Subduction
Subduction is when a big piece of the ground slowly slides under another piece and goes deep down.
Think of it this way
Two cutting boards lie flat on a table, pushed together at one edge. When you press them toward each other, one board slides under the other and drops lower. The top board rides up while the bottom board goes down and under. This is how one ground layer slowly sinks beneath another.
Explaining subduction by grade level
Think of two flat cards on a table. Push them at each other. One card slides under the other. Earth's ground works the same way. Big pieces of rock push at each other. One slides under and sinks deep down where it is very hot.