The words irruptive, eruptive sound the same but have different meanings and spellings. Why do irruptive, eruptive sound the same even though they are completely different words?
The answer is simple: irruptive, eruptive are homophones of the English language.
That erupts
That is accompanied by eruptions
Produced by eruption
An eruptive rock
Irrupting or tending to irrupt.
Geology Intrusive.
Definitions from Wiktionary, Creative Commons Attribution/Share-Alike License, from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").