Why are glaciers such effective agents of erosion and deposition?

They are changing the landscape forms through erosion. 
the resulting landforms include glacial cirques, striations, glacial horns, trim lines, U - shaped valleys, sheepback (roches moutonnees), overdeepening (basins or valley were the water can be accumulated) etc

Magma forms by partial melting of upper mantle and crust. partial melt means that only a fraction of the available material forms a melt, and that the remainder stays solid. the partial melt rises because of its lower density and ascends through he crust. this is so conclusive mean the answer would be magma igneous rocks or balistic magma.

Economy is the answer