Question

How do continents grow by accretion?

Answer 1

Hint: The uppermost solid layer of earth is called the lithosphere. It is composed of the crust and the upper mantle. The lithosphere is further divided into pieces called tectonic or lithosphere plates.

Complete answer: The process by which material is added to tectonic plates or landmass is called accretion.Tectonic plates keep moving because of the convection cell in the mantle. Areas where two or more more lithospheric or tectonic plates collide are called convergent boundaries. Convergent boundaries lead to subduction of one plate under another.
-Tectonic plates move due to heat generated by radioactive decay of elements in the mantle. Convection cells lead to some material escaping to the surface and brings some cooler material down to the mantle. Convection cells result in bringing material to the surface creating a new crust. This new crust becomes denser as it is pushed by newer crust. Subduction takes place when a dense crust collides with a less dense crust. Due to gravity, the subduction slab is driven into the mantle.
-Subducted plate floats in the asthenosphere(lower mantle ). As ocean crust gets recycled into the upper mantle, the lighter material accumulates along continental margins thereby increasing the area of continental landmass.

Note: Continents also grow by deposition of sediments along coastline and riverbanks. Landmass accretion in riverbanks and deltas by alluvium deposits is an example of the same.