Seafloor spreading describes a geological process where new oceanic crust forms at mid-ocean ridges and gradually moves away. This process is a component of plate tectonics, explaining how Earth’s rigid outer layer reshapes. It occurs primarily at divergent plate boundaries, where tectonic plates separate.
Earth’s Internal Engine
Earth’s tectonic plates, including seafloor spreading, are driven by internal forces within the planet. Beneath the lithosphere, a hotter, pliable layer called the asthenosphere lies within the upper mantle. This asthenosphere behaves like a viscous fluid, allowing for slow, convective movements. Heat from Earth’s core and radioactive decay within the mantle creates temperature differences that drive these convection currents.
Hot, less dense material from the deep mantle rises towards the surface, while cooler, denser material sinks. This circulation acts like a conveyor belt, exerting forces on the overlying lithospheric plates, causing them to move. The upwelling of hot mantle material creates new crust at the surface.
New Crust from the Deep
New oceanic crust primarily forms at mid-ocean ridges, vast underwater mountain ranges. These ridges form where two oceanic plates pull apart, creating a rift. As plates diverge, pressure decreases on the underlying mantle, causing it to partially melt and generate magma. This molten rock then rises towards the surface through fractures and fissures within the thinning crust.
Upon reaching the seafloor, magma erupts as lava and rapidly cools in the cold ocean water, solidifying into new oceanic crust. This crust is predominantly basalt. As more magma rises and solidifies, it pushes the previously formed crust away from the ridge axis. This outward movement expands the ocean basin, with spreading rates varying from 0.1 to 17 centimeters per year.
Uncovering the Evidence
Scientists have gathered evidence to support the theory of seafloor spreading. One piece of evidence comes from the symmetrical patterns of magnetic striping found on the ocean floor. As new crust forms at mid-ocean ridges, iron-rich minerals within the cooling magma align with Earth’s magnetic field, recording its polarity. Since Earth’s magnetic field periodically reverses, these reversals are preserved as alternating stripes of normal and reversed magnetism, mirroring each other on both sides of the ridge.
Another indicator is the age of the ocean floor rocks. Radiometric dating of rock samples reveals that the youngest oceanic crust is consistently found directly at the mid-ocean ridges. The age of the crust progressively increases with increasing distance from the ridge, providing direct confirmation of the outward movement of the seafloor. Furthermore, measurements show a higher heat flow from Earth’s interior at mid-ocean ridges compared to other oceanic areas. This elevated heat flow is consistent with the upwelling of hot magma and the formation of new crust at these active spreading centers.