What is the difference between the adding of new rock at a divergent plate boundary and volcanic activity?

They never really separate because magma continuously moves up from the mantle into this boundary, building new plate material on both sides of the plate boundary.

This submarine volcanic activity accounts for roughly 75% of the average annual volume of magma that reaches the Earth’s crust. Most submarine volcanoes are found on ridges thousands of meters below the ocean surface.

Explanation:

when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.

the principles and empirical processes of discovery and demonstration considered characteristic of or necessary for scientific investigation, generally involving the observation of phenomena, the formulation of a hypothesis concerning the phenomena, experimentation to test the hypothesis, and development of a conclusion that confirms, rejects, or modifies the hypothesis.