I can't comment on anything to do with geology, but no, IB Experimental Psychology involves very little of what I'd call 'biology' - the more biological psychology course is IB Neuroscience. It's certainly not uncommon for physicists to take psychology (though the majority of those taking it tend to be biologists and chemists, I'd say there was a fair range - even a few SPSers and philosophers such as yours truly) and in fact one of the supervisors I had, Hugh Greenish, was a physicist who became interested in hearing and is now one of the department's PhD students - go figure.
If you're on facebook you might want to send a message to Martin Smith of Sidney, who is a geologist that took experimental psych last year (after changing from both Plant Science and HPS) so can probably give you a fair response in terms of reassurances/regrets about combining geology and psychology.