Volcanic landscapes
[see further information (in German) on volcanism and related topics
under:
http://www.uni-muenster.de/MineralogieMuseum/Vulkane/Vulkan-10.htm
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Volcanisms is not randomly distributed over the world but is
concentrated near plate boundaries:
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Where plate subduction or seafloor spreading takes
place.
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At distinct hot spots: where deep mantle plumes reach Earth‘s
surface.
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Fig.1 Major volcanic regions |
(
Source: FAO, 2001.) |
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Landforms in volcanic regions are strongly influenced by the
chemical and mineralogical composition of the materials that were deposited
during eruptive phases. Volcanic rocks and magmas are grouped according to
their silica contents in 3 main categories:
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Rhyolite = 65 – 75 % SiO2
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Andesite = 65 – 55 % SiO2
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Basalt = 55 – 45 % SiO2
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The division based on the silica content is done because the
SiO2-content correlates with the viscosity of magmas and
the type of volcanism.
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Rule of thumb: the higher the
SiO2-content, the more acid and viscous the magma and
the more explosive the volcanic eruptions.
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World Resource Base (WRB) distinguishes 3 landforms of volcanic
regions based on the magma composition:
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Major landforms in regions with basaltic volcanisms.
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Major landforms in regions with andesitic
volcanisms.
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Major landforms in regions with rhyolitic
volcanisms.
Major landforms in regions with basaltic
volcanisms:
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Basaltic volcanism occurs where basic mantle material reaches the
surface:
a. Divergent plate margins (sea floor spreading): the midocean
ridge (for example Island, Canary Islands)
b. Hot spot areas: Hawaii (is the top of the largest shield
volcano in the world) or the continental rift valleys (East African rift
valley)
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Basaltic magma is little viscous and gases escape easily, i.e.
basaltic volcanism produce little ash and pyroclastic rocks (tephra). The
low-viscosity lava (fast moving fluid magma) can flow over large
distances.
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Fig.2 Basaltic magma |
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Fig.3 The low-viscosity lava |
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The resulting shield volcanoes are comparatively flat.
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Fig.4 Shield volcanoes |
(
Source: FAO, 2001.) |
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Most eruptions are fissure eruptions and take place along
extensional cracks in the Earth‘s crust (Ethiopia, India, Siberia).
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Fig.5 Fissure eruption |
(
Source: FAO, 2001.) |
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In the past enormous flood basalts ocurred through fissure
eruptions which covered hundreds of square kilometers. Examples are the Parana
plateau (S-America), Ethiopia, Siberia, India (Deccan Traps are the largest
continental flood basalt and cover about 500.000
km2).
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Fig.6 Flood basalts worldwide |
(
Source: FAO, 2001.) |
Major landforms in regions with andesitic
volcanisms:
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Characteristic appearance at convergent plate boundaries where
plate subduction takes place:
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Fig.7 Plate subduction at convergent plate
boundaries |
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a. Cordillera-type mountain belts (Andes)
b. Island arcs (Philippines and Japan)
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Major concentrations are found around the Pacific rim (andesite
line) and is sometimes called the ring of fire.
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Fig.8 Ring of fire |
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The classical volcano type associated with andesitic volcanism is
the stratovolcano (= stratified volcano).
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Fig.9 Stratovolcano, Stromboli, Italy |
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Lava flows are more viscous and extend only a few kilometers.
Hence, stratovolcanos have steeper slopes than shield volcanoes. They are
composed of alternating layers of lava and pyroclastic rock of andesitic
composition.
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Magma holds an intermediate position between basaltic and
rhyolitic magmas (SiO2-content) and thus produces both
pyroclastics (= tephra) and lavas.
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Fig.10 Tephra of 2-64 mm diameter (lapilli), Kilauea
Caldera Hawaii |
(
Source: US Geological Survey) |
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Active, large and high stratovolcanoes can produce devastating
volcanic mudflows (lahars).
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Fig.11 Lahar |
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Major landforms in regions with rhyolitic
volcanisms
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Produced by partial melting of the continental crust and contains
acid (SiO2-rich) magma.
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Magmas are very viscous and withstand very high gas pressures. As
a result, the eruptions are very violent. The underlying magma chambers can
empty itself completely and huge craters are formed in this way
(calderas).
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Fig.12 Caldera Lake Toba, Sumatra,
Indonesia |
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The high viscosity of the lava precludes lava flow; a lava dome
is formed instead (e.g. Obsidian Dome, USA).
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Main extrusive products of rhyolitic volcanism are:
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These ignimbrites often fill depressions and valleys of tens or
hundreds of meters depth. ignimbrite surfaces are flat and featureless (in
contrast to surfaces of lava flows).
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