Seafloor spreading over the past 100 to 200 million years has caused the Atlantic Ocean to grow from a tiny inlet of water between the continents of Europe, Africa, and the Americas into the vast ocean that exists today. 173, 174, 195 convergent boundary, 158 core: definition of, 141, 142, 173. At a divergent plate boundary - also known as a constructive plate boundary, the plates move apart from one another. A divergent plate boundary is formed where tensional tectonic forces result in the crustal rocks being stretched and finally split. This rate may seem slow by human standards, but because this process has been going on for millions of years, it has resulted in plate movement of thousands of kilometers. One of the most amazing places on the Earth’s is where two massive tectonic plates are moving apart. The rate of spreading along the Mid-Atlantic Ridge averages about 2.5 centimeters per year (cm/yr), or 25 km in a million years. While the location of these plate boundaries are well defined from satellite-derived bathymetry, significant regions remain unmapped at the high-resolutions. This submerged mountain range, which extends from the Arctic Ocean to beyond the southern tip of Africa, is but one segment of the global mid-ocean ridge system that encircles the Earth.
Perhaps the best known of the divergent boundaries is the Mid-Atlantic Ridge. This age progression could only be explained by the continuous formation of new oceanic crust at the ridges and gradual spreading-apart of the plates over time. A tectonic boundary where two plates are moving away from each other and new crust is forming from magma that rises to the Earths surface between the two.
(NOAA) The evidence for sea-floor spreading came with the discovery that oceanic crust is youngest near the ridge and becomes becomes progressively older away from the spreading center. It gets progressively younger toward the mid-Atlantic ridge, where oceanic crust is forming today. The crust near the continental margins (blue) is about 200 million years old.
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