The statement that oceanic crust cannot be older than 200 million years is based on the process of plate tectonics and the concept of seafloor spreading. Oceanic crust is continuously being created at mid-ocean ridges through volcanic activity, where magma rises to the surface, cools, and solidifies, forming new crust. As this process occurs, older crust is pushed away from the ridge and eventually subducted beneath continental plates or other oceanic crust in subduction zones.

Since the theory of plate tectonics suggests that the oldest oceanic crust is typically no more than around 200 million years old, this implies that older oceanic crust has been recycled back into the Earth’s mantle through subduction. This process effectively renews the oceanic crust, preventing it from accumulating to ages much older than 200 million years.

However, it’s important to note that there are exceptions to this general rule. Some fragments of ancient oceanic crust, known as ophiolites, can be found in certain geological settings, such as mountain belts, where they have been preserved and uplifted through tectonic processes. These ophiolites provide valuable insights into the history and evolution of oceanic crust, but they are relatively rare compared to the ongoing formation and recycling of younger oceanic crust at mid-ocean ridges and subduction zones.