How to reset my old iMac not properly starting up

I'm trying to reset my old iMac for cleaning all the data before throwing it. When I try to switch it on the screen is showing the apple, a bar never reaching to the end and the screen seems a bit like a chess table with many black and white squares (see attached picture). I've tried already the 10 seconds powering, I've tried Command/Control + Powering, I've tried Control R. Any possible idea on what to do?

Thanks