The rocky beach at Green Point used to be deep underneath the crust of the Earth and is one of the few places in the world you can stand on what was once the Earth’s mantle. While the Earth’s mantle is only 30 miles (45km) below your feet most everywhere else on Earth, it is over a thousand degrees and you would be smushed beyond any recognition if you tried to go there. However, here, on this beautiful beach in an obscure corner of Washington State, the plate tectonics has brought the mantle to you!
The rock is a type of metamorphic rock called serpentinite. According to Northwest Geology, this rock originally came from the upper portion of the earth’s mantle, below the oceanic crust. Notice the strangely reddish-orange rocks at Green Point. That’s made up of dunite and peridotite, both of which contain high amounts of the mineral olivine. Peridotite also contains some iron-bearing minerals, part of what give it that rusty coloring.
The high amounts of chromium limit the growth of trees and other vegetation in some areas of the park. It isn’t clear how this sliver of the mantle came to the surface here in this beautiful park, but it most likely happened when the San Juan Islands and other bits of exotic terranes were smushed onto the North American continent by the looming Cascadia megathrust fault looming to the west.
Know Before You Go
The park is just beyond the Ferry terminal to the San Juan Islands and is free to enter.