Chapter 7
Oceanic Spreading Ridges and Sea Floor
Creation
Abstract The Spreading Ridge System is one of the most striking features
exposed on the seafloor. It is a belt of volcanoes that surrounds the planet both
under the oceans and in subaerial terrain. The Spreading Ridge System is where
plate divergence occurs when ascending hot material rising from the mantle creates the sea floor. The spreading ridges extend around the World for a total length
of 70,000 km and a width of about 1,000–2,000 km. This system is cut by tectonic
and magmatic discontinuities such as fracture zones and disrupted spreading ridge
segments. When compared to a human body, the spreading ridge system is like a
continuous ‘‘backbone’’ and the ‘‘rib-bones’’ are the fracture zones emanating
from both sides of the ridge axis.
At the base of the lithospheric plate, at 70–100 km depth within the crust,
diverging forces push the plates apart from each side of the ridge axis in order to
make space for new molten crust to be extruded. As the crust created on the ridge
axis is moved away from its point of origin, it cools and becomes less ductile,
which causes it to break due to the tectonic constraints related to spreading. The
speed of crustal creation at a ridge axis is not the same everywhere, as indicated by
the different spreading rates that have been measured (Fig. 7.1). The different rates
of spreading have influenced the ridge landscape, as well as the rocks’ morphology
and structure. Thus, on a slow spreading ridge (total rate \3 cm/yr) such as the
Mid-Atlantic Ridge (MAR), the axis of spreading along its strike has created a
large depression, graben or rift, sometimes more than 1 km deep and about
15–30 km wide. On the other hand, faster rates of spreading such as in the Pacific
Ocean cause the axial graben to be much smaller and the ridge itself is more
volcanically active than the MAR axis. Thus, the East Pacific Rise will be less
affected by tectonic breaks because of the larger volume of lava erupted. Also the
East Pacific Rise (EPR) is more affected by off-axial volcanism (See Chap. 9),
which constructs more individual volcanoes than on the MAR. The MAR segments behave as more rigid structures and show better defined tectonic breaks than
the EPR. Also the East Pacific Rise does not follow any continental contour line
because it is detached from continental masses whereas the slower spreading
R. Hekinian, Sea Floor Exploration, Springer Oceanography,
DOI: 10.1007/978-3-319-03203-0_7,
Ó Springer International Publishing Switzerland 2014
165
Oceanic Spreading Ridges and Sea Floor
Creation
Abstract The Spreading Ridge System is one of the most striking features
exposed on the seafloor. It is a belt of volcanoes that surrounds the planet both
under the oceans and in subaerial terrain. The Spreading Ridge System is where
plate divergence occurs when ascending hot material rising from the mantle creates the sea floor. The spreading ridges extend around the World for a total length
of 70,000 km and a width of about 1,000–2,000 km. This system is cut by tectonic
and magmatic discontinuities such as fracture zones and disrupted spreading ridge
segments. When compared to a human body, the spreading ridge system is like a
continuous ‘‘backbone’’ and the ‘‘rib-bones’’ are the fracture zones emanating
from both sides of the ridge axis.
At the base of the lithospheric plate, at 70–100 km depth within the crust,
diverging forces push the plates apart from each side of the ridge axis in order to
make space for new molten crust to be extruded. As the crust created on the ridge
axis is moved away from its point of origin, it cools and becomes less ductile,
which causes it to break due to the tectonic constraints related to spreading. The
speed of crustal creation at a ridge axis is not the same everywhere, as indicated by
the different spreading rates that have been measured (Fig. 7.1). The different rates
of spreading have influenced the ridge landscape, as well as the rocks’ morphology
and structure. Thus, on a slow spreading ridge (total rate \3 cm/yr) such as the
Mid-Atlantic Ridge (MAR), the axis of spreading along its strike has created a
large depression, graben or rift, sometimes more than 1 km deep and about
15–30 km wide. On the other hand, faster rates of spreading such as in the Pacific
Ocean cause the axial graben to be much smaller and the ridge itself is more
volcanically active than the MAR axis. Thus, the East Pacific Rise will be less
affected by tectonic breaks because of the larger volume of lava erupted. Also the
East Pacific Rise (EPR) is more affected by off-axial volcanism (See Chap. 9),
which constructs more individual volcanoes than on the MAR. The MAR segments behave as more rigid structures and show better defined tectonic breaks than
the EPR. Also the East Pacific Rise does not follow any continental contour line
because it is detached from continental masses whereas the slower spreading
R. Hekinian, Sea Floor Exploration, Springer Oceanography,
DOI: 10.1007/978-3-319-03203-0_7,
Ó Springer International Publishing Switzerland 2014
165