Convergent plate boundaries Objective to be able to explain the formation and key features of these zones.
Destructive plate margins When plates collide due to convection currents/slab pull in the asthenosphere oceanic crust is subducted (dives down) under the continental plate into the upper mantle because oceanic crust is denser than continental crust. The descending oceanic plate starts to melt at depths of 100km due to heat from friction and the increased heat with depth. By 700km deep, the plate is completely melted. This zone of melting is called the Benioff zone.
Key features Earthquakes: As oceanic crust descends, friction with the overlying continental crust leads to stresses building up and causes major earthquakes. Destructive margins are some of the most seismically active zones in the world, with shallow to deep focus earthquakes along the subduction zone. Fold mountains: Rocks scraped off the descending plate and folding of the continental crust help to create young fold mountain chains on the leading edge of continental masses, usually out of sedimentary rocks. As plates collide, the continental landmass is uplifted, compressed and buckled and folded.
Ocean Trenches: Deep ocean trenches are found along the seaward edge of destructive margins. They are long narrow depressions on the ocean floor running parallel to the plate boundary. They mark the point where one plate begins to descend beneath another at a subduction zone and can reach great depths.
Volcanoes: Friction created by the descending slab of the ocean floor also generates enormous heat, leading to partial melting of the crust. Magmas made from the melting of the old ocean floor ae less dense than the mantle so they try to rise up through fissures (cracks) in the buckled continental crust and burning their way through overlying rock until they reach the surface. Where volcanoes erupt they help to create the young fold mountains. Rising magma incorporates older continental crust which means they are more rich in silica. They are more viscous and flow less easily leaving intrusive features such as batholiths and extrusive features such as volcanoes. Stickier lavas often block off their own vents until they erupt violently forming composite volcanoes. 80% of all active volcanoes are found above subduction zones.
Example: The Nazca and South American Plate boundary: Young fold mountains: Andes Ocean Trench: Peru- Chile Trench (8km deep) Volcanoes: Chimborazo and Cotopaxi (6000m high) Draw a SIMPLIFIED version of this.
Oceanic - Oceanic collision 2 oceanic plates converging = still subduction. The one that is subducted may be slightly denser or moving more quickly. Subduction processes much the same as oceanic continental subduction, however where volcanoes erupt they form submarine volcanoes which may later grow into island arcs. They can form curving lines of new volcanic land known as island arcs and deep ocean trenches. Over time, island arcs may develop to become major landmasses (Japan is an example) Shallow to deep focus earthquakes e.g. 2004 earthquake which caused the Boxing Day Tsunami where the Australian Plate is being subducted under the Eurasian Plate. Example: Marianas Trench (over 10km deep) and the Marianas islands in the Western Pacific where the Pacific Plate is being subducted under the smaller Philippine Plate.
Continental v continental (collision) Continental plates are lower density than the asthenosphere beneath them and have similar densities to each other therefore they will not be subducted. Instead they will collide. As the plates move towards each other, their edges and the sediments between them are forced up into fold mountains. As there is no subduction, there is no volcanic activity. Shallow focus earthquakes triggered by the movement of the plates. Material is also forced downwards to form deep mountain roots.
The sub-continent of India moved NE by sea floor spreading of the Indo-Australian Plate until it collided with the Eurasian plate 40million years ago. The collision formed the Himalayan mountain chain, home of the highest mountain on Earth, Mount Everest (8,848m) The Himalayas are constantly changing because these highly folded and faulted regions do not become seismically quiet after first impact. At this extreme altitude, weathering and erosion reduce mountain height but plate movements cause isostatic uplift. Scientists believe that Everest is increasing in height by up to 2.5cm per year. The region experiences many earthquakes e.g. Sichuan in China 2008.
Draw a SIMPLIFIED version of the 3 diagrams, that you will remember.
Conservative Sometimes called passive or slip margins. Occur where the direction of plate movement of two plates is parallel or nearly parallel. No crust is destroyed or created. Frequent seismic activity as the build up of friction as plates pass each other is released as earthquakes. E.g. LA 1994 and San Francisco 1906 and 1989 Not associated with volcanoes. The San Andreas fault in California where the North American plate moves NW by 1cm a year only to be passed in the same direction by the faster moving Pacific Plate.
(a) Identify three limitations with the data evidence in Fig. 5. [3] (b) Explain the features and processes at divergent plate boundaries.[6] Remember key terms, words, technical terms, specific locations