<u>Gravity </u>is the principal <u>driving force </u>of <u>plate tectonics </u>(second one is convection<u>)</u>. It causes different density plates to move on the Earth's surface. However, when a <u>denser plate coincides the less denser plate, the high density plate subducts</u> below the <u>lesser density plate</u>. The process, therefore, is called <u>subduction</u>. During this collision of plates, <u>shearing resistance increases</u> and all <u>pressures come at the edge of the plate</u>. The process continues and the lithosphere drags the rest of the plate. The portion of plate below the less denser plate then reaches the mantle. Here, the edge of plate is destroyed due to high temperature of mantle as well as pressure.
There are a lot of functions to the digestive system like absorbing nutrients, get rid of waste but the main function is to process organic molecules so they can enter cells.