Volcanic Structures

Volcanic plugs are formed as a result of congealed magma and fragmented volcanic rock materials in the feeding conduits of a volcano upon the cessation of volcanic activity.Upon the ceasing of volcanic eruptions, the central conduit of the volcano would be plugged with hard, solidified lava.
They are believed to lie atop a body of magma which could be in the liquid or completely solid form, depending on the state of activity of the volcano. They are commonly funnel-shaped and taper downwards into bodies that are elliptical in shape or elongated, disc-like forms.
If the volcano is composed of weaker materials, erosion is able to remove weaker materials on the volcanic cone, leaving the resistant volcanic conduit standing tall as a volcanic plug.



Devils Tower National Monument, Wyoming in USA is a volcanic plug that rises 386m above the surrounding terrain and the summit is 1,558m above the sea level.

Craters are found near the summit of most volcanoes. A caldera is a funnel-shaped depression opening upwards where volcanic gases, ash and lava are ejected from the volcano. A crater is less than 2km in diameter and formed via outward explosion.

However, many shield volcanoes have a much larger depression known as a caldera, which is am elongated crater at the summit of the volcano.  Calderas are circular, steep walled basins that my be several kilometres or more in diameter. They are the resultant features of the partial emptying of a magma chamber and due to inward collapse. Rapid ejection of magma during a large lava or tephra eruption can cause the emptying or partial emptying of the magma chamber. The unsupported roof of the chamber would then succumb and collapse under its own weight. The caldera may later hold a lake when the caldera cools, enabling snow and rain to accumulate, such as Crater Lake in Oregon, USA and Lake Toba in Indonesia.Subsequent events of minor eruptions can result in the production of small volcanic islands within the lake.



Crater Lake in Oregon occupies a circular caldera 8km in diameter and was attributed to a great eruption by Mount Mazama.





The figure shows the formation of Crater Lake due to the eruption of Mount Mazuma in 5677 B.C.E, which emptied a substantial volume of viscous magma, leaving a void inside the weakened mountain and the eventual failure of the unsupported top into the emptied magma chamber.













With reference to:

• Hyndman, D. (2006) Natural Hazards and Disasters
• Abott, P.L. (2007) Natural Disasters
• http://pubs.usgs.gov/dds/dds-72/site/intro.htm