Divergent Plate Boundaries
Divergent plate boundaries are locations where plates
are moving away from one another. This occurs above rising
convection currents. The rising current pushes up on the
bottom of the lithosphere, lifting it and flowing laterally
beneath it. This lateral flow causes the plate material
above to be dragged along in the direction of flow. At the
crest of the uplift, the overlying plate is stretched thin,
breaks and pulls apart.
Divergent Plate Boundary - Oceanic: (see illustration
above) When a divergent boundary occurs beneath oceanic
lithosphere, the rising convection current below lifts the
lithosphere producing a mid-ocean ridge. Extensional forces
stretch the lithosphere and produce a deep fissure. When
the fissure opens, pressure is reduced on the super-heated
mantle material below. It responds by melting and the new
magma flows into the fissure. The magma then solidifies
and the process repeats itself.
The Mid-Atlantic Ridge is a classic example of this type
of plate boundary. The Ridge is a high area compared to
the surrounding seafloor because of the lift from the convection
current below. (A frequent misconception is that the Ridge
is a build-up of volcanic materials, however, the magma
that fills the fissure does not flood extensively over the
ocean floor and stack up to form a topographic high. Instead,
it fills the fissure and solidifies. When the next eruption
occurs, the fissure most likely develops down the center
of the cooling magma plug with half of the newly solidified
material being attached to the end of each plate.
Visit the Interactive Plate Boundary Map
to explore satellite images of divergent boundaries between oceanic plates. Two locations are marked: 1) the
Mid-Atlantic Ridge exposed above sea level on the island of Iceland, and 2) the Mid-Atlantic Ridge between North
America and Africa.
Effects that are found at a divergent boundary between oceanic
plates include: a submarine mountain range such as the Mid-Atlantic
Ridge; volcanic activity in the form of fissure eruptions;
shallow earthquake activity; creation of new seafloor and
a widening ocean basin.
Divergent Plate Boundary - Continental: (See Illustration
above.) When a divergent boundary occurs beneath a thick
continental plate, the pull-apart is not vigorous enough
to create a clean, single break through the thick plate
material. Here the thick continental plate is arched upwards
from the convection current's lift, pulled thin by extensional
forces,and fractured into a rift-shaped structure. As the
two plates pull apart, normal faults develop on both sides
of the rift and the central blocks slide downwards. Earthquakes
occur as a result of this fracturing and movement. Early
in the rift-forming process, streams and rivers will flow
into the sinking rift valley to form a long linear lake.
As the rift grows deeper it might drop below sea level allowing
ocean waters to flow in. This will produce a narrow, shallow
sea within the rift. This rift can then grow deeper and
wider. If rifting continues a new ocean basin could be produced.
The East Africa Rift Valley is a classic example of this
type of plate boundary. The East Africa Rift is in a very
early stage of development. The plate has not been completely
rifted and the rift valley is still above sea level but
occupied by lakes at several locations. The Red Sea is an
example of a more completely developed rift. There the plates
have fully separated and the central rift valley has dropped
below sea level.
Visit the Interactive Plate Boundary Map
to explore satellite images of divergent boundaries between continental plates. Two locations are marked within the rift valley of East Africa and another location is marked within the Red Sea.
Effects that are found at this type of plate boundary include: a rift valley sometimes occupied by a long linear lakes or a shallow arm of the ocean, numerous normal faults bounding a central rift valley and shallow earthquake activity along the normal faults. Volcanic activity sometimes occurs within the rift.
Contributor: Hobart King