Seafloor Spreading and Plate Tectonics: Geological Processes

Seafloor Spreading

By 1960, it was established that the composition of the ocean floor is primarily basaltic. Mid-ocean ridges are areas where new oceanic crust is created. Seafloor basalts are older the further they are from the ridge; their age is highest near continents and minimum along the ridge itself. The rate of expansion is estimated at a few millimeters per year. Seismic surveys revealed that it is not just the crust, but the entire lithosphere, that moves via convection currents.

Banded Magnetic Seabed

The Earth's magnetic field is unstable, and its polarity reverses periodically, causing the magnetic poles to swap positions. Currently, the north magnetic pole is in the Antarctic region, and the south magnetic pole is in the Arctic. The formation of crystals with residual magnetism occurs as follows:

• Lava formation: Magnetite crystals form within the lava. While the lava is extremely hot, these crystals remain disordered.
• Cooling process: As the lava cools, the crystals orient themselves according to the Earth's magnetic field, acquiring remanent magnetism.
• Reversed magnetism: A rock formed during a period when the magnetic field was reversed will retain that reversed residual magnetism.

In the last five million years, there have been more than twenty such reversals. These magnetic field inversions are recorded in volcanic rocks containing minerals like magnetite.

Plate Tectonics

Plate tectonics refers to the lithosphere being dragged by the convective motion of the mantle beneath it.

Evidence of Tectonic Plates

In the 1950s, physicist and seismologist Hugo Benioff discovered that earthquakes along the Pacific coast of North America had a peculiar distribution: shallow seismic foci were close to the shoreline, while deeper ones occurred inland, as if along a slope. This area is known as the Benioff plane. According to plate tectonic theory, the Benioff plane corresponds to the surface of a Pacific lithospheric plate sinking into the mantle beneath the continent. The distribution of volcanoes and earthquakes coincides with the edges of lithospheric plates, and mountain ranges are a result of plate collisions.

Lithospheric Plates

There are two types of lithosphere that differ in composition and physical properties: oceanic and continental.

Types of Plates Based on Composition

• Oceanic: Composed only of oceanic lithosphere.
• Continental: Composed only of continental lithosphere.
• Mixed: Contain both types of lithosphere.

Small fragments of lithosphere, known as microplates or litosferocalastos, move as they are pushed by the much larger plates surrounding them.

Movements at Plate Edges

The edges of the plates are areas characterized by intense geological activity, including volcanism and seismic events.