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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…

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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…

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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…

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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.

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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…

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deposted, the ash, pumice and rock fragments may deform (flatten) and weld together because of
the intense heat and the weight of the overlying material.

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…

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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:…

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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…

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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…

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lower; bubbles of steam rise and force water to the surface. 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…

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