What Is Fossilization?
We're not talking trilobites and Tyrannosauruses here: Language fossilization refers to the process in the learning of a secondary
language in which the student has more and more difficulty furthering his fluency in the language, until eventually, the student can learn no more. The language, for all intents and purposes, has been set in stone in the mind of the learner at this last point. Some potential for learning small superficial aspects of the language might still exist, such as vocabulary, but conceptual understanding of the material will not develop any further. Fossilization, thus, is a sort of stagnation in secondary language acquisition that cannot be overcome.
Why Does Fossilization Happen?
There's no real rule determining when certain users may begin to fossilize. It varies widely by the individual and by the environment in which the language is learned.
Fossilization most often occurs in an inadequate learning environment. This usually means learning a language in a classroom, as opposed to learning it in the country where it is natively spoken. Many aspects of a language simply cannot be taught in a classroom, where one generally learns a highly academic version of the tongue, as opposed to the colloquial language.
However, fossilization can still occur despite complete immersion in a foreign language environment for decades, a well documented phenomenon among, for example, immigrants. Clearly, this is not the only issue at stake.
Fossilization often means that certain aspects of the language were learned incompletely or incorrectly, such as grammatical features like conjugating verbs in the wrong fashion or using the wrong vocabulary, in such a manner that they cannot be unlearned and replaced with correct usage.
Fossilization may also consist of a sort of subconscious clinging to aspects of the learner's mother tongue, for instance, with syntax and phonology. This may reflect an inability to similarly “unlearn” characteristics of a mother language for the purpose of learning another; the native language so deeply hardwired into the brain that its paradigms cannot be replaced when attempting to learn a new and foreign language.
Critical Period Hypothesis
In the critical period hypothesis, or CPH, it is argued that language fossilization is inevitable in those individuals who are learning a language beyond this critical period, which ends roughly at puberty. CPH as generally accepted allows for virtually unlimited learning of superficial elements of a language such as grammar and vocabulary, but fossilization will still occur when it comes to the more intuitive aspects of a language, such as cadence, pronunciation and idiom.
However, language fossilization has been noted to varying degrees among those still well within the critical period in certain individuals. This may be a minority of individuals, yes, but it proves that those within the CPH are not universally invulnerable to the effect.
The Risk of Fossilization
Fossilization is, again, not well understood, and therefore someone's risk to develop fossilization is also not understood. The current understanding is that those who are learning the language within a native environment are less likely to fossilize, and at a minimum have a higher threshold at which they fossilize. Learning a language at a younger age, following the CPH, also seems to reduce the risk of fossilization. Still, the debate goes on among psycholinguists: keep an eye out for further developments!
For more information, check out this lesson on Interlanguage and Fossilization. For a more technical approach, check out this article on Fossilization, automatization and second language acquisition.