The Mid-Ocean Ridge and rift valleys, such as the one that runs through eastern Africa, occur along boundaries where plates are spreading apart. New oceanic crust is created as the plates separate and molten rock rises up from the mantle and fills the space. The earthquakes and the volcanic eruptions along the Mid-Ocean Ridges are a direct result of this process.
Click on a diagram on the right to learn more.
When two plates carrying continents collide, the continental crust buckles and rocks pile up, creating towering mountain ranges. The Himalayas were born when the Indian subcontinent smashed into Asia 45 million years ago. The Himalayas are still rising today as the two plates continue to collide. The Appalachian Mountains and Alps also formed in this way.
Plates Slide Past One Another
Plates grinding past each other in opposite directions create faults called transform faults. Powerful earthquakes often strike along these boundaries. The San Andreas Fault is a transform plate boundary that separates the North American Plate from the Pacific Plate. This fault system is largely responsible for the devastating earthquakes in Los Angeles and San Francisco.
When an ocean plate collides with another ocean plate or with a plate carrying continents, one plate will bend and slide under the other. This process is called subduction. A deep ocean trench forms at this subduction boundary. As the subducting plate plunges deep into the mantle, it gets so hot it melts the surrounding rock. The molten rock rises through the crust and erupts at the surface of the overriding plate. The result is either a volcanic mountain range such as the Cascades and Andes, or chains of islands called island arcs such as Japan and the Aleutian Islands.