Plate Boundary Volcanism Reference: Tarbuck and Lutgens Pages Volcanoes are associated with two of the three types of plate boundaries, these being convergent and divergent boundaries. Very little volcanic activity is seen at transform fault boundaries.  3) Interior of tectonic plates. 2) Subduction zones 1) Ridges (or spreading centers)  Volcanism associated with plate tectonic activity are found in three areas on Earth;

Subduction Zone Volcanism  At trenches, ocean floor bends and moves downward into the upper mantle. At depths of 100 km partial melting of the ocean crust and mantle takes place.  Basaltic and andesitic magmas are produced at subduction zones.  After great quantities of magma are produced, the molten rock moves upward toward the surface because it is less dense than the surrounding rock.  Ocean – Ocean convergent boundaries and Ocean – Continent convergent boundaries have this type of volcanism.

Subduction Zone Volcanism  Ocean crust subducts beneath ocean crust.  Basaltic magmas are produced and burns upward toward the surface forming a chain of volcanoes called a “volcanic island arc” parallel to the trench.  Examples include; Islands of Japan. 1) Ocean – Ocean Volcanism Ocean Plate Asthenosphere

Subduction Zone Volcanism  Ocean crust subducts beneath continental crust.  Andesitic and Granitic magmas burns upward into the continental crust adding to mountain systems.  This type of volcanism is very explosive and is mainly found surrounding the Pacific Ocean. 2) Ocean – Continent Volcanism  Examples include; volcanoes in the Andes mountains. Ocean Plate Continental Plate Asthenosphere

Subduction Zone Volcanism  At a depths of 100 km, the subducting ocean crust starts to melt. This generates magmas that are thick and contains large amounts of gases. As a result, subduction eruptions at ocean-continent boundaries are very explosive and produce composite volcanic cones.  most of the world’s volcanoes are of this type and border the Pacific Ocean, called the Pacific Ring of Fire. 2) Ocean – Continent Volcanism Ocean Plate Continental Plate Asthenosphere

Sample Problem With the aid of a diagram describe the volcanic features formed at a ocean-ocean convergent boundary. Ocean Plate Asthenosphere Answer: At an ocean-ocean convergent boundary, one oceanic plate subducts beneath another and melts when it descends into the mantle at approximately 100km depth. This molten material moves up through the oceanic plate located above and erupts onto the ocean floor. This forms a chain of volcanic islands parallel to the trench called a “volcanic island arc.”