The lava pushed up by volcanic activity around the main island of Taiwan was mainly composed of andesite. Molten andesite has a relatively higher content of silicon dioxide, so is more sticky. Thus it more readily forms into conical mountains such as the Tatun volcanic mountains of northern Taiwan. The basalt rocks of Penghu form a stark contrast to the geology of the main island of Taiwan.
台灣本島的火山活動主要由安山岩組成，安山岩漿的二氧化矽含量較高，較為黏稠，容易堆積成台北大屯山群的圓錐狀火山，因此矽含量較低的澎湖玄武岩，是台灣本島地質非常好的對比。