Volcanoes are caused by movement of tectonic plates within the earth's crust or along ocean floors, and produce primary and secondary effects upon eruption. Volcanoes may form when tectonic plates move away from one another, or when they collide. Volcanoes form on land and beneath the sea. Active volcanoes produce physical primary effects along with long-term secondary effects.
Volcanoes are caused by movement of tectonic plates deep within the earth’s surface and along its ocean floors. Volcanoes are classified in two distinct categories: composite volcanoes and shield volcanoes. Composite volcanoes form from sticky and acidic lava; shield volcanoes, in contrast, form from basic lava. Over time, both types of volcanoes gradually increase in size. Inside their domes, gases and hot air accumulates and leads to increasing pressure. Eventually, volcanoes relieve themselves of these burdens through volcanic eruptions, which produce primary and secondary effects. The possibility for a future volcanic eruption depends on the volcano’s history of activity. Volcanoes may be active (meaning they may erupt at any given time) dormant (with the potential to erupt) or extinct (failing to erupt in the past 10,000 years, and unlikely to do so again).
Most volcanic activity occurs at the boundaries of the planet’s tectonic plates. These plates are essentially large sheets or blocks that occasionally shift positions and move around. The movement of these plates influences the type of volcano that forms as a result, which in turn dictates the volcano’s shape. One type of movement is called spreading plate margins. As the name indicates, this movement involves tectonic plates moving away from each other. When plates move away from each other at divergent tectonic plate margins, they produce basic lava (as opposed to acidic lava). Divergent plate movement generally occurs deep within the sea floors throughout the earth’s oceans. In addition to moving apart, tectonic plates may also move together. This movement is called subducting plate margin movement. Essentially, it involves a slow-moving collision between two massive tectonic plates. At subducting plate margins, one plate is usually pushed beneath the shelf of a bordering plate. As one plate is forced downward, the seawater or dirt, soil and earthen debris is often forced down with it. More violent eruptions generally occur as a result. Volcanoes that form from overlapping plates often assume the classic clone shapes that many people associate with volcanoes. In addition to forming from different types of tectonic plate movements, volcanoes form in different locations along plate boundaries. Some develop far away from plate boundaries, and are called intraplate. These volcanoes generally form above hot mantle pits or plumes, which may produce large plumes stemming from great depths. Eventually, overlying plates move away from these volcanic hotspots, leaving dormant volcanoes in their wake.
Volcanic eruptions produce two types of effects: primary and secondary. Primary effects refer to immediate events stemming from the eruption itself, while secondary effects are conditions and situations resulting from primary effects. Primary effects generally produce physical changes. This category includes volcanic gases, lava flows and pyroclastic flows, which are essentially quickly-moving avalanches of hot ash, rock fragments and gas. Secondary effects are physical and economic in nature: eruptions may create landslides and floods, and interrupt power and water supplies and routine functioning of surrounding towns, cities and communities.