Convergent Boundaries

From IslandWood Education Wiki
Jump to: navigation, search

Places where plates crash or crunch together are called convergent boundaries. Plates only move a few centimeters each year, so collisions are very slow and last millions of years. Even though plate collisions take a long time, lots of interesting things happen. For example, in the drawing below, an oceanic plate has crashed into a continental plate. Looking at this drawing of two plates colliding is like looking at a single frame in a slow-motion movie of two cars crashing into each other. Just as the front ends of cars fold and bend in a collision, so do the "front ends" of colliding plates. The edge of the continental plate in the drawing has folded into a huge mountain range, while the edge of the oceanic plate has bent downward and dug deep into the Earth. A trench has formed at the bend. All that folding and bending makes rock in both plates break and slip, causing earthquakes. As the edge of the oceanic plate digs into Earth's hot interior, some of the rock in it melts. The melted rock rises up through the continental plate, causing more earthquakes on its way up, and forming volcanic eruptions where it finally reaches the surface. An example of this type of collision is found on the west coast of South America where the oceanic Nazca Plate is crashing into the continent of South America. The crash formed the Andes Mountains, the long string of volcanoes along the mountain crest, and the deep trench off the coast in the PacificOcean.  Another example of a convergent plate boundary is right here in Washington.  The Juan de Fuca plate, an oceanic plate, and the North American plate are colliding, with the denser oceanic plate subducting beneath the lighter continental plate.  Associated with these plate boundaries are the mountain ranges of Washington.  The Cascades are the volcanic arch associated with subduction as the colder ocean plate lowers the melting point of mantle rocks and magma rises to the surface.  The Olympic mountains are an accretionary wedge, composed of sedimentary and igneous rocks scraped off of the ocean plate as it subducts beneath the continent.

Geology and Weather05.jpg