Ridge-push
Encyclopedia
Ridge push or sliding plate force is a proposed mechanism for plate motion in plate tectonics
. Because mid-ocean ridges lie at a higher elevation than the rest of the ocean floor, gravity causes the ridge to push on the lithosphere
that lies farther from the ridge.
As molten magma
rises at a mid-ocean ridge
it heats the rocks around it. The heat expelled by the magma expands the lithosphere
and asthenosphere
at the ridge, pushing them above the surrounding ocean floor. As time passes, the heated rocks cool and their density
increases. When this happens gravity pulls the weighty lithosphere away from the mid-ocean ridge, down the sloping asthenosphere, which, in turn, allows new molten magma to well up. This fresh magma will eventually become new lithosphere.
The cooling rock exerts a force on spreading lithospheric plates, helping to drive their movements. The force is called ridge push. It can be calculated by the principle of isostasy
, making a balance between the mid-ocean ridge and the old ocean floor. Notably, in such a calculation, the force depends only on the temperature of the asthenosphere at the ridge (the same in all 'normal' ridges, though higher at hotspots
, and not on the spreading rate of the ridge.
Another force, that for some plates exceeds ridge-push, is slab pull
, where the weight of a subducting
slab pulls the plate at the surface along.
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
. Because mid-ocean ridges lie at a higher elevation than the rest of the ocean floor, gravity causes the ridge to push on the lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
that lies farther from the ridge.
As molten magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
rises at a mid-ocean ridge
Mid-ocean ridge
A mid-ocean ridge is a general term for an underwater mountain system that consists of various mountain ranges , typically having a valley known as a rift running along its spine, formed by plate tectonics. This type of oceanic ridge is characteristic of what is known as an oceanic spreading...
it heats the rocks around it. The heat expelled by the magma expands the lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
and asthenosphere
Asthenosphere
The asthenosphere is the highly viscous, mechanically weak and ductilely-deforming region of the upper mantle of the Earth...
at the ridge, pushing them above the surrounding ocean floor. As time passes, the heated rocks cool and their density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
increases. When this happens gravity pulls the weighty lithosphere away from the mid-ocean ridge, down the sloping asthenosphere, which, in turn, allows new molten magma to well up. This fresh magma will eventually become new lithosphere.
The cooling rock exerts a force on spreading lithospheric plates, helping to drive their movements. The force is called ridge push. It can be calculated by the principle of isostasy
Isostasy
Isostasy is a term used in geology to refer to the state of gravitational equilibrium between the earth's lithosphere and asthenosphere such that the tectonic plates "float" at an elevation which depends on their thickness and density. This concept is invoked to explain how different topographic...
, making a balance between the mid-ocean ridge and the old ocean floor. Notably, in such a calculation, the force depends only on the temperature of the asthenosphere at the ridge (the same in all 'normal' ridges, though higher at hotspots
Hotspot (geology)
The places known as hotspots or hot spots in geology are volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the mantle elsewhere. They may be on, near to, or far from tectonic plate boundaries. There are two hypotheses to explain them...
, and not on the spreading rate of the ridge.
Another force, that for some plates exceeds ridge-push, is slab pull
Slab pull
The Slab pull force is a tectonic plate force due to subduction. Plate motion is partly driven by the weight of cold, dense plates sinking into the mantle at trenches. This force and the slab suction force account for most of the overall force acting on plate tectonics, and the ridge push force...
, where the weight of a subducting
Subduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...
slab pulls the plate at the surface along.