One seafloor plate slides under another seafloor plate and begin to melt back into magma. what is the name of this process?

When a plate is pulled under another plate and melts into magma it is called subduction.

It is the doctrine that the us had the right and duty to expand throughout north america. it just means they believed they had the right to expand westward and they did! they stated with the 13 colonies and then they kept moving gaining different territories throughout the years