The study of fossils has helped geologists divide Earth's history into four eons and a number of finer-scale subdivisions: eras, periods, and epochs. Each geologic division represents a distinct timespan in the history of Earth along with characteristic life forms.
Hadean
Archaean
Proterzoic
Phanerzoic

a mid-ocean ridge or mid-oceanic ridge is an underwater mountain range, formed by plate tectonics. this uplifting of the ocean floor occurs when convection currents rise in the mantle beneath the oceanic crust and create magma where two tectonic plates meet at a divergent boundary.a geomagnetic reversal is a change in a planet's magnetic field such that the positions of magnetic north and magnetic south are interchanged.