History of the Earth
Ketaki Gaikwad
Earth Forms...
Asteroids and Comets...
Oxygen enters the atmosphere...
3.5 billion years ago early bacterial life introduced oxygen to the atmosphere. As the first free oxygen was released through photosynthesis by cyanobacteria, it was initially soaked up by iron dissolved in the oceans. Once the iron in the oceans was used up, the iron oxide stopped being deposited and oxygen was able to start building up in the atmosphere about 2.4 billion years ago.
Snowball Earth...
Snowball Earth describes a theory that for millions of years the Earth was almost entirely covered in ice, stretching from a pole to a pole. This freezing happened over 650 million years ago in the Pre-Cambrain. Life could only cling on in ice-free areas, or where sunlight managed to sneak through the ice to allow photosynthesis.
Ordovician-Silurian mass extinction...
443 million years ago the third largest extinction in Earth's history, the Ordovician-Silurian mass extinction happened. This mass extinction had two dying times separated by hundreds of thousands of years. During the Ordovician, most life was in the sea, 85% of sea life was wiped out. A huge ice sheet in the southern hemisphere caused climate change and a fall in sea level. It also messed with the balance of the oceans.