The words eruptive, irruptive sound the same but have different meanings and spellings. Why do eruptive, irruptive sound the same even though they are completely different words?
The answer is simple: eruptive, irruptive are homophones of the English language.
That is accompanied by eruptions
Produced by eruption
An eruptive rock
Irrupting or tending to irrupt.
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.