Seafloor spreading
Just below the Earth’s crust the mantle is very solid – further down it is liquid and able to move.
In I/GCSE Physics, CONVECTION CURRENTS in the mantle cause magma (molten rock) to rise to the surface.
The force is strong enough to move the solid part of the mantle and the tectonic plates.
When the magma reaches the surface, it hardens to form new areas of oceanic crust (SEAFLOOR), pushing the existing floor outwards. This process is called SEAFLOOR SPREADING. It results in seafloor spreading by a few centimetres each year.
Constructive plate boundaries
New oceanic crust is continuously forming at the crest of an oceanic ridge and old rock is gradually pushed further outwards.
The Earth has a MAGNETIC FIELD which changes polarity (reverses) every million years or so. Combined with seafloor spreading, this produces stripes of rock alternating polarity. Geologists can work out how quickly new crust is forming from the widths of the stripes. This occurs at CONSTRUCTIVE PLATE BOUNDARIES, where the plates are moving apart.
The Earth’s crust, together with the upper region of the mantle, consists of huge blocks of rock called TECTONIC PLATES.
Refer toI/GCSE Physics,volcanoes, mountains and earthquakes occur at the edges of tectonic plates – their creation depends on the direction the plates are moving.
If the plates are MOVING APART, as at mid-ocean ridges, VOLCANOES are produced as molten magma is allowed to escape
If the plates are MOVING TOWARDS EACH OTHER, the edges of the plates crumple, and one plate ‘dives’ under the other – this is called SUBDUCTION. It produces MOUNTAINS.
This is also part of the ROCK CYCLE, because the plate that dives under the other one becomes part of the mantle and emerges much later from volcanoes and in seafloor spreading.
If the plates are MOVING SIDEWAYS, stresses build up at the plate boundary. When the stress reaches some critical value, the plates slip suddenly, causing an EARTHQUAKE.
That's all~ Thanks for watching.
