This weird formation is well-named, pointing directly upward. It's the one geological formation that interests virtually every visitor. It is comprised of a soft volcanic material, easily eroded, that has resulted in the unusual shape. Before the erosion it was a "tuff cone" which results from hardening and compaction of volcanic ash into "tuff". Many tuff cones dot the Galapagos landscape but none as eroded as Pinnacle Rock.
"...probably the most famous (and most photographed) landmark in the Galapagos..."
Barry Boyce on Pinnacle Rock A Traveler's Guide to the Galapagos Islands 1994