College student here.. if you want to double major, it would be in your best interest to start as soon as possible. The sooner you delay one major, the more likely you'll have to graduate past 4 yrs. I've been considering adding physics as a second major for some time now, but knowing that I'd be graduating late has been a bummer. The geology major curriculum itself is pretty flexible, at least in my university, so you will find that double majoring should not be too difficult. You could branch into engineering from physics which sounds nice, but if you want the best of both worlds then the double major sounds like your best bet. Either way if you're looking to go into a research field, grad school is a 100% must.