Final answer:
The material for new crusts comes from the Earth's mantle, through a process called subduction. This results in the melting of the subducted material which rises and solidifies at the Earth's surface, forming new crusts.
Step-by-step explanation:
The principle that 'matter is neither created nor destroyed' is known as the Law of Conservation of Mass. This is especially true in the Earth's system, including in the creation of new tectonic plates. The material for new crusts comes from the earth's mantle through a process called subduction, where one tectonic plate slides beneath another and gets forced down into the mantle due to the pressure. Over time, due to intense heat and pressure, this subducted material melts and forms magma. This magma rises through the cracks in the Earth's crust (often at mid-ocean ridges for oceanic crust) and solidifies when it cools, thus forming new crust material.
Learn more about Plate Tectonics