1. Plate Boundary Volcanism Reference: Pages 505-508 & 557 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;

2.  Basaltic magmas are produced at spreading centers. Spreading Center (Rift) Volcanism  Rift volcanism can occur in two areas; 1) beneath the oceans (Mid Atlantic Ridges) 2) within continental plates (African Rift Zone) 1) Beneath Oceans  At ridges, ocean floor moves apart creating a rift (crack) through which basaltic magma moves up through and forms new ocean floor. MohoMagmaContinental Plate Oceanic Plate Ocean

3.  Shield volcanoes are formed along ridges when basaltic lava flows on the ocean floor. In some case these volcanoes can rise above sea level and form a volcanic Island. Example; Surtsey near Iceland. Spreading Center (Rift) Volcanism 1) Beneath Oceans  The greatest volume of volcanic rock is produced within oceanic ridges where seafloor spreading is active. Example; along the Mid Atlantic Ridge. MohoMagmaContinental Plate Oceanic Plate Ocean

4.  When land becomes separated, water moves in between and forms a sea, for example, Mediterranean Sea. These seas may eventually form an ocean. Spreading Center (Rift) Volcanism 2) Within Continents  Some spreading centers located within continents, cause the land mass to separate. African rift zone. Rift Valley

5.  Shield volcanoes are produced by these eruptions. Rift Eruptions  Rift eruptions generally flow out smoothly and fluidly because the basaltic lava generated in the upper mantle contains few gases and has very little silica. Gases and a high percentage of silica cause lavas to be viscous and explosive.  Rift eruptions produce the greatest volume of volcanic rock.

6. Sample problem Describe a spreading center and briefly describe what volcanic activity is present at these locations. Answer: A divergent boundary, sometimes refered to as spreading centers, displays tensional forces caused by mantle convection causing oceanic plates to move apart. Basaltic magmas which are very fluid magmas with little dissolved gases erupt quietly onto the ocean floor adding to the diverging oceanic crust. New sea floor is formed at spreading centers.