The Mid-Ocean Ridge –
Part 2
, Spreading Rates
• The MOR is where the seafloor spreads from due to extension of the
tectonic plates.
• The space that opens is filled with magma: lava flows, dykes and the
remains of magma chambers.
• If there is not enough magma to fill the space, the crust fractures,
stretching to form normal faults.
• The rate of spreading is found by radiometrically dating ocean floor
basalts known distances from the MOR, and using pelagic microfossils
to find the age of sediments at known distances from the MOR.
• Each MOR has its own style of faulting, volcanic activity and spreading
rate, depending on the strength of the oceanic crust, and is dependent
on how cold and brittle the upper part of the lithospheric plate is.
• The rate of spreading governs how much heat is carried upwards by
magma and how quickly it is lost from the top of the magma chamber,
mostly by hydrothermal activity.
, Slow-Spreading Ridges
• For example: Mid-Atlantic Ridge
• South Atlantic – 4cm/year
• North Atlantic – 2cm/year
• Rift valleys 20km wide and 3km deep.
• Rugged mountains at the crest.
• Some parts of the ridge are covered by young lava flows and pillow
eruptions, other parts show faulting instead.
• Rough abyssal hills on either side of the ridge.
• There is insufficient partial melting to maintaining a magma chamber,
just a mush zone (melt within the softened mantle).
• Magma chambers have long periods of inactivity and are
discontinuous in extent along the rift valley. Each eruption is a distinct
event.
• Some sections have numerous small volcanic cones.
Part 2
, Spreading Rates
• The MOR is where the seafloor spreads from due to extension of the
tectonic plates.
• The space that opens is filled with magma: lava flows, dykes and the
remains of magma chambers.
• If there is not enough magma to fill the space, the crust fractures,
stretching to form normal faults.
• The rate of spreading is found by radiometrically dating ocean floor
basalts known distances from the MOR, and using pelagic microfossils
to find the age of sediments at known distances from the MOR.
• Each MOR has its own style of faulting, volcanic activity and spreading
rate, depending on the strength of the oceanic crust, and is dependent
on how cold and brittle the upper part of the lithospheric plate is.
• The rate of spreading governs how much heat is carried upwards by
magma and how quickly it is lost from the top of the magma chamber,
mostly by hydrothermal activity.
, Slow-Spreading Ridges
• For example: Mid-Atlantic Ridge
• South Atlantic – 4cm/year
• North Atlantic – 2cm/year
• Rift valleys 20km wide and 3km deep.
• Rugged mountains at the crest.
• Some parts of the ridge are covered by young lava flows and pillow
eruptions, other parts show faulting instead.
• Rough abyssal hills on either side of the ridge.
• There is insufficient partial melting to maintaining a magma chamber,
just a mush zone (melt within the softened mantle).
• Magma chambers have long periods of inactivity and are
discontinuous in extent along the rift valley. Each eruption is a distinct
event.
• Some sections have numerous small volcanic cones.
