Plates in Motion

As the Earth's crustal plates move they must meet along their edges. At this location they interact in one of three ways: they spread apart from each other, one goes under or subducts under the other, or they slide alongside each other. During VISIONS ‘05 we will be working over the boundary of the Pacific Plate and the Juan de Fuca Plate, which is a seafloor spreading zone.

Spreading Centers

Spreading is made possible by the addition of crustal material from underneath the plates. Volcanoes bring lava up from below and form new seafloor, in this case along the Juan de Fuca Ridge, on the western edge of the Juan de Fuca Plate. At the same time the eastern edge of the plate is subducting under the North American Plate forming the chain of volcanoes, such as Mt. St. Helens and Mt. Rainier, on the western edge of continent. This is somewhat like a very slow moving conveyor belt in which material is added on one end and removed on the other.

Spreading Rates

As spreading rates go, the Juan de Fuca ridge is sort of medium paced. Although the rates vary, the Mid-Atlantic Ridge located in the center of the Atlantic Ocean is slow, about 2-4 cm/year, while at the Juan de Fuca ridge the rate is about 6 cm/year. A fast spreading zone, like the East Pacific Rise located in the South Pacific, moves as much as 11 cm/year! As the plates move away from each other, they create a valley down the center of the volcano. It is this axial valley that houses the hydrothermal vents in which we are so interested.

Hydrothermal Vent Fields

Presently, scientists have identified five hydrothermal vent fields in the axial valley of the Endeavour Segment on the Juan de Fuca ridge. These fields form when large amounts of seawater circulate through the porous and permeable rocks that form the seafloor. As the water migrates deeper into the rocks, it is heated and reacts with the minerals that form these rocks. These zones are called recharge zones (where the seawater seeps in). In the zone in which the water reacts with minerals the temperatures are high, often more than 350 degrees C. It is in this zone that hydothermal vent fluids are thought to acquire their final chemical signatures. Because of the increased buoyancy at higher temperatures, the fluid rises rapidly to the surface through the discharge zones (in other words, the hydrothermal vents).

Black Smokers

The structures known as black smokers form when the superheated water emerges on the seafloor and meets near-freezing ocean water. The minerals in the water separate, or precipitate, out and become solid particles. These particles can either build the chimney of the black smoker (much as a bricklayer builds the walls of a house) or rise in a plume of hot water that can look like black smoke.