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;

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.

Moho Magma Continental Plate

Oceanic PlateOcean

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) Volcanism1) Beneath Oceans

The greatest volume of volcanic rock is produced within oceanic ridges where seafloor spreading is active. Example; along the Mid Atlantic Ridge.

Moho Magma Continental Plate

Oceanic PlateOcean

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

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.

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.