Plate Margins  We must now look at the 3 main types of plate margin: Constructive. Destructive. Conservative.  Each type of plate margin has different rocks and types of hazard Credit: Hawaii Natural History Association

Plate Margins Credit: U.S. Geological Survey

Destructive Plate Margins  May also be called convergent plate margins.  Why?  What makes them converge?  There are 3 types of destructive plate margins depending on what types of plate are involved: Oceanic v oceanic Oceanic v continental Continental v continental  Each will produce different hazards and rocks. Credit: Hawaii Natural History Association

Golden Rule 1 Why do plates/rocks melt: 1. Because the temperature has increased. 2. Because the pressure is decreased. 3. Because it is saturated in water. If one or more of these situations exists then the plate/rock will melt.

Golden Rule 2  What kind of magma is produced when a plate/rock melts?  Remember how we classify igneous rocks. Acid: > 66% quartz rich Intermediate: 52 – 66 % Basic: 44 – 52%quartz poor Ultrabasic: < 44%  Quartz is one of the first minerals to melt, so  When the initial magma melts the one it produces will be richer in SiO 2 and so will move towards the acid end of the scale.  If you melt an ultrabasic rock it will produce a basic magma.  If you melt a basic rock the magma will be more intermediate.

Golden Rule 3  The more SiO 2 in a magma the more explosive will be the volcanic eruptions.  Why?  Because the magmas are more viscous/sticky.  Gasses come out of solution when a magma reaches the surface (P drops) but they cannot escape easily.  They therefore expand putting pressure on the magma chamber.  Eventually the P is so great that the top/plug is blown off or the side slips and a violent eruption occurs.

Oceanic v Oceanic collision Credit: U.S. Geological Survey

Oceanic v Oceanic collision  The features/landforms: Subduction zone: describes the whole area where subduction is taking place. Ocean Trench: is the deep valley formed in the ocean floor as the subducted plate bends. Benioff zone: zone of earthquakes set off by the solid lithospheric plate forcing it’s way through the mantle. Island Arc: An arc of islands running parallel to the trench/subduction zone created by volcanic activity fed by magma from the melting subducted plate.

Oceanic v Oceanic 2 The processes:  Subduction: Occurs where a piece of oceanic lithosphere bends and sinks beneath another plate. –Why do oceanic plates subduct? –Because oceanic crust is denser (3.0). Credit: U.S Geological Survey

Oceanic v Oceanic 3  Seismic activity: As the cold/solid oceanic plate sinks through the mantle there is frictional resistance. When the resistance is overcome and the plate moves the energy is released as seismic waves = earthquake. This is the Benioff zone. Credit: U.S. Geological Survey

Oceanic v Oceanic 4  Melting: As mentioned earlier there are 3 reasons why crust may melt. Which is occurring in this situation? –Increased T as the plate sinks deeper. –Also the plate is saturated in water. The melting oceanic crust is basic so what kind of magma will form when it melts? More SiO 2 rich and so will move towards being intermediate. This magma will move upwards through the overlying mantle and thin crust to the surface. Credit: U.S. Geological Survey

Oceanic v Oceanic 5  Explosive volcanic activity.  Basic/intermediate magma will be fairly violent.  Enough volcanic activity will occur to build volcanic islands above sea level in an arc parallel to the plate margin (island arc). Credit: U.S. Geological Survey

Oceanic v Oceanic examples  Montserrat/Caribbean arc: Soufriere Hills.  Aleutian islands.  Indonesia: Krakatoa  Philippines: Pinatubo  Kamchatka. Credit: U.S. Geological Survey

Oceanic v Continental Crust collision  The features/landforms:  Many are the same: Trench. Subduction zone. Benioff zone Continent based volcanoes. Fold mountains. Credit: Hawaii Natural History Association

Oceanic v Continental Crust 2  The processes.  Again very similar to ocean v ocean with a subtle difference. Subduction (but only the oceanic crust will subduct). Seismic activity. Melting: Mountain building (orogenesis) Credit: Hawaii Natural History Association

Oceanic v Continental Crust 3  What happens to the melt as it enters the continental crust?  Remember that the magma is now intermediate(ish).  It travels up through the continental crust that has what composition?  Acid.  The hot magma melts the continental crust and adds SiO 2 rich crust to the intermediate magma to make magma that is very intermediate or acid. Credit: U.S. Geological Survey

Oceanic v Continental Crust 4  Acid/intermediate magma will produce what kind of volcanic activity? Very explosive and dangerous.  Mountain building (orogenesis). This involves crust being compressed and either folding or faulting. Fault movement sets off earthquakes.  There will therefore be earthquakes linked to both the Benioff zone and orogenesis. Credit: U.S. Geological Survey

Oceanic v Continental Examples  Volcanoes along the Andes: Popacatapetl (Mexico) Nevada del Ruiz (Columbia)  The Rockies/Cordillera: Mt. St. Helens Credit:/ U.S. Geological Survey Eruptions of Mt. St. Helens,

Continental v Continental Crust Credit: U.S. Geological Survey

Continental v Continental Crust  The features/landforms. Mountain ranges  The processes: Mountain building (orogenesis). No subduction because continental crust will not subduct. Credit: U.S. Geological Survey Everest

Continental v Continental Crust  Will there be volcanoes?  The crust is up to 90 km thick and so is hot enough at it’s base to melt.  However, the melt produced is so viscous and the distance to the surface so far that magma will not make it to the surface.  Therefore NO VOLCANOES. Credit: U.S. Geological Survey

Continental v Continental Crust  Will there be earthquakes? Orogenesis is still taking place. All the energy is being put into deforming the rocks.  So YES there will be many large earthquakes.

Conservative Plate Margins  This is where 2 plates are sliding past each other horizontally but are not being created or destroyed.  There is only one really good example in the world: California, San Andreas Fault. Credit: U.S. Geological Survey (Photographer – Robert E Wallace Credit: U.S. Geological Survey

Conservative Plate Margin  What kind of volcanic activity will there be?  None.  Why not?  Nothing is melting.  Will there be any earthquakes?  Yes and very large. Credit: U.S. Geological Survey

Constructive Plate Margin May also be called: divergent plate margins. Why? New crust is generated along ocean ridges. Examples are: The Mid Atlantic Ridge and the East Pacific Rise. Credit: U.S. Geological Survey

Constructive Plate Margin Credit: U.S. Geological Survey

Constructive Plate Margin Along conservative margins there is neither a loss nor a gain and the plates slip past each other, these margins are marked by transform faults. Mid ocean ridges are characterised by: Active volcanoes (submarine volcanoes and in places emergent volcanic islands) Small earthquakes Lack of sediment High heat flow Credit: U.S. Geological Survey

Constructive Plate Margin The new crust formed at this plate margin, along with a layer of the upper most part of the mantle, moves away from the ridge and new material is added to the trailing edge. Credit: U.S. Geological Survey

Constructive Plate Margin  Iceland forms example of Mid Ocean Ridge that has grown into a volcanic complex above sea level Credit: U.S. Geological Survey

Passive Plate Margins  Nothing is happening.  2 pieces of crust are fused together.  Like the N. American continental plate and the Atlantic oceanic plate.  However, in time the margin may become active.  It is already starting to subduct in the Caribbean arc region (Montserrat). Credit: Hawaii Natural History Association