The mid oceanic ridge is the longest mountain chain in the solar system that is located underwater along the earth’s oceans. It is an opening in the earth’s crust between plate tectonics and is 70,000 Km long and 5 – 30Km wide, in which the length is double the circumference of earth. This mountain ridge can be thought of as a crack in the earth’s crust that allows molten rock within the earth (magma) to flow from the mantle up to the surface (30 – 60Km). Once the magma reaches the surface it cools and solidifies under the water and forms a new section of the ocean floor. This process, called “seafloor spreading”, is continually happening, causing the ocean floor to be pushed outward from the ridge as new magma fills in the cracks and solidifies.
Plate tectonics: Divided sections of the earths crust that gradually move overtop of the asthenosphere, in which the rubbing and collision of these plate tectonics is a leading cause of earthquakes
Asthenosphere: The layer of the earth below the crust that has a liquid-solid composition (similar to plastic)
An image displaying the distribution of the Mid-oceanic
ridge around the world as white lines
Frederick Vine and Drummond Matthews were two geologists that were analyzing the magnetic properties of rock formations in the Indian ocean in the 1960s. They found that seamounts along the ocean floor that contained magnetic minerals within them had differing magnetic properties, more specifically, that the magnetic fields were inverted. Every few million years, the magnetic field that surrounds the earth will naturally flip/invert, causing the magnetic north pole to be flipped to the south pole (reference magnetic reversals for more information). It was found that this phenomenon, coupled with seafloor spreading, was the reason that Vine and Drummond had detected these magnetic differences.
An image displaying the seafloor spreading process, with each band of solidified
magma representing an inversion in the earth's magnetic field (Tivey, 2007)
To further explain this observation, once molten rock from within the earth is cooled at the mid oceanic ridge, some rock types will inherit a magnetic field direction based on the earth's magnetic field. Magma is extremely hot and is above a threshold called the curie temperature (150 – 300 °C for primary magnetic minerals within the upper oceanic crust), in which temperatures above this threshold do not inherit any magnetic properties. Therefore, many molten rock types within the earth's mantle do not have magnetic properties due to the elevated temperatures.
Curie temperature: The temperature threshold at which a substance will lose its magnetic properties, values below this threshold will maintain their magnetic properties. It is important to note that the curie temperature differs for each substance.
However, once the magma cools it drops below the curie temperature and any magnetic minerals within the rock will orient themselves to be in line with the earth’s magnetic field lines. Consequently, the natural inversion of the earth’s magnetic field causes bands of solidified rock to form on the ocean floor, with each band representing a period in time when the earth's magnetic field has flipped (reference above image).
