Question

why do tectoic plates move? WHAT FORCE OR PRESSURE CAUSE THIS MOVEMENT? WHICH LAEYS OF THE EARH PLAYS ROLE IN THIS PROCESS?

Answer 1

Short answer
Tectonic plates move because the solid outer shell of the Earth (the lithosphere) rides on slowly flowing, hotter rock beneath it. That slow flow and the differences in density and elevation of plate parts generate forces that push and pull plates. The ultimate energy source is heat inside the Earth (from radioactive decay and leftover formation heat) that drives mantle convection.

Main forces and how they work
- Mantle convection (the engine): Heat from the deep Earth makes hotter mantle rock rise and cooler rock sink. This slow circulation of the mantle creates horizontal drag on the base of plates and sets up the overall pattern of motion.
- Slab pull (often the largest force): Where oceanic plates sink into the mantle at subduction zones, the cold, dense, sinking slab pulls the rest of the plate behind it into the mantle.
- Ridge push (gravitational sliding): At mid-ocean ridges the lithosphere is hotter and higher. Gravity causes the elevated lithosphere to slide away from the ridge crest, pushing plates apart.
- Basal drag / mantle drag: Flowing mantle exerts shear stresses on the base of the plates and can either help drive or resist plate motion depending on the flow direction.
- Slab suction, trench rollback and other local forces: Complex interactions near subduction zones can add additional push/pull components.

Which layers are involved
- Lithosphere: the rigid outer shell (crust + uppermost mantle). This is what we call tectonic plates.
- Asthenosphere: the upper part of the mantle directly beneath the lithosphere. It is hotter, weaker and can flow on geological timescales, allowing lithospheric plates to move.
- Deeper mantle: convection cells extend down into the deeper mantle and help set large-scale flow patterns that influence plate motions.
- Core: not a plate layer, but the core supplies heat that maintains mantle convection (through convective heat transfer and by producing heat via radioactive elements in the mantle).

Observational evidence
- GPS and geodetic measurements show plates moving at rates of about a few millimetres to several centimetres per year.
- Seismic imaging and tomography show slabs sinking and upwelling mantle material.
- Patterns of earthquakes, volcanoes and mid-ocean ridges match the predictions of plate motions driven by the forces above.

Summary
Heat from Earth’s interior drives mantle convection; differences in density and elevation (cold sinking slabs and elevated ridges) produce slab pull and ridge push; and the lithosphere rides on the weaker, flowing asthenosphere. The combination of these processes produces the horizontal motion of tectonic plates.