Types of Volcanoes

The formation of volcanoes is done through geological processes that take place beneath the surface of Earth, mainly plate tectonics and the flow of magma. Magma is hot rock found under the crust and when it accumulates pressure it may upsurge towards the surface through cracks or weak areas of the crust.

Diagram showing different volcano types and their structures

Plate Tectonic Settings

The lithosphere of the earth is separated into a number of tectonic plates that slide gradually over the semi-melting mantle. Volcanic eruptions are usually found in convergent as well as divergent plate boundaries. At convergent boundaries, subduction creates magma that rises to form explosive volcanoes, typical of the Pacific Ring of Fire. At divergent boundaries, plates separate and magma rises to fill the void, creating non-explosive eruptions along mid-ocean ridges (USGS, 2020).

Major Volcano Types

Table 1: Characteristics of Major Volcano Types
Volcano Type	Shape	Eruption Style	Example
Shield Volcano	Broad, gently sloping	Non-explosive, fluid lava flows	Hawaiian Islands
Composite (Stratovolcano)	Steep-sided, conical	Highly explosive	Mount Fuji, Mount St. Helens
Cinder Cone	Steep-sided, small	Short-lived, explosive bursts	Paricutin, Mexico

Shield Volcanoes

The widely occurring type is the shield volcano which is described by broad sides which are gently sloping that are the result of lava accumulation of low viscosity. Shield volcanoes have non-explosive eruptions that allow lava flows over long distances. These volcanoes are often associated with hotspots like the ones in Hawaii (USGS, 2020).

Formed by low-viscosity basaltic lava
Lava flows can travel long distances
Typically not associated with pyroclastic flows

Composite Volcanoes (Stratovolcanoes)

Composite volcanoes are slender and steep-sided volcanoes formed by layers of lava, ash and volcanic debris. Convergent plate boundaries are commonly connected with them and they are highly explosive. Pyroclastic flows, ash clouds, and lahars are typical volcanic hazards associated with composite volcanoes, making them particularly dangerous to nearby populations (Sigurdsson, 2015).

Alternating layers of lava and pyroclastic material
Associated with subduction zone volcanism
Produce the most destructive eruptions

Cinder Cone Volcanoes

The third significant type is the cinder cone volcano which is the most basic and smallest type of volcano. Volcanic fragments ejected in the air during eruption fall back to the ground around the vent forming cinder cones. These volcanoes tend to be steep sided and eruptions tend to be very short lived. Though not as destructive as composite volcanoes, they can be rather dangerous on a local scale (USGS, 2020).

Hotspot Volcanism

Other than plate boundaries, volcanoes also occur at hotspots, or regions of high heat in the earth's mantle. Hotspots are fairly stationary as tectonic plates pass over them. The Hawaiian Islands were formed when the Pacific Plate passed above a mantle hotspot. Hotspot volcanoes usually produce fluid lava flows rather than violent eruptions (Sigurdsson, 2015).

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