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Convergent Boundaries

Dictionary

Converging Boundaries:

Convergent plate boundaries (saw tooth - with the teeth on the side of the over-riding plate) occur where two plates are colliding together.  Two continental plates will crumple and form mountain ranges because neither is dense enough to sink into the mantle.  However, ocean plates will form subduction zones as they sink back into the mantle.

 

Convergent continent-continent e.g.  The Himalayas Mountains formed from the collision of the continent of India with the continent of Eurasia.

Convergent ocean-continent e.g. The Nazca Plate is subducting under the less dense South American Plate

Convergent ocean-ocean e.g. At the Aleutian trench, the Pacific Plate is subducting under the Eurasian Plate.

 

1. Continent - continent Convergent Boundary

 

Shallow to moderate earthquakes, mountain building

As the continent of India collided with Eurasia, a shallow ocean between the two continents subducted below Eurasia.   Once the two continents touched, the subduction stopped and the two continents crumpled together forming the highest mountains in the world.  Marine fossils high in the Himalayas are remnants of that ancient ocean.   Although volcanoes would have resulted from the earlier subduction in the area, all volcanic activity has ceased since the subduction ended.  Shallow to moderate depth earthquakes and high, folded mountains are characteristic of this continent-continent convergent boundary.

 

2. Continent - ocean Convergent Boundary

Shallow and deep earthquakes, volcanoes, mountain building

 

When ocean plates collide with continental plates, the denser ocean plate will sink or subduct beneath the continental plate.  As the ocean plate sinks into the mantle, an ocean trench marks the start of subduction.  The sinking plate is cold and brittle and causes shallow to moderate to deep earthquakes as it sinks lower down into the mantle.  In fact, subduction zones are the only localities on Earth where deep earthquakes (300 to 700 km depths) occur.  Temperature increases one degree Celsius for each kilometer of depth.  As ocean plates subduct, any water within the ocean crust is eventually turned into steam.  The rising steam melts the crust and magma rises to the surface in these areas, forming volcanoes.  The Nazca plate subducting below the South American plate is a typical subduction zone.  An ocean trench marks the start of subduction.  Volcanoes occur further to the east; and earthquakes range from shallow near the trench to deepest further east where the ocean plate is deep within the mantle.    Another example is the Juan de Fuca plate subducting below the North American plate.  Ocean trenches, shallow to deep earthquakes and volcanoes characterize these boundaries.

Example: West coast of South America, Pacific North-west as the Juan de Fuca Plate subducts under the North American Plate

 

3. Ocean - ocean Convergent Boundary

Shallow and deep earthquakes, volcanoes in island arcs, formation of ocean trenches

 

When two ocean plates collide, one will subduct below the other.  The start of subduction is marked by an ocean trench and shallow earthquakes.  As the ocean plate subducts and any water in the rock turns to steam, the rock above the steam will melt forming magma to rise and form volcanic islands.  Moderate to deep earthquakes mark the sinking of the subducting plate into the mantle.  If the subducting plate is large, a long trench will mark the subduction zone and a line or arc of volcanic islands will be formed.  These are called volcanic island arcs.

Examples include the Aleutian Island arc, the Philippine Island arc and the Japan Island arc. 

 


 

 

 

 

 


 

 

 

 

 

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