Geology in action: A mid-ocean ridge you can visit on vacation!
UPDATE Nov 2009: Science magazine only keeps their news articles free for a short time. The link below requires paid access to read. Wikipedia has an article on the volcano.
The Great Rift Valley is a place where one of Earth's tectonic plates is actively dividing in two -- the eastern tip of Africa and the Arabian peninsula is separating from the rest of Africa. In several million years there will be a brand new ocean (not a "sea") like the Atlantic, and the Great Rift Valley will become its mid-ocean ridge.
This African volcano is essentially an example of the undersea volcanoes that run down the spreading center along the middle of the Atlantic (Iceland is a set of them that are above the ocean surface). If you want to see what was happening when the Atlantic Ocean was forming, and Brazil was being broken away from West Africa, go visit Ol Doinyo Lengai in Tanzania.
ADDED: Besides the world's only active (on land!) natrocarbonatite volcano, the Great Rift Valley also has examples of sheeted dikes forming and breaking through to the surface.