GEOLOGIC TIME SCALE The geologic time scale is used by geologists and other scientists to map the timing and relationships between events that have occurred during the history of the Earth.Based on radiometric dating techniques, the Earth is estimated to be about 4,570 million years (4570 "Ma") old. The geological time scale is a means of mapping the history of the earth. It combines estimates of the age of geological formations as provided by radiometric dating techniques with the direct evidence of sequences and events in the rock record as assembled by geologists.

Pre Cambrian The Precambrian is the name given for the first super eon of Earth’s history. This division of time — about seven-eighths of Earth's history — lasted from the first formation of the planet (about 4.6 billion years ago) to the geologically sudden diversification of multicellular life known as the Cambrian Explosion (usually dated at about 542 million years ago). The Precambrian is usually considered to have three eons: the Hadean, the Archean and the Proterozoic. Some scientists recognize a fourth eon, which they call the Chaotian, which predates the others and is the time of the first formation of our solar system.

HADEON EON The Hadean Eon after the Greek god Hades, who ruled the underworld, because during most of the Hadean period the surface of the Earth must have been like our image of Hell. The Hadean Eon began when the planet Earth first began to form, about 4.5 billion years ago. At first there was just a cloud of gas and dust, and then the Sun formed, and gradually the planets formed. Around 45 million years after the planets first began to form, the Moon formed. Probably a large planetoid, about the size of Mars, crashed into the Earth. Little bits of hot rock splashed off during the crash and orbited around the Earth. Eventually these bits joined together, cooled off, and became the Moon.

ARCHAEAN EON At the end of the Hadean Eon and the beginning of the Archaean Eon, about 3.8 billion years ago, Earth was still about three times as hot as it is today, but it was no longer hot enough to boil water. Most of the Earth was covered with oceans, and Earth's atmosphere was mainly carbon dioxide with very little oxygen in it. Just a little bit of land was forming as volcanoes began to poke out of the water. Most of the rocks were igneous or metamorphic like granite or quartz. But the earliest sedimentary rocks like sandstone also formed, mainly in the oceans, during this time. About this time - around the beginning of the Archaean Eon, about 3.8 billion years ago - the earliest living cells formed on Earth. These cells all lived in the oceans, which were probably much warmer and more acidic than they are now. By about 3.5 billion years ago, these early cells had evolved into simple prokaryote cells. For the rest of the Archaean Eon, there were only prokaryote cells on Earth (and the vast majority of cells on Earth are still prokaryotes).

By three billion years ago, some of these prokaryote cells evolved to be able to make their own food out of sunlight, water, and carbon dioxide. We call this process photosynthesis. Cells that got their energy by photosynthesis excreted (pooped out) oxygen, and once a lot of cells were photosynthesizing there started to be more and more oxygen on Earth. But during the Archaean Eon almost none of that oxygen was in the atmosphere - instead, iron and sulfur rocks mixed with early oxygen atoms to make rusty red rocks and limestone.

Proterozoic Eon An age which marks the evolution of multicellular life Age: 542 – 2,500 million years ago Subdivisions Neoproterozoic (540-1,000 Ma) Mesoproterozoic (1,000-1,600 Ma) Paleoproterozoic (1,600-2,500 Ma) Proterozoic Eon began, trillions of prokaryote cells lived in Earth's oceans. Some of these cells could photosynthesize their energy from sunlight, water, and carbon dioxide. These photosynthesizing cells pooped out a lot of oxygen, and by about 2.3 billion years ago, the iron and sulphur rocks of Earth had soaked up all the oxygen they could use. Now there began to be more and more left-over oxygen floating around in Earth's atmosphere.

Paleoproterozoic

MESOPROTEROZOIC

NEOPROTEROZOIC