As oceanic plates spread away from the mid-ocean ridges where they form, the deep layers of peridotite are altered by seawater into serpentine minerals (more about that). The product is 20 percent larger than the original rock, so the result is a relatively soft serpentine slurry. When old ocean crust enters subduction zones, this process accelerates and large mud volcanoes form. Examples in the Marianas Trench are tens of kilometers across.
This roadcut near Lake Berryessa, in northern California, exposes a preserved serpentinite mud volcano. It contains polished boulders (like this one)in a scaly matrix. The whole exposure is still a soft, incompetent rock that is slowly moving downhill today as a landslide, some 100 million years after it erupted in a subduction zone.