How close are なりつつある and なっている?

Question

語学学習は私の新しい習慣の一部になりつつあるのでしょうか？

"Is learning languages becoming a part of my new daily routine?"

なりつつある here seems similar to なっている... are they often interchangeable? Would I be able to change them back and forth without changing the meaning of this sentence much? This is my first contact with this suffix so if anyone has warnings about its usages or beginner's mistakes, please feel free to espouse.

wwwjdic defines つつある as :to be doing; to be in the process of doing

http://nhg.pro.tok2.com/qa/kousei-7.htm

Answer 1

They are not interchangable.

• なりつつある: in the process of becoming
• なっている: have (already) become

Answer 2

Usually, (ます形)つつある and ～ている are interchangeable. However, when it comes to 瞬間動詞 such as 死ぬ, なる (both are 瞬間動詞の接近型), they are not.

As なる has been well explained, let me use 死ぬ as an example.

• 死につつある：in the process of being dead; dying
• 死んでいる：is dead