Once they'd seen Old Faithful the inlaws thought they'd done geysers. So they were less than impressed when we took them out at dusk to sit on a cold hard wooden bench for 30 minutes, in order to wait for another. Little did they realise, but Grand Geyser is well worth the wait!
The eruption lasted around 10 minutes and we saw two distinct phases. First the massive shooting high into the sky bit. It's big. See how tiny the very tall trees seem by comparison.
Then the jet on the left set going, and a rainbow formed in the evening light, while the main part was a little less vigorous.