If when ice causes rocks to crack by expanding in openings is the most important form of mechanical weathering, then what is the most important part of mechanical weathering?

Answer choices:
- roots and shoots
 - when they liquify
 - roots to grow
 - when it melds into soil
 - volcanic rock cracking
 - a tree falling
 - the looseness of soil
 - ice causes rocks to crack

 

        ice causes rocks to crack  