After listening to some Iron Butterfly (never 'eard of 'em afore), I'd describe that as progressive folk psychedelia, of the type that Hawkwind and Jethro Tull did, with a little Latin-esque jazz fusion thrown in from the Santana school.
It could be described as being as pretentious as my description!
Jethro Tull was around at the time but Hawkwind had not been formed.
We were the UK's Hippies. Pink Floyd was still a Rock Band with pretencions.
The Music, several early Led Zep tracks are well known for this, usually started with a cool quiet section which then morphed into a heavy Rock section. Before the Who did their infamous Leeds Concert.