Solve this riddle." i am the first on earth, the second in heaven, i appear two times in a week, you can only see me once in a year although i am in the middle of the sea, what am i?

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.