GEOGRAPHY PROJECT2012

MADE BY

MATTHEW SPITERI 3.2

DIFFERENT TYPES OFVOLCANOES

Composite VolcanoesComposite volcanoes, also called Strato volcanoes, they are formed by alternating layers of lava and rock fragments.  This is the reason they are called composite.      Strato-volcanoes often form impressive, snow-capped peaks which are often exceeding 2500m in height, 1000sq.km in surface, and 400km3 in volume.      Between eruptions they are often so quiet they seem extinct.  To witness the start of a great eruption requires luck or very careful surveillance.      Composite volcanoes usually erupt in an explosive way.  This is usually caused by viscous magma.  When very viscous magma rises to the surface, it usually clogs the crater pipe, and gas in the crater pipe gets locked up. Therefore, the pressure will increase resulting in an explosive eruption.      Although Strato-volcanoes are usually large and conical, we can distinguish different shapes of them: concave , pyramidal, convex-concave, helmet-shaped , collapse caldera, nested , multiple summits, elongated along a fissure .

Different Shapes of Composite Volcanoes.

Shield VolcanoesShield volcanoes are huge in size.  They are built by many layers of runny lava flows. Lava spills out of a central vent or group of vents.  A broad shaped, gently sloping cone is formed.  This is caused by the very fluid, basaltic lava which can't be piled up into steep mounds.

Shield volcanoes may be produced by hot spots which lay far away from the edges of tectonic plates.   Shields also occur along the mid-oceanic ridge, where sea-floor spreading is in progress and along subduction related volcanic arcs.      The eruptions of shield volcanoes are characterized by low-explosivity lava-fountaining that forms cinder cones and spatter cones at the vent.  Famous shield volcanoes can be found for example in Hawaii.

Cinder Cones

  A Cinder cone is a steep conical hill formed above a vent.  Cinder cones are among the most common volcanic landforms found in the world.  They aren't famous as their eruptions usually don't cause any loss of life.  Cinder cones are chiefly formed by Strombolian eruptions.  The cones usually grow up in groups and they often occur on the flanks of Strato volcanoes and shield volcanoes.      Cinder cones are built from lava fragments called cinders.  The lava fragments are ejected from a single vent and accumulate around the vent when they fall back to earth.      Cinder cones grow rapidly and soon approach their maximum size.  They rarely exceed 250m in height and 500m in diameter.

The shape of a cinder cone can be modified during its (short) life.  When the position of the vent alters, aligned, twin or secant cones develop.  Nested, buried or breached cones are formed when the power of the eruption varies.

Spatter ConesWhen hot erupting lava contains just enough explosive gas to prevent the formation of a lava flow, but not enough to shatter it into small fragments the lava is torn by expanding gases into fluid hot clots, ranging in size from 1cm to 50cm across, called spatter.      When the spatter falls back to Earth the clots weld themselves together and solidify forming steep-sided accumulations. These accumulations focused on an individual vent are called spatter cones.

Different shapes of Spatter Cones

Complex VolcanoesComplex Volcanoes can also be called as compound volcanoes In fact all volcanoes could be complex volcanoes since all of them are made up of multiple flows, ash layers, domes, cones, etc. in varying amounts.      However, when we call a volcano a complex volcano it is because we mean the "system" of those volcanoes is not "simple".  Caldera complexes for instance have often got a large caldera with many subsidiary vents and deposits, some of which could be considered "volcanoes" in their own right.      Frankly speaking, a volcano that consists of a complex of two or more vents is reckoned as a compound or complex volcano.

These are all different types of Complex Volcanoes

How does Volcanoes erupt ???????An eruption begins when pressure on a magma chamber forces magma up through the conduit and out the volcano's vents.  When the magma chamber has been completely filled, the type of eruption partly depends on the amount of gases and silica in the magma.  The amount of silica determines how sticky (level of viscosity) the magma is and water provides the explosive potential of steam.

1. low water, low silica - runny lava flows (not viscous)

2. low water, high silica (very viscous) - pasty lava - often building domes.

3. high water, low silica(not viscous) - fountain of runny lava

4. high water, high silica (very viscous) - explosion

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