Question

What is a constructive force caused by the convection in the mantle

Answer 1

Explanation:

As the convection current increases,  it pushes magma at a divergent boundary between two plates.  This forms an undersea mountain range (the Mid-Ocean Ridge) as this magma cools, and further the melting magma welds itself to the plates, creating new crust.  Crust is specifically killed in subduction zones at a convergent margin.  Nevertheless, the melting of the falling plate triggers the explosion of volcanoes on the underlying surface (such as the Andes produced by the collapse of the Nazca plate below the South American plate), which also creates new material on a terrestrial rock recycling.

Even it may seem that the convergent boundaries are destructive, but they are not actually because they give birth to new material from their own death.