Vulcanicity

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15/01/2015
Vulcanicity
Volcano activity tends to be:
Along ocean ridges where plates are moving apart e.g. the Mid Atlantic Ridge, Iceland has
been formed from volcanic activity
Associated with rift valleys e.g. African Rift Valley has a large number of volcanoes along it
including Mt Kenya and Mt Kilamanjaro
On or near subduction zones ­ the line of volcanoes or ring of fire surrounds the Pacific
Ocean ­ most violent activity
Over hot spots like middle of Pacific Ocean give rise to Hawaiian islands
Locating famous volcanoes sheet
Volcanoes are generally located along the
edges of continents e.g. Mt St Helen's, which is
so due to plate boundaries. Anomalies occur in
the middle of plates due to hot spots e.g.
Hawaii.
Volcanoes:
These are openings in the Earth's crust through which lava, ash and gases erupt
Molten rock beneath the Earth's crust is referred to as magma but once ejected it is lava
The pressure exerted upon hot rocks in the mantle keeps them semi solid
Fissures and fractures in the crust create low pressure areas that allow some material
beneath the crust to become molten and rise
If the reach the surface they are extrusive, there are minor and major extrusive landforms,
e.g. Volcanoes and Geysers respectively
If they remain within the crust they are intrusive and only reach the surface through
erosion
Extrusive landforms:
These vary reflecting the material erupted, the nature of the eruption and the time
elapsed since the last activity
Basaltic (basic) lava originates from upward movement of mantle material. Most common
along constructive boundaries but also found at hotspots
Andesitic (intermediate) lava typical at destructive boundaries, here crust is being
destroyed
Rhyolitic (acid) lava is found at destructive and collision margins

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Lava Characteristics Types Characteristics
Basaltic Low viscosity Pahoehoe Least viscous
52% silica (low) lava Rate of advance can be slow
Lets out gases (ropey) Cool surface and flows underneath
Runny
Effusive eruptions A'a lava A few meters thick
Shield, submarine, (jagged) Upper rubbly part
mid-ocean ridges & Lower solid lava
hot spots, rift systems Cools slowly
Andesiti Intermediate
c Medium silica
Lets out some gases
Explosive then runny lava eruptions
Mt Fuji, Japan
Destructive margins
Rhyoliti Acid lava
c High in silica
Lets out few…read more

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Eruptions are explosive and
unpredictable. E.g. Mt Etna,
Italy.
Acid or dome Rhyolitic Steep sided volcanoes, formed
Continental crust from very viscous lava. As the
lava cannot travel far, it builds
up convex cone shaped
volcanoes. Lava may solidify in
the vent and be revealed later
by erosion. Eruptions are
explosive and unpredictable.
E.g. Puy de Domes, France.…read more

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America, and the Cascade Range in North America. Composite volcanoes in the latter category are
present in the Pacific Ring of Fire, and also on the Japanese Islands.…read more

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Primary effects of volcanoes:
1. Lava:
Basaltic, andesitic and rhyolitic
2. Tephra:
Fragments of volcanic rock and lava that are blasted into the air by explosions or carried upward
by hot gases in eruption columns or lava fountains. It includes large dense blocks and bombs, and
small light rock debris such as scoria, pumice, reticulate and ash.…read more

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Example: Philippines, 15th September 1984, pyroclastic flows descend the south-eastern flank of
Mayon Volcano, Philippines. Maximum height of the eruption column was 15km above sea level,
and volcanic ash fell within about 50km toward the west.
Nuée ardente (glowing cloud): a French term introduced in 1904 to describe pyroclastic flows
erupted on Mount Pelée on the Island of Marginique. When viewed at night or in low light,
pyroclastic flows may appear to glow red.…read more

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Secondary effects of volcanoes:
1. Lahars/mudflows:
These are closely associated with volcanic activity; rain brings soot and ash back to ground, which
becomes a heavily saturated mudflow when combined with melted snow and ice. The resulting
flow picks up sediment and turns it into a potentially hazardous mudflow.
Example: Volcanic muds devastated the Columbian town of Armero after the eruption of Nevado
del Ruiz in 1985.
2. Flooding:
Under ice volcanoes can cause catastrophic melting.
Example: Grimsvotn glacial burst in 1996/Eyjafjallajokull in 2010
3.…read more

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Minor forms extrusive activity
Solfatara:
Small volcanic areas without cones, produced by gases (mainly sulphurus) escaping to the suface
e.g. around Bay of Naples, Italy
Geysers:
These occur when water heated by volcanic activity explodes onto the surface e.g. Old Faithful,
Yellowstone National Park/Strokkur, Haukadalur geothermal area, Iceland
Hot springs/boiling mud:
Sometimes the water, heated below the surface does not explode onto the surface. If this water
mixes with surface deposits, boiling mud is formed. This is common in Iceland.…read more

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A fountain of both hot (often boiling) and cold water and steam ejected from the ground at
considerable force. Heights range from being less than 1m to rising over 100m. They can last for
several minutes and make a loud roaring sound as they rise upwards at a very rapid rate.
Examples:
These are located in the Haukadalur geothermal area.…read more

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Pressure is reduced deeper in the
system, so the boiling point reduces. Eventually, the water in the bottom of the reservoir is
converted into superheated steam, this occupies a much greater volume than water thus the
water in the upper part of the reservoir is forced out violently ­ forming the geyser. The eruption
continues until either the temperature drops below boiling or the reservoir runs out of water. The
fountain can be followed by a period of steam.…read more

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