So apparently the Phantom Ship at Crater Lake National Park isn't a dike. The article in the Medford Mail Tribune does not make it completely clear, but seems to indicate that the island is the remnant of lava flows, and that its existence is more a matter of chance than the result of harder, more resistant intrusive rock withstanding weathering and erosion. The above picture is from a wikimedia source, not the AP picture accompanying the article; AP is apparently pretty picky about copyright infringement.
I'm sorry to see that Southern Oregon University is canceling its geology program; I have fond memories of that campus and its students from the 80's. Jad D'Allura led two field trips on Klamath Mountain Geology for OSU groups when I was an undergraduate, and I bumped into him briefly when I was in Death Valley over spring break in 1993. Good guy, and great trip leader.