Origin of Earth
Formation of the Solar System
The Solar System's formation is believed to have begun approximately 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. This collapse, which was likely triggered by a shock wave from a nearby supernova, resulted in a rotating disk of gas and dust known as the solar nebula.
The majority of the collapsing mass collected in the center, forming the Sun, while the rest flattened into a protoplanetary disk out of which the planets, moons, asteroids, and other small Solar System bodies formed. This model, known as the nebular hypothesis, is accepted by most scientists as the explanation for the formation of the Solar System.
Early Earth
The Earth is estimated to have formed around 4.54 billion years ago, approximately one-third the age of the universe, by accretion from the solar nebula. Volcanic outgassing likely created the primordial atmosphere and then the ocean, but the early atmosphere contained almost no oxygen. Much of the Earth was molten because of frequent collisions with other bodies which led to extreme volcanism.
Hadean and Archean Eons
The Hadean Eon represents the time before a reliable (fossil) record of life; it began with the formation of the planet and ended 4.0 billion years ago. The following Archean Eon saw the evolution of the first life forms. During these times, the Earth's crust had cooled enough to form a solid surface, but there were still high levels of volcanic activity and large amounts of water vapor in the atmosphere, which, along with the volcanic gases, likely created a greenhouse effect.
Proterozoic Eon
The Proterozoic Eon was a period of relative stability in the Earth's climate. The first supercontinent, Rodinia, formed around 1 billion years ago, followed by the supercontinent Pannotia, which formed around 600 million years ago. The first known glaciations occurred during the Proterozoic, one beginning shortly after the formation of Rodinia and another near the end of the Proterozoic.
Phanerozoic Eon
The Phanerozoic Eon, the current eon in the geologic time scale, covers the last 541 million years of Earth's history, during which abundant life has existed on the planet. It is subdivided into three eras: the Paleozoic, Mesozoic, and Cenozoic, which are further subdivided into a number of periods. The Phanerozoic Eon is marked by the proliferation and diversification of multicellular life and the establishment of complex ecosystems.