I’ve been listening to geologists talk about oceanic core complexes for a couple of weeks now. Boy, was it confusing at the beginning! Here’s the gist of it. At faster spreading ridges magma is injected into the gap as the tectonic plates move apart and so no faults are needed to accommodate the plate spreading. However, the southwest Indian Ridge is an ultra-slow spreading ridge and the crust and uppermost mantle is cold so there isn’t enough magma coming out of the mantle to fill the “gap” caused by the spreading tectonic plates, and so big faults are needed to accommodate the plate spreading and consequently ocean floor is made differently here.
At the slowest spreading ridges, large faults develop parallel to the ridge allowing blocks of crust to slide out from underneath the ridge axis. Water seeps into the cracks associated with the fault and chemically changes the rock in the fault zone making it weaker. Slipping at the weakened rock areas and stretching (like taffy) down deep in the fault allows a huge block of seafloor to slide out and rotate and roll over forming an enormous (maybe 30 -50 km across) domal mountain. This exposes deep rocks from the crust and mantle. These undersea mountains are called oceanic core complexes and form large domes with small ridges like a corrugated roof. Analyzing rock samples from oceanic core complexes can help us understand how Earth’s ocean crust formed in this area.
Scientists aboard the R/V Thomas G. Thompson have already identified six oceanic core complexes through examination of the maps made from multi beam data! Why is this exciting? It’s exciting because we can use the maps to identify dredge locations on the oceanic core complexes to collect rocks that came from below the ridge axis. Examination of these rocks helps us explain how the Earth’s crust grows at slow spreading ridges. The surfaces of oceanic core complexes are among the few places on Earth where the mantle is exposed. Normally mantle is well buried beneath 7 km of Earth’s crust in the ocean and by 40km of crust in the continents.
We’ve got a piece of the rock! -- Mantle rock that is.