Volcanism at La Reforma began during the Pliocene, with ash flows and subaqueous pumice flows. Later activity became subaerial and generated pillow lava. Of the local volcanoes, La Reforma was the first to emerge from the sea. Four to five million years ago, basaltic dykes formed. Volcanic activity at La Reforma occurred between 1.6 and 1.4 million years ago, and at least four ignimbrites have been found there.