Final answer:
The four major tectonic plates involved in the expansion of the Atlantic Ocean are the North American, Eurasian, South American, and African Plates, moving apart due to mantle convection. The Juan de Fuca Plate off the West coast of North America is subducting under the North American Plate, contributing to the formation of the Cascade Range and its volcanic activity.
Step-by-step explanation:
The four major tectonic plates that are moving away from one another and thus enabling the Atlantic Ocean to open up between them are the North American Plate, the Eurasian Plate, the South American Plate, and the African Plate.
The Mid-Atlantic Ridge is a divergent tectonic plate boundary located along the floor of the Atlantic Ocean, and it's the region where these four plates are moving apart. This ridge represents a location where new oceanic crust is formed as magma rises from below the Earth's surface, cools, and solidifies. The relative movement of these plates is due to the process of mantle convection, which drives the plates' apart at a rate comparable to the growth of human nails.
Along the West coast of North America, from the Gulf of California northward to British Columbia, Canada, we encounter various types of plate boundaries. The boundary begins with a transform boundary, the San Andreas Fault, where the Pacific Plate and the North American Plate slide past each other. Farther north, near the coast of Washington and Oregon, is the Juan de Fuca Plate. This small plate is bounded by a divergent boundary to the west, where it moves away from the Pacific Plate, and a convergent boundary to the east, where it subducts beneath the North American Plate. This subduction results in the formation of the Cascade Range, which includes volcanoes like Mt. St. Helens.
The ultimate fate of the Juan de Fuca Plate is to be fully subducted beneath the North American Plate. As it continues to move and sink, the plate's material will melt and may contribute to volcanic activity in the region. This ongoing subduction process is integral to the geology and seismic activity of the Pacific Northwest.