Remnant of an early Earth
Earth’s mantle is a convecting solid. It is hot enough that the material in the mantle flows viscously on a geologic timescale; over millions of years it churns and mixes. How effective is it at mixing? Over the 4.5 billion year history of the Earth, the mantle should have overturned completely several hundred times.
Imagine taking a tube of Aquafresh (the tri-colored toothpaste), emptying it into a jar, and then stirring it a hundred times. Would you be able to find any part of the toothpaste’s original colors? This article tells the story of a group of scientists basically doing exactly that for the Earth.