When it comes to land and sea, it is easy to think of the winding coastline separating the two completely. However, in the eyes of earth scientists, the difference between continental crust and ocean crust has nothing to do with whether they are submerged by sea water. The continental crust includes not only the terrestrial crust, but also part of the crust below shallow seas closer to the coastline. These areas are called marginal seas and continental shelves. The upper layer of the continental crust is dominated by granite, while the lower layer is dominated by rocks such as basalt. The oceanic crust mainly refers to the part of the crust under the deep-sea ocean basin. There are a small amount of thin deep-sea sediments on the surface. In addition, it is mainly composed of basalt and other materials.
Scientists have found some rocks in the crust of ancient continents around the world, many of which are more than 3.8 billion years old, which is almost as old as the earth; and in the ocean crust on the seabed, the oldest rocks discovered by scientists are no more than 200 million years old. Why can't we find "old" rocks on the ocean floor? This is directly related to the "life journey" of the ocean crust. The theory of seabed spreading and plate tectonics believes that the ruptured plate is driven by mantle convection to move to both sides of the rift, and a large amount of magma from the asthenosphere is ejected from the rift. The rift became a center of ocean expansion, called the mid-ocean ridge. The mid-ocean ridge continues to eject hot magma, which cools, consolidates and settles, so new ocean crusts are "born"; new ocean crusts continue to be produced, pushing out the previously formed ocean crusts, and at the same time these old ocean crusts will continue to cool down and become denser and denser. This is the "growth" of the ocean crusts; When the expanding ocean crust collides with the continental crust, it will subduct under the continental crust and re-enter the earth's interior to be melted. The place where the ocean crust subducts is called the trench, which is undoubtedly the "death canyon" of the ocean crust. The oceanic crust on the seabed follows this life journey repeatedly. Since this cycle usually does not exceed 200 million years, we naturally cannot find older oceanic crust rocks.
Unlike the oceanic crust, once formed, the continental crust can rarely be subducted into the earth's interior to "remelt". This is because the density of the continental crust is relatively low. When it encounters the oceanic crust, the oceanic crust always "sinks" underneath. Only a small part of the continental crust collides with other continental crusts, melting deep in the collision zone. As a result, the continental crust can remain unchanged for a long time, and the oldest rocks on earth are thus preserved inside some very stable continental crusts.