Convergent Plate
Boundaries
,Summary of What Happens at Convergent Plate Boundaries
• As the Plate Tectonic theory was developed, it was quickly recognised
that if new lithosphere is being created at divergent boundaries,
then it must be destroyed elsewhere. So what happens when plates
converge?
• Volcanic Activity – around the edges of ocean basins there are island
arcs. Along the edges of some continents there are fold mountains
with chains of andesite volcanoes. As the subducing plate heats up,
flux melting occurs along the top surface of the descending oceanic
crust, producing basaltic magma. As this less dense, hot (1,200°C)
magma rises, it may melt and mix with silicic continental curst to
erupt intermediate, andesitic lavas at the surface.
• Batholiths – where continental crust is melted by magma most of the
material will remain separate because of the different viscosities.
Melted silicic material forms large granite batholiths deep within fold
mountains, later exposed by erosion.
• Trenches – deep-ocean trenches are long, narrow, linear, deepest
parts of the Earth’s surface. They are formed as the ocean floor is
dragged down where two plates converge, indicating the boundary.
, Summary of What Happens at Convergent Plate Boundaries
• Fold Mountain Belts (Orogenic Belts) – fold mountains form on the
edges of the continents parallel to the subduction zone. They are
compressional features made of folded and faulted sediments that
have been scraped off the descending plate onto the non-subducting
plate. High pressures and temperature results in regional
metamorphism.
• Benioff Zone – as the oceanic plate descends, the sloping plane of
the boundary is marked by deep-focus earthquakes along the Benioff
zone. At higher levels, where the subduction is beginning, the
earthquakes are generated along the boundary itself partly due to
friction. Further down, the foci occur within the plate as the interior
part of the plate remains colder and more rigid, while the edges of
the plate heat up.
Boundaries
,Summary of What Happens at Convergent Plate Boundaries
• As the Plate Tectonic theory was developed, it was quickly recognised
that if new lithosphere is being created at divergent boundaries,
then it must be destroyed elsewhere. So what happens when plates
converge?
• Volcanic Activity – around the edges of ocean basins there are island
arcs. Along the edges of some continents there are fold mountains
with chains of andesite volcanoes. As the subducing plate heats up,
flux melting occurs along the top surface of the descending oceanic
crust, producing basaltic magma. As this less dense, hot (1,200°C)
magma rises, it may melt and mix with silicic continental curst to
erupt intermediate, andesitic lavas at the surface.
• Batholiths – where continental crust is melted by magma most of the
material will remain separate because of the different viscosities.
Melted silicic material forms large granite batholiths deep within fold
mountains, later exposed by erosion.
• Trenches – deep-ocean trenches are long, narrow, linear, deepest
parts of the Earth’s surface. They are formed as the ocean floor is
dragged down where two plates converge, indicating the boundary.
, Summary of What Happens at Convergent Plate Boundaries
• Fold Mountain Belts (Orogenic Belts) – fold mountains form on the
edges of the continents parallel to the subduction zone. They are
compressional features made of folded and faulted sediments that
have been scraped off the descending plate onto the non-subducting
plate. High pressures and temperature results in regional
metamorphism.
• Benioff Zone – as the oceanic plate descends, the sloping plane of
the boundary is marked by deep-focus earthquakes along the Benioff
zone. At higher levels, where the subduction is beginning, the
earthquakes are generated along the boundary itself partly due to
friction. Further down, the foci occur within the plate as the interior
part of the plate remains colder and more rigid, while the edges of
the plate heat up.
