200 million years ago, sedimentary strata were being pushed along the sea floor as the result of seafloor spreading.
Instead of this the strata were scraped off the crust and squeezed against sediments in the volcanic sea to the west.
The collision was also causing a great thickening of the crust, pushing the rocks down into the top of the mantle.
The high pressures and temperatures of this occurrence metamorphosed the lower rocks into the Haast Schists.
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