<P> The mid-ocean ridge systems form new oceanic crust . As crystallized basalt extruded at a ridge axis cools below Curie points of appropriate iron - titanium oxides, magnetic field directions parallel to the Earth's magnetic field are recorded in those oxides . The orientations of the field in the oceanic crust preserve a record of directions of the Earth's magnetic field with time . Because the field has reversed directions at irregular intervals throughout its history, the pattern of geomagnetic reversals in the ocean crust can be used as an indicator of age . Likewise, the pattern of reversals together with age measurements of the crust is used to help establish the history of the Earth's magnetic field . </P> <P> The mid-ocean ridges of the world are connected and form the Ocean Ridge, a single global mid-oceanic ridge system that is part of every ocean, making it the longest mountain range in the world . The continuous mountain range is 65,000 km (40,400 mi) long (several times longer than the Andes, the longest continental mountain range), and the total length of the oceanic ridge system is 80,000 km (49,700 mi) long . </P> <P> Increased rates of sea - floor spreading (i.e. the expansion of the mid-ocean ridge) has caused global (eustatic) sea - level to rise over very long timescales (millions of years). </P> <P> The high sea levels that occurred during the Cretaceous period (144--65 Ma) can only be attributed to plate tectonics since thermal expansion and the absence of ice sheets by themselves cannot account for the fact that sea levels were 100--170 meters higher than today . </P>

How do ocean floor features such as trenches and mid ocean ridges form