Download Lesson 10 - Rift Volcanism

Plate Boundary Volcanism
 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.
 Volcanism associated with plate tectonic
activity are found in three areas on Earth;
1) Ridges (or spreading centers)
2) Subduction zones
Reference:
Pages 505-508 & 557
3) Interior of tectonic plates.
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.
 Basaltic magmas are produced at spreading centers.
Ocean
Moho
Oceanic Plate
Magma
Continental Plate
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.
 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.
Ocean
Moho
Oceanic Plate
Magma
Continental Plate
Spreading Center (Rift) Volcanism
2) Within Continents
 Some spreading centers located within continents,
cause the land mass to separate. African rift zone.
Rift Valley
 When land becomes separated, water moves in
between and forms a sea, for example, Mediterranean
Sea. These seas may eventually form an ocean.
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.
 Shield volcanoes are produced by these eruptions.
 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.