Huayu Islet

The volcanic rock geology of the Penghu Islands is entirely basalt, with the sole exception of Huayu Islet, which is composed of andesite. Andesite is a rock formed from volcanic magma that cools rapidly after eruption; it has high viscosity and flows slowly, typically accumulating in conical shapes near the volcanic crater, unlike the mesa topography formed by basalt. Formed approximately 65 million years ago, it is the oldest and westernmost island in Penghu.