Seafloor Spreading: Why the Ocean Floor is Getting Bigger

Seafloor Spreading: Why the Ocean Floor is Getting Bigger
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The ocean floor is spreading at a rate of five centimeters per year in the Atlantic Ocean, so what? When the seafloor expands, it causes all kinds of havoc at the edges of the Earth’s tectonic plates, which result in volcanic eruptions, earthquakes and even coastal flooding. Yep, that insignificant five-centimeter movement could be responsible for the next big ecological disaster that is plastered all over the 6 o’clock news.

What Causes the Spreading

In the center of the seafloor there are vast mountain ranges that have formed where the tectonic plates in the ocean meet. These are found near the center of all of the major oceans. The space between the plates has allowed magma to bubble up into the sea for millions of years forming the mid-oceanic ridges. As the magma makes its way to the surface, it pushes the tectonic plates apart. The magma bursts through the earth’s crust and is instantly cooled by the seawater. This process creates hard pockets of lava rock that prevent the plates from returning to their original positions. This is one of the major components of the theory of plate tectonics.

What is Plate Tectonics

Plate Diagram

The theory of plate tectonics, sometimes confused with the earlier theory of continental drift, is a comprehensive explanation of the composition of the Earth’s crust and how it is not a single solid sphere but a collection of individual plates of hardened rock that float on top of a viscous liquid mantle. The best way to envision this was put forth by the National Oceanic and Atmospheric Administration’s New Millennium Observatory on their site about tectonics. They suggested imagining a chocolate covered caramel candy bar that has been left in a hot area for a while. The chocolate breaks into pieces and moves on the flowing caramel center.

What is interesting about this theory is that the interactions of the plates can have catastrophic consequences for the immediate environment in areas where two plates meet. The areas where the plates meet are called plate boundaries. There are three types; convergent, divergent and transform fault.

A convergent boundary is one where when two pieces of tectonic plate come together one is forced beneath the other back into the mantle. This leads to a destruction of plate area. Mountain ranges are often formed when plates come together in this way.

A divergent boundary occurs when two plates move away from each other due to the force of magma pushing up between them. It is in this area that new seafloor is created.

A transform fault boundary occurs when two plates slide next to each other. When plates do this, it often causes earthquakes. This type of movement is called a slip fault. The San Andreas Fault in California is a prime example.

So, new sea floor is being created from divergent boundaries in the mid-ocean ridges, but how much new seafloor are we talking about?

How Fast is the Seafloor Spreading

The latest magnetic imaging result suggests that the Atlantic Ocean is expanding at a rate of between 5 and 10 centimeters a year while in the Pacific some areas are expanding as much as 13 centimeters. Because the rate of expansion is not constant throughout the world, or even within areas of a single mid-ocean ridge, there is no simple way to express the amount of seafloor gained in a particular period of time. The amount of the spreading isn’t nearly as important as the consequences of the spread.

What Does This Mean for the World

Because the size of the earth isn’t increasing, it must be true that as one plate spreads some other plates must be shrinking. The Atlantic and Pacific seafloors are both expanding and this means that the boundaries of their respective plates are forming either convergent or transform fault boundaries. As we have already learned, movement along these faults can cause earthquakes and volcanic activity. The “ring of fire” around the edge of the Pacific Ocean is a direct effect of the plates expanding and creating turbulence in the crust. The Atlantic Ocean seafloor is expanding at a much slower rate and thus isn’t experiencing the severe reactions that the Pacific Rim is.

Another possibility that isn’t discussed much in the literature on the subject is that although the seafloor is expanding, it is forcing sea level to rise. The edges of the oceanic tectonic plates are being driven up over the continental plates causing the Ocean to have less holding capacity. This, in turn, causes water to overflow into the coastal areas of the continents.

With the seafloor spreading at the current rate, we can expect to continue to see volcanic and seismic activity around the Pacific Rim and perhaps begin to see low level flooding in coastal regions that could affect major cities around the globe.