According to the U.S. Geological Survey, geologists have identified four types of volcanoes. Cinder cones are the simplest type of volcano, followed by shield volcanoes, composite volcanoes and lava domes.
Cinder cones, such as Mexicos' Paricutin, consist simply of loose tephra, or volcanic debris. They tend to be fairly small, up to 1000 feet in height and a mile across, with steep sides and a small crater. Shield volcanoes, such as the Hawaiian Islands and Iceland, are formed almost entirely of cooled lava flows and can be tens of thousands of feet high and miles across at the base. Composite volcanoes are a blend of both lava flows and tephra, and are generally similar in size to shield volcanoes. Examples of composite volcanoes include Vesuvius, Vulcano and Mount St. Helens. Lava domes are rounder in shape and form when lava erupts very slowly.