Volcanoes

• Volcanoes are classified by shape and size. These are determined by such factors as the volume and type of volcanic material ejected, the sequence and variety of eruptions, and the environment.

• Among the most common types are shield volcanoes, Composite cones and cinder cones.

• Shield volcanoes have a low, broad profile created by highly fluid basalt flows that spread over wide areas.

• The fluid basalt cannot build up a cone with sides much steeper than 7 degrees. Over thousands of years, however, these cones can reach massive size.

• The Hawaiian Islands are composed of shield volcanoes that have built up from the sea floor to the surface some 5 kilometres (3 miles) above.

• Peaks such as Mauna Loa and Mauna Kea rise to more than 4,145 meters (13,600 ft) above sea level.

• Hawaii is the largest lava structure in the world, while Mauna Loa, if measured from the sea floor, is the world's largest mountain in terms of both height and volume.

Mauna-loa, Hawaii

• Almost half the world’s volcanoes are composite volcanoes, formed along destructive margins where subduction takes place.

• They include Nevado del Ruiz, Colombia; Mt Pinatubo, Philippines; Chances Peak, Soufriére Hills, Montserrat; Mt St Helens in NW USA’s Cascade Range; and Popocatapetl, Mexico.

• They are composed of alternating layers of lava and pyroclastic material (ash and rock debris).

• When a quiet lava flow ends, it creates a seal of solidified lava within the vent of the volcano.

• Pressure gradually builds up below, setting the stage for a violent blast of pyroclastic material.

• These alternating cycles repeat themselves, giving composite (strato) volcanoes a violent reputation.

Alternate layers of lava and pyroclastic material

Nevado del Ruiz, Colombia

Popocateptl, Mexico

Chances Peak, Soufriere Hills, MontserratLava dome at night

Chances Peak, Soufriere Hills, Montserrat

Haiti

Florida (USA)

Plymouth, Catholic Church Buried In Lahar Deposits, Montserrat

Lahar Deposits In Residential Area, Plymouth, MontserratJanuary 2006

Location of centre of Harris following Feb.11 event. Arrow indicates

remains of school.

http://www.photovolcanica.com/VolcanoInfo/Soufriere%20Hills/Soufriere%20Hills.html

• Check out this website on Montserrat

The volcano was really warming up

Every few minutes a new eruption sent hundreds of thousands of tonnes of ash, rocks, super heated gases and steam into the atmosphere

Montserrat viewed from a nearby island

• A cinder cone is a conical hill of mostly cinder-sized pyroclastics containing large amounts of gases.

• The profile of the cone is determined by the steepest angle at which debris remains stable and does not slide downhill.

• Larger cinder fragments, which fall near the summit, can form slopes exceeding 30 degrees. Finer particles are carried farther from the vent and form gentle slopes of about 10 degrees at the base of the cone.

• These volcanoes tend to be explosive but may also extrude some lava.

• Cinder cones are numerous, occur in all sizes, and tend to rise steeply above the surrounding area. Those occurring on the flanks of larger volcanoes are called parasitic (or secondary) cones.

An example of a cinder cone is Paricutín in Mexico.  It was born on February 20, 1943 in a

corn field and grew to 300 feet in 5 days. It is now 3000 metres. It reached almost 1500 metres in its

first year. It has been dormant since 1952.

Parucutîn, Mexico

• Hot gas and ash that are pushed out of a volcanic vent form a jet. The force with which the gas is pushed out diminishes quickly away from the vent, but where the ash and gas are hot they rise through the atmosphere as a plume.

• As the plume rises it mixes with, and heats, cold air to become more buoyant and rise still further.

• Under certain conditions the ash cloud may become unstable and collapse down the side of the volcano to form a pyroclastic flow, with temperatures typically at 1,000C and speeds of up to 700k/hr (450mls/hr). These pyroclastic flows are very dense and do the most damage and cause the most loss of life. They can knock down houses and uproot massive trees.

• The large quantities of gas (including water vapour) and fine ash particles that are emitted often result in violent rainstorms. The deluges of rain upon the volcano slopes, which may be augmented by melting ice, help to mobilise ash and debris flows to form lahars or mudflows.

Pyroclastic flow, Soufriere Hills, Montserrat

The day after the pyroclastic flow down the Soufriere Hills – everything in its path obliterated and a new land area (delta) formed in the sea

• Volcanic activity typically alternates between short active periods and much longer dormant periods.

• An extinct volcano is one that is not erupting and is not likely to erupt in the future.

• A dormant volcano, while currently inactive, has erupted within historic times and is likely to do so in the future.

• An inactive (extinct) volcano is one that has not been known to erupt within historic times.

• Such classification is arbitrary, however, since almost any volcano is capable of erupting again.

• Volcanoes occur along plate margins that diverge (constructive margins) such as along the Mid Atlantic Ridge, and along destructive margins such as Soufriere Hills, Montserrat where subduction occurs.

• Styles of eruption and types of lava are associated with different kinds of plate boundaries.

• Most lavas that issue from vents in oceanic divergence zones and from mid-oceanic volcanoes are fluid basaltic.

• Where ocean plates collide, the rock is more likely to be andesite which contains silica that makes it more viscous (less fluid).

• Near the zone where an ocean plate and continental margin converge, consolidated ash flows are found.

• Nearly 1,900 volcanoes are active today or known to have been active in historical times.

• Of these, almost 90 percent are situated in the Pacific Ring of Fire. This belt partly coincides with the mountain ranges of western North and South America, and the volcanic island arcs fringing the north and western sides of the Pacific basin.

• Look at the map page 15 to get a better idea of the ditribution of volcanoes around the world.