In the process of L2 acquisition, IL continually evolves into an ever-closer approximation of the TL, and ideally, a learner’s IL should continue to advance gradually until it becomes equivalent, or nearly equivalent, to the TL. However, it has been observed that somewhere in the L2 learning process, such an IL may reach one or more temporary restricting phases during which the development of the IL appears to be detained (Nemser, 1971; Selinker, 1972; Schumann, 1975). A permanent cessation of progress toward the TL has been referred to as fossilization.
Fossilization includes such items as pronunciation, vocabulary usages, and grammatical rules. It has also been noticed that adult L2 learners’ IL systems, in particular, have a tendency, or propensity, to become stagnated or solidified i.e., the language learners make no further progress in IL development toward the TL, and become permanently fossilized, in spite of the amount of exposure to the L2.
The concept of fossilization in SLA research is so intrinsically related to IL that Selinker (1972) considers it to be a fundamental phenomenon of all SLA.