The most typical and most common type of volcanoes on continents are steep and conical stratovolcanoes with a relatively small summit crater at their top. The strata (layers) of these volcanoes were built by the eruption and flows of viscous lavas, tephra, and pyroclastic flows and deposits. Infrequent eruptions over hundreds of thousands of years form the regular history of any stratovolcano. The composition of the expelled material varies from mafic to felsic, but only the intermediate to felsic magma types may generate highly explosive eruptions. Some parasitic vents may appear on the flanks and slopes of stratovolcanoes, where lava domes or cinder cones may form.