CHAPTER 2 Plate Boundaries

Divergent Boundaries When two or more plates separate from each other, it creates a divergent plate boundary. Divergent boundaries are also called constructive or separating boundaries as new crust is created between them. The boundary between the Eurasian and North American Plates is an example of a major divergent boundary. Divergent boundaries lead to rifting and seafloor spreading

Divergent Boundaries Divergent Boundaries are formed as convection currents move the plates away from each other. As plates separate, magma rises from the mantle to fill the gap left between them. This leads to the formation of ocean ridges and volcanoes along the plate boundary. Iceland is an example of a newly formed volcanic island. It forms part of the Mid-Atlantic Ridge.

Divergent Boundaries Rift valleys are formed when continental plates are pulled apart by convection currents. East Africa is rifting apart from the rest of the African continent. The Red Sea has formed as a result of water from the Indian Ocean rushing in to fill the north part of the rift valley. East African Rift Valley

Divergent Boundaries Seafloor spreading occurs when separating oceanic plates allow magma to rise through the gap before cooling to form new oceanic crust. As new seafloor is created, and oceans are widened. The Atlantic began to open up roughly 130 million years ago and continues to grow wider today. The red lines show the mid-ocean ridges of the world’s oceans. The light blue areas bordering the continents are continental shelves (the edges of continents that have been submerged by the ocean).

Convergent Boundaries Convergent plate boundaries are formed where convection currents cause two or more plates to collide with one another. There are three types of convergent plate boundary: Oceanic-oceanic, Continental-oceanic and Continental-continental.

Oceanic-Oceanic When two oceanic plates collide, the heavier plate subducts underneath the lighter plate. The plate sinks into the mantle and melts. Magma from the melting plate rises upwards through the overlying plate. Eventually, the magma reaches the surface, forming volcanoes. As material from the volcanoes build up, volcanic arc islands are formed, e.g. the Mariana Islands in the Pacific Ocean The friction caused between a subducting and an overlying plate leads to strong earthquakes occurring along the boundary.

Continental-Oceanic When a continental and an oceanic plate collide, the heavier oceanic plate subducts into the mantle. Ocean trenches and strong earthquakes are created along a continental-oceanic plate boundary. Volcanic arcs are also formed along the continental plate as magma from the subducting plate is forced upwards. Fold mountains are also formed along the edges of the non-subducting (continental) plate.

Continental-Continental When two continental plates collide, neither plate is subducted. This is because continental plates are too light to be forced downward. The movement is mainly upwards. This process is called uplift. The Indo-Australian Plate is colliding with the Eurasian Plate. This has led to the formation of the Himalaya Mountains. Earthquakes are common along continental-continental plate boundaries as friction between the plates causes the crust to vibrate

Passive Boundaries Passive plate boundaries are created where plates slide past each other. The best-known passive boundary in the world exists between the North American and Pacific Plates. Pressure builds over time as the faster-moving Pacific Plate tries to jolt free from the slower-moving North American Plate. This pressure builds until one plate slips or jolts free. The sudden movement of the plate triggers powerful earthquakes. The movement of the plates has created a large crack in the crust that is over 1300 km long. This is known as the San Andreas Fault