How Eruptions Work

A few things you should know beforehand:

The basic premise of a volcano eruption is when magma, or melted rock, comes up from inside the Earth and breaks Earth’s surface.

The Earth’s outer layer, or lithosphere, is broken up into several tectonic plates, each of which contains the Earth’s crust and a dense layer of the Earth’s mantle; the plates are about 100 km in thickness.

There are three types of tectonic plate boundaries (places where the plates meet)

            Convergent plate boundaries are where plates push together and form deep trenches, if in the ocean.

            Divergent plate boundaries are where plates pull apart, allowing ridges to form.

            Transform plate boundaries are where plates slip past other; little significant   volcanic activity goes on here

These boundaries are where most of the world’s volcanoes occur.

            -At convergent plate boundaries, spreading center volcanism occurs.  Magma            from below the lithosphere comes to the surface and pushes up to form ridges,        such as the Mid-Atlantic Ridge.  The pressure generated by the rising magma     tends to create fissures in the Earth’s crust, through which the magma erupts and            creates new crust

            -At divergent plate boundaries, what are commonly called subduction zones are           created.  One of the plates is pushed under at these boundaries, sending the            lithosphere into the mantle; this is called subduction.  Water carried by the   subducted plate is released into the mantle, lowering the temperature at which the           mantle melts.  The magma then rises to form volcanoes at the surface.

            -At hot spots, magma rises to the surface at a place not near a plate boundary.

            These spots are thought to be formed by the action of convection.

Convection is the displacement of something hot by something cold.When something is heated, it expands and becomes less dense.  In the case of volcanoes, the cooler mantle pushes upwards and pushes heated magma to the surface.

            Most volcanoes have pockets or chambers of magma below the surface.  The magma has gases distilled in it by the high pressures underground.  As magma is pushed to the surface by convection and by the force of buoyancy, the pressure decreases, allowing the gas to form bubbles.  As the magma gets closer and closer to the surface, more bubbles form, increasing the magma’s speed upward even more.  The thicker (the more silica) the magma, the more bubbles that are trapped, making for a more explosive eruption.

            Buoyancy plays a part in the eruption also.  The buoyant force is calculated by:

                        Magnitude of buoyant force=weight of fluid displaced

            The magma is less dense than the surrounding rock, making it buoyant and         allowing it to rise

Classes of Eruptions  

Several types of volcanic eruptions exist. The type of eruption that occurs is affected by the composition of the magma present and the amount of water vapor. Certain types of eruptions are usually associated with certain types of volcanic landforms.

Hawaiian eruptions-The calmest type of eruption, Hawaiian eruptions generally emit thin basaltic lava.  These eruptions are set apart by their steady flow of lava, which builds up large shield volcanoes, and low amount of tephra produced.

Strombolian eruptions-Named after the island Strombolia, near Sicily, they are typified by periodic short and explosive eruptions of basaltic lava that can go from a few meters to several hundred meters in height.  The tephra emitted by the eruptions usually build a scoria cone.

Plinian eruptions-Named after a Roman who recorded an account of the Vesuvius eruption in 79 A.D., these eruptions usually involve felsic magna and propel ash and pumice many kilometers into the air from a composite or stratovolcano, which falls to cover a large area.  Pyroclastic flows and mudslides can also occur during the course of a Plinian eruption 

Vulcanian eruptions-Named after the Roman god Vulcan, these eruptions commence as a series of short, violent bursts, then settles into a smaller and more sustained eruption pattern.  Less tephra is produced than in a Plinian eruption, and the height of the eruption ranges from 5-10 km.  

Hydrovolcanic eruptions-These eruptions happen when magma touches ground or surface water, and usually occur underwater in the ocean in varying forms of violence and with different types of magma.

This diagram shows the amount of explosiveness relative to the height of the eruption column for the various types of eruptions.  As you can see, the ultraplinian and plinian has the highest explosiveness, while the hawaiian has the lowest.