenously). The build-up of heat in the Earth's interior heat became so great that the earth's crust heated up to over 2000 degrees and melted. After the crust melted, the heavy elements such as iron "fell" towards the Earth's interior. The Earth redistributed its elements into layers or zones, such that most of the iron and heavier elements concentrated in the core, while mostly lighter elements ended up in the crust. This violent event is known as the Great Iron Catastrophe. Because the oldest crust of the Earth was destroyed by this event, no rocks older than 3.8 billion years have ever been found. Geologists assumed that the Earth is the same age as meteorites, which have been radiometrically age-dated at 4.6 billion years.

6.     THE GREAT RAIN - Much of the Earth's water came from the interior of the planet, through intense volcanic activity. Water molecules were part of the minerals of the deeply-buried rocks. Through volcanic eruptions, these water molecules escaped into the atmosphere initially as steam. Eventually, condensation of this volcanic steam lead to rainfall. In order to fill the ocean basins, it may have rained for thousands of years. Rain of Comets: Recent evidence suggests that a significant portion of Earth's water may also come from the crash-landing of comets, which are large balls of ice and dust that travel through outer space.

7.     FIRST LIFE ON EARTH arose from the oceans, often called "the cradle of life." The oldest fossil remains are age-dated at 3.8 billion years, and consisted of simple one-celled creatures known as blue-green algae and bacteria. These simple life forms were able to manufacture their own food by converting sunlight into energy and storing the energy inside their cells.