Oceanic Ridges: Formation, Characteristics, and Plate Tectonics
Oceanic Ridge Characteristics and Plate Tectonics
The oceanic ridge system is an undersea mountain range, rising between 2 and 3 km above the abyssal plain. It extends for approximately 65,000 km, periodically interrupted by laterally moving fractures called transform faults. The central depression is known as the rift valley.
Age and Sedimentation
The age of the ocean floor is less than 185 million years (Ma). No rocks currently exist within the ridges that are older than this. The older oceanic crust separates from the ridge symmetrically on either side. The thickness of sediments increases with the age of the seafloor, with the ridge itself being devoid of sediments. Sediment thickness increases with distance from the ridge.
Seafloor Spreading and Lithosphere
The lithosphere extends as new seafloor is created at the ridges, spreading outward on either side. All oceanic lithosphere originates at these ridges.
Destruction of Oceanic Lithosphere
According to the theory of plate tectonics, lithosphere returns into the Earth’s interior at subduction zones.
Distribution of Volcanoes and Earthquakes
Volcanoes and earthquakes occur in geologically active areas, while other regions are more stable. Earthquakes occur when blocks of rock fracture along activated or old faults, involving the movement of one block of material over another.
Lithospheric Plates
The lithosphere is divided into rigid pieces called lithospheric plates, ranging in thickness from 50 to 200 km and varying greatly in surface area. There are seven major plates and a dozen smaller ones.
Plate Boundaries
The edges of lithospheric plates are of three types:
- Ridges: Boundaries where new oceanic lithosphere is generated from materials originating within the Earth.
- Subduction Zones: Boundaries where lithosphere is destroyed, offsetting the generation at the ridges.
- Transform Faults: Boundaries where lithosphere is neither created nor destroyed, but one plate moves laterally relative to another.
Renewal of Oceanic Lithosphere
Oceanic lithosphere is continually renewed, while continental lithosphere is more permanent. The creation of oceanic lithosphere at ridges and its destruction at subduction zones explains the relative youth of the ocean floor.
Plate Movement
Lithospheric plates move over the plastic materials of the sublithospheric mantle at speeds of several centimeters per year, in different directions. This movement causes continental drift, collisions, and friction between plates. Consequently, major geological activity, such as volcanoes, earthquakes, and ridge formation, occurs at plate boundaries.
Driving Forces
The movement of lithospheric plates is driven by thermal energy from within the Earth, aided by gravitational energy. The temperature difference between the hot core and the cold lithosphere generates convection currents in the mantle, causing plate movements.
Earth’s Changing Plates
Throughout Earth’s history, not only has the position of lithospheric plates changed, but also their shape, size, and number. The activity of ridges and subduction zones alters the size of the plates. Additionally, plates can break apart and merge.