The earth’s crust is subjected to forces of expansion and compression as it floats upon the deeper, fluid mantle. In this case, this region was getting stretched and large blocks of the crust collapsed over time, forming a trough. The faulting that accompanied these events allowed magma to flow to the surface, filling the trough with heavy basalt and causing it to subside still further.
These were ancient events, preceding the appearance of life on land and quite possibly preceding the advent of any life at all in the shallow, saline seas that formed in similar rifts as the crust gradually divided into separate land masses.