Mud Volcanoes in the Mariana Forarc

How they form and what come out of them!

In the Mariana forearc, two forms of "faulting" occur.

The first occurs because large seamounts on the subducting plate force sections of the overriding plate (the plate that stays on top) to get "pushed" up. You can imagine the subducting plate as large conveyor belt with big irregular bumps along it. As the bumps get passed down they push up against the overriding plate and force chunks of it to go up.

The second and even more important reason for faulting is because of the shape of the Mariana arc.
Over the past 45 million years or so, the arc has split in two, twice! Each time the east side has change shape and become more bowed out toward the east. The middle is bowing faster than the north and south ends, and this causes a crescent bulge that has been constantly growing eastward. As the bulge increases, the forearc stretches to fill its increasing length. This stretching creates faults that go all the way down into the mantle. Keep this in mind because these faults will be crucial to the formation of mud volcanoes.

Within the subducted plate, water and other fluids are stored within in it in two ways. The first is the water that fills the small pores (spaces between grains) of seafloor sediment and small cracks in various rocks. The second is water that is bound up in the mineral structure of certain minerals such as clays.

These minerals are called "hydrated minerals." As the plate is subducted down into the mantle, it is heated up and put under pressure. This heat and pressure drives the water out of the small pores, and from the clay minerals. As the fluids come out of the subducting plate they begin to rise because they are lighter than their surroundings, and so they get pushed upwards through the upper mantle below the Mariana forearc. The faults created by the stretching and extension, provide excellent passage ways for the fluids to rise back up through the overriding mantle
As the water rises through the mantle, some of it reacts with the minerals around it and forms serpentine. The serpentine, like the water, is now less dense than its surrounding mantle material. Therefore, it too, starts to rise. The serpentine gets squeezed through the faults like toothpaste being squeezed out of a tube. As the serpenine reaches the seafloor, it oozes out and over time creates the large mud volcanoes that we are studying.