Our Earth is approximately 4.6 billion years old. The oldest continental crust was recently found in Australia and contained pieces of zircon that dates back to about 4.4 billion years. The oldest oceanic crust dates back to only 180 to 200 million years old. What would account for the large discrepancy in age between the oldest continental crust and the oldest oceanic crust?

Answer 1

Young Earth formed by the accretion of cold particles within a cloud of dust and gas. The Earth was then heated by the impact of asteroids, comets, and other falling debris. This heat, combined with gravitational compression and heat from decaying radioactive elements accumulating deep within the newly assembled planet, caused Earth to partially melt. Gravity pulled most of the dense iron and nickel inward to form the planet's core, while the lighter elements-silicon, magnesium, aluminum, and oxygen-bonded compounds-rose to the surface and formed the crust. We can consider the formation of a permanent crust as the "birthday" of Earth some 4.6 billion years ago.
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The ocean basins formed soon after crustal formation. If the ocean basins are genuinely ancient, and if the processes that produce the sediments have been operating for most or all of that time, the age of sediments on the ocean floor should be great like that found on the oldest continental crust, but they are not. The young spreading ridges are almost free of sediment, and the oldest edges of the basins support layers of sediment 15 to 20 times thinner than the age of the ocean itself would suggest. The oldest sediments of the ocean basins are rarely more than 180 million years old. The reason is that sediments are subducted at a plate's leading edge.