On granite, they sometimes rout out grooves and insert steel bars for support. These need to go back into the counter behind the edge at least as far as they project. Then, they're epoxied in. If you can talk to the fabricator, see what they suggest and will warrantee. A corbel or two can do it, but it will be a kneebanger and restrict your available space.
Another possibilty is have a steel flat support fabricated. There are shelf brackets that you can buy that will hold far more than that 80#; I've got some on my wall in the garage that are rated for close to 1/2T.
What you don't want is to leave it unsupported, as it is highly likely some kid or adult will decide it is a good place to sit some day and snap it off.