Here we discuss the major volcanic landforms and how they are formed, and in some cases, later modified. Most of this material will be discussed with reference to slides shown in class that illustrate the essential features of each volcanic landform. Shield volcanoes are composed almost entirely of relatively thin lava flows built up over a central vent. Most shields were formed by low viscosity basaltic magma that flows easily down slope away form the summit vent. The low viscosity of the magma allows the lava to travel down slope on a gentle slope, but as it cools and its viscosity decreases, its thickness builds up on the lower slopes giving a somewhat steeper lower slope.
Most shield volcanoes have a roughly circular or oval shape in map view.
Very little pyroclastic material is found within a shield volcano, except near the eruptive vents, where small amounts of pyroclastic material accumulate as a result of fire fountaining events. Shield volcanoes thus form by relatively non-explosive eruptions of low viscosity basaltic magma. Vents for most shield volcanoes are central vents, which are circular vents near the summit. Hawaiian shield volcanoes also have flank vents, which radiate from the summit and take the form of en-echelon fractures or fissures, called rift zones, from which lava flows are emitted. This gives Hawaiian shield volcanoes like Kilauea and Mauna Loa their characteristic oval shape in map view.