What should we do about the bark splitting open at the base of our 50 year...
We cannot say what the problem is from the photo. Many things can cause bark splitting--from harmless slime flux (you can put that in our website search box) to boring insects.
We'd recommend that you contact an arborist certified by the International Society of Arboriculture (ISA). They are on the staffs of many tree service companies and can come out to your site and diagnose the problem for free. You may want to get more than one opinion.
To find a certified arborist, go to www.treesaregood.org and follow the prompts. They will give you arborists in your area and county.