Seismic and other data have now shown that this
is the deepest rift on the planet. How deep was the rift? In some
places more than 35 km, so deep that it almost split North America
into two separate continents. An astounding 20 to 25 km depth of
the gigantic rift valley were filled with interfingered lava flows
and sediment carried in from the surrounding highlands (shown by
the layered reflections below 10 km depth); then another 10 or more
kilometers of sediments filled in after the rift had formed.
Below the rift, the crystalline crust had thinned to just one quarter
of its original thickness. This enabled molten rock from even greater
depths to force its way up — providing the enormous volume
of volcanics that have been found, and perhaps completely fracturing
the crust in some places.
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