The Volcano Adventure Guide: Excellent information and background for anyone wishing to visit active volcanoes safely and enjoyably. The book presents guidelines to visiting 42 different volcanoes around the world.
Illustration of the main types of plate boundaries
Types of subduction zones
Oceanic-oceanic plate collision, subduction and formation of an island arc.
Oceanic-continental plate collision, subduction and formation of a volcanic arc.
There are 2 main types of subduction zones:Oceanic-oceanic plate boundaries: If the subducting plate subducts beneath an adjacent oceanic plate, an island arc is formed. Examples include the Aleutians, the Kuriles, Japan, and the Philippines, all located at the northern and western borders of the Pacific plate. Oceanic-continental plate boundaries: if the subducting plate subducts beneath continental lithosphere, then a similar belt of volcanoes will be generated on the continent. These are called volcanic arcs. Examples include the Cascade volcanic arc of the U.S. Pacific northwest, and the Andes volcanic arc of South America, but also the Hellenic arc in the southern Aegean.
The second important tectonic setting where many volcanoes occur is along or near converging plate boundaries. At most such boundaries, where two plates collide, the heavier of the two - usually an oceanic one - sinks (or is pulled) under the other plate, a process called subduction. Subduction consumes lithosphere and since the surface of the earth is a constant, it compensates for the amount of lithosphere created at divergent plate boundaries.
When the (typically very old) oceanic crust sinks back into the mantle in a subduction zone, it comes progressively under greater pressure and temperature. Its rocks contain significant amounts of water, carbon dioxide and other fluids which are released into the overlaying mantle wedge. Melting the mantle by adding fluids This addition of fluids lowers the melting point of the mantle (in a similar way as adding salt lowers the melting point of ice). As a result, the mantle rocks in the wedge overlying the subducting slab produce partial melts = magmas. As the magmas are lighter than the mantle and start to rise above the subduction zones to produce a linear belt of volcanoes parallel to the oceanic trench. The best example are the subduction zones around the Pacific Ocean, often called the "Ring of Fire". Magmas change composition The magmas in subduction zone volcanoes are often explosive, because they arrive at the surface as very sticky (viscous) and gas rich. Why? On their long way up to the surface, these magmas can (and typically do) undergo a variety of processes, such as cooling and partially crystallizing when they pool and cool in magma chambers at different depths. This lets them cool down, which results in the partial crystallization of the magmas. As different crystals grow from the magmas, the remaining fluid magma changes its chemical composition from the original hot basaltic (silica-poor, iron/magnesium rich) to progressively more silica-rich compositions such as andesitic, dacitic or even rhyolitic.
Explosive fragmentation of sticky, gas-rich magma erupted at Krakatau volcano, a typical subduction-zone volcano.
Volcanoes in subduction zones are typically explosive. Sometimes, they are also called "gray volcanoes" (as opposed to "red volcanoes"), because their eruptions often produce gray ash plumes rather than red hot fluid lava flows. As magmas in subduction zones are typically richer in silica, they are also much more viscous. At the same time, they still contain most of their fluids (mostly water, carbon & sulfur dioxide). At the surface, these fluids will form bubbles, but sometimes are unable to escape the sticky magma other than by explosive fragmentation.
Top 20 quakes in 2016: Several magnitude 7 quakes occurred in 2016 (but none above M8): Papua New Guinea, NZ, Ecuador, Solomon Islands, Sumatra, Chile, Alaska and others - nearly all major earthquakes were located near active subduction zones.
See the top 20 list of largest quakes in 2016 with this map.
Vulkane Italiens - order online: 88 pages, brilliant color photos, German language - this beautifully illustrated guidebook is a perfect introduction and companion for all interested in Italy's rich and varied volcanic areas.
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