At face value, diagenesis and lithification seem like the same process. If a very basic sense, both of these terms describe the action of sediment becoming stone. However, lithification is just one part of the diagenesis of rock.
Diagenesis describes the entire process of sediment changing over the course of time since it was deposited, up until it is fully lithified. It also describes not just the process of sediment becoming rock, but all changes that happen to the sediment overtime, regardless of whether it is becoming more solid.
On the other hand, lithification is the very specific process of sediment becoming stone.
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